Showing papers in "Quarterly Journal of the Royal Meteorological Society in 2000"

A turbulence scheme allowing for mesoscale and large‐eddy simulations

Abstract: The paper describes the turbulence scheme implemented in the Meso-NH community research model, and reports on some validation studies. Since the model is intended to perform both large-eddy and mesoscale simulations, we have developed a full three-dimensional scheme, based on the original method of Redelsperger and Sommeria. A prognostic equation for the turbulent kinetic energy is used, together with conservative variables for moist non-precipitating processes. A particularity of the scheme is the use of variable turbulent Prandtl and Schmidt numbers, consistently derived from the complete set of second-order turbulent-moment equations. The results of three idealized boundary-layer simulations allowing detailed comparisons with other large-eddy simulation (LES) models are discussed, and lead to the conclusion that the model is performing satisfactorily. The vertical flux and gradient computation can be run in isolation from the rest of the scheme, providing an efficient single-column parametrization for the mesoscale configuration of the model, if an appropriate parametrization of the eddy length-scale is used. The mixing-length specification is then the only aspect of the scheme which differs from the LES to the mesoscale configuration, and the numerical constants used for the closure terms are the same in both configurations. The scheme is run in single-column mode for the same three cases as above, and a comparison of single-column and LES results again leads to satisfactory results. It is believed that this result is original, and is due to the proper formulation of the parametrized mixing length and of the turbulent Prandtl and Schmidt numbers. In fact, a comparison of the parametrized mixing length with the length-scale of the energy-containing eddies deduced by spectral analysis of the LES shows interesting similarity.

North atlantic oscillatiodannular mode: Two paradigms—one phenomenon

Abstract: The North Atlantic Oscillation (NAO), as defined in the studies of Sir Gilbert Walker ca. 1930, and the zonal-index cycle, as elaborated by investigators at the Massachusetts Institute of Technology some twenty years later, are different interpretations of the same entity, whose time variations are well represented by the leading principal component of the northern hemisphere sea-level pressure field. The NAO paradigm envisions this phenomenon as involving a unique teleconnection pattern in the Atlantic sector that varies on interannual and longer time-scales in association with large-scale atmosphere-ocean interaction. In contrast, the zonal-index-cycle paradigm posits the existence of independent, fundamentally zonally symmetric (or ‘annular’) modes of variability in the northern and southern hemispheres, both of which fluctuate on intraseasonal as well as interannual time-scales. Spontaneous interactions between the zonally symmetric flow and the eddies are viewed as being largely responsible for the variability of the annular modes at the higher frequencies, and a variety of different mechanisms including, but by no means limited to, atmosphere-ocean interaction are viewed as potentially capable of forcing them at the lower frequencies. The NAO and ‘annular mode’ paradigms offer contradictory interpretations of the causal linkages that are responsible for the observed correlations between North Atlantic climate variability and variations in a diverse array of zonally averaged quantities. They suggest different research agendas and they evoke quite different images in the popular press. It is argued that the two paradigms cannot be equally valid and that it is in the interests of the community to come to a consensus as to which of them is more appropriate. Rules of evidence are proposed as a basis for making that decision.

The Met. Office global three‐dimensional variational data assimilation scheme

Abstract: The Met. Office has developed a variational assimilation for its Unified Model forecast system, which contains a grid-point mode) that is run operationally in global, mesoscale, and stratospheric configuration. Key characteristics of the design are: •a development path from three-dimensional to four-dimensional variational assimilation; •global and limited-area configurations; •variational analysis of perturbations; •and a carefully designed, well conditioned background term. The background term is implemented using a sequence of variable transforms to independent balanced and unbalanced variables, to vertical modes, and to spectral coefficients. The coefficients used are based on statistics from differences of one- and two-day forecasts valid at the same time. The covariance model represents many of the features seen in the covariances of forecast differences. The three-dimensional variational data assimilation (3D-Var) system was implemented in the operational global forecast system on 29 March 1999. In parallel trials, the 3D-Var system gave a 2.7% improvement in a composite skill score (verified against observations and weighted according to the importance of each field).

Skill and relative economic value of the ECMWF ensemble prediction system

Abstract: SUMMARY The economic value of the European Centre for Medium-Range Weather Forecasts (ECMWF) operational ensemble prediction system (EPS) is assessed relative to the value of a perfect deterministic forecast. The EPS has substantial relative value throughout the medium range. Probability forecasts derived from the EPS are of greater benefit than a deterministic forecast produced by the same model. Indeed, for many users, the probability forecasts have more value than a shorter-range deterministic forecast. Based on the measures used here, the additional information in the EPS (reflecting the uncertainty in the initial conditions) provides a benefit to users equivalent to many years’ development of the forecast model and assimilation system. The impact of ensemble size on forecast value is considered. The difference in performance between ensembles with 10 and with 50 members may appear relatively small, based on standard skill measures, yet the larger ensembles have substantial benefit to a range of users. Further increases in ensemble size may be. expected to provide additional value.

Atmosphere-ocean thermal coupling in the North Atlantic: A positive feedback

Abstract: In this article, it is examined whether a positive atmosphere-ocean feedback is found over the North Atlantic through a series of general circulation model (GCM) experiments. A 60-year integration of an atmospheric GCM (AGCM) coupled with a 50 m deep motionless ocean was conducted. The coupled run was compared with two uncoupled runs, one with prescribed mid-latitude climato-logical sea surface temperatures (SSTs) and the other with daily SSTs derived from the coupled experiment. The uncoupled atmosphere forces the slab ocean, to obtain SST responses in these runs. Consistent with previous studies, the coupled atmosphere-ocean fields show interannual variances larger than those in the uncoupled runs, due to reduction in the local thermal damping. Patterns of maximum atmosphere-ocean covariability show the North Atlantic Oscillation (NAO) and tripole SST anomalies both in the coupled and uncoupled fields, indicating a dominant role of the atmosphere in generating the SST anomalies. On the other hand, analyses of the temporal variability in the three runs suggest an active role of SST anomalies in determining the polarity of the air-sea coupled variability longer than the interannual time-scale. A combined analysis of forcing SST, upper-air height, and the response SST anomalies in the uncoupled run identified that a patch of positive SST anomalies in the mid-latitude band around 40°N effectively excite the positive phase of the NAO, which in turn reinforces the tripole SST anomalies. This relationship was further confirmed by a nine-member ensemble AGCM experiment forced by the relevant SST anomalies. Since the forcing and response SST anomaly patterns bear some resemblance, these results manifest positive feedback at work in the coupled atmosphere-ocean patterns. The processes responsible for this positive feedback were elaborated by a series of linear model experiments. The local thermal adjustment of the atmosphere to the SST anomalies results in increased precipitation over the Gulf Stream region. Associated diabatic heating applied to a linear baroclinic model yields a positive height response to the east, which highly influences the southern part of the NAO. This stationary response in turn induces a northward deflection in the storm track activity, leading to an eddy vorticity feedback that tends to force the positive phase of the NAO.

Propagation mechanisms for the Madden-Julian Oscillation

Abstract: The Madden-Julian Oscillation (MJO) is examined using 20-years of outgoing long-wave radiation and National Centers for Environmental Prediction/National Center for Atmospheric Research re-analysis data. Two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested. The first is a local mechanism operating over the warm-pool region. At the phase of the MJO with a dipole structure to the convection anomalies, there is enhanced tropical convection over the eastern Indian Ocean and reduced convection over the western Pacific. Over the equatorial western Indian Ocean, the equatorial Rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge. This lends to suppress ascent in the boundary layer and shuts off the deep convection, eventually leading to a convective anomaly of the opposite sign. Over the Indonesian sector, the equatorial Kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough, which will tend to favour convection in this region. The Indonesian sector is also influenced by an equatorial Rossby wave response (of opposite sign) to the west of die reduced convection over the western Pacific, which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough. Hence, convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial Rossby wave response, and expand to the east via their equatorial Kelvin wave response. The second mechanism is global, involving an anomaly completing a circuit of the equator. Enhanced convection over the tropical western Pacific excites a negative mean-sea-level pressure (m.s.l.p.) anomaly which radiates rapidly eastward as a dry equatorial Kelvin wave at approximately 35 m s−1 over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the m.s.l.p. anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle.

The ecmwf operational implementation of four-dimensional variational assimilation. III: Experimental results and diagnostics with operational configuration

Abstract: The first two papers of this series describe the development of the operational four-dimensional variational assimilation (4D-Var) configuration implemented at the European Centre for Medium-Range Weather Forecasts (ECMWF) The basic features are a 6-hour incremental 4D-Var set-up with two minimization steps, using very simplified physics in the first minimization and a more complete set of linear physics in the second This paper describes the validation of this configuration Prior to implementation, 12 weeks of experimentation showed a consistent improvement relative to 3D-Var After an additional 6 weeks of encouraging parallel operation with the then current operational suite, 4D-Var with physics was introduced in operations at ECMWF in November 1997 The difference in scores is statistically significant, and the fast-growing components of the 4D-Var analysis errors are shown to be smaller than their 3D-Var counterparts The performance of this new operational assimilation system is studied for the month of January 1998, for which the 4D-Var analyses exhibit more realistic baroclinic waves than the 3D-Var, especially in the Pacific area A case-study illustrates the improvement one can expect in forecast terms in the mid latitudes The 4D-Var system improved the forecast skill in the Tropics in general Observing-system experiments show that the current 4D-Var operational system benefits from the assimilation both of satellite data and conventional observations

Deriving cloud overlap statistics from radar

Abstract: The predictions of general-circulation models (GCMs) are sensitive to the assumed cloud overlap within a vertical column of model grid boxes, but until now no reliable observations of the degree of cloud overlap have been available. In this note we derive the overlap characteristics of clouds from 71 days of high vertical resolution 94 GHz cloud radar data in the UK. It is found that, contrary to the assumption made in most models, vertically continuous clouds tend not to be maximally overlapped. Rather, the overlap of clouds at two levels tends to fall rapidly as their vertical separation is increased, and for levels more than 4 km apart, overlap is essentially random. A simple inverse-exponential expression for the degree of overlap as a function of level separation is proposed that could, once results become available from a variety of other locations and seasons, be implemented in current GCMs with relatively little difficulty.

A probability and decision-model analysis of PROVOST seasonal multi-model ensemble integrations

Abstract: A probabilistic analysis is made of seasonal ensemble integrations from the PROVOST project (PRediction Of climate Variations On Seasonal to interannual Time-scales), with emphasis on the Brier score and related Murphy decomposition, and the relative operating characteristic. To illustrate the significance of these results to potential users, results from the analysis of the relative operating characteristic are input to a simple decision model. The decision-model analysis is used to define a user-specific objective measure of the economic value of seasonal forecasts. The analysis is made for two simple meteorological forecast conditions or ‘events’, E, based on 850 hPa temperature. The ensemble integrations result from integrating four different models over the period 1979–93. For each model a set of 9-member ensembles is generated by running from consecutive analyses. Results from the Brier skill score analysis taken over all northern hemisphere grid points indicate that, whilst the skill of individual-model ensembles is only marginally higher than a probabilistic forecast of climatological frequencies, the multi-model ensemble is substantially more skilful than climatology. Both reliability and resolution are better for the multi-model ensemble than for the individual-model ensembles. This improvement arises both from the use of different models in the ensemble, and from the enhanced ensemble size obtained by combining individual-model ensembles; the latter reason was found to be the more important. Brier skill scores are higher for years in which there were moderate or strong El Nino Southern Oscillation (ENSO) events. Over Europe, only the multi-model ensembles showed skill over climatology. Similar conclusions are reached from an analysis of the relative operating characteristic. Results from the decision-model analysis show that the economic value of seasonal forecasts is strongly dependent on the cost, C, to the user of taking precautionary action against E, in relation to the potential loss, L, if precautionary action is not taken and E occurs. However, based on the multi-model ensemble data, the economic value can be as much as 50% of the value of a hypothetical perfect deterministic forecast. For the hemisphere as a whole, value is enhanced by restriction to ENSO years. It is shown that there is potential economic value in seasonal forecasts for European users. However, the impact of ENSO on economic value over Europe is mixed; value is enhanced by El Nino only for some potential users with specific C/L. The techniques developed are applicable to complex E for arbitrary regions. Hence these techniques are proposed as the basis of an objective probabilistic and decision-model evaluation of operational seasonal ensemble forecasts.

Predictability and the relationship between subseasonal and interannual variability during the Asian summer monsoon

Abstract: The relationship between subseasonal and interannual variability of the Asian summer monsoon has been investigated through analysis of the dominant modes of variability in the 40-year NCEP/NCAR Re-analysis, with complementary satellite and surface-based precipitation data. The hypothesis that the characteristics of monsoon subseasonal variability (i.e. weather regimes) are modulated on interannual time-scales in a systematic and therefore predictable manner has been tested. The null hypothesis is that predictability of the seasonal mean monsoon behaviour requires that the effects of the slowly varying components of the climate system be correctly simulated. An interannual mode of monsoon variability has been identified which is closely related to the observed seasonal mean all-India Rainfall (AIR). A counterpart of this mode has also been identified at subseasonal time-scales which projects strongly on to the daily AIR, confirming that a common mode of monsoon variability exists on sub-seasonal and interannual time-scales. It has been shown that the temporal behaviour of this subseasonal mode, as described by the probability distribution function (PDF) of the principal component time series, does not show any evidence of bimodality, the shape of the PDF being Gaussian. Further, it has been shown that the mean of the PDF is systematically and significantly perturbed towards negative (positive) values in weak (strong) monsoon years as categorized in terms of the seasonal mean AIR. This translation in the mean of the PDF, rather than a change in shape of the PDF, suggests that anomalous monsoons are not associated with changes in weather regimes. Further analysis has confirmed that low-frequency modulation of the basic state is primarily responsible for these shifts in the subseasonal PDFs, supporting the null hypothesis that predictability of the seasonal mean monsoon requires that the effects of the slowly varying components of the climate system be correctly simulated. Thus, model improvements to reduce systematic errors in the mean simulation and the response to low-frequency boundary forcing may improve the prospects for dynamical seasonal prediction. However, the results indicate that only a subset of the subseasonal modes are systematically perturbed either by the El Nino Southern Oscillation or in weak vs. strong monsoon years, suggesting that predictability is likely to be limited by the chaotic, internal variability of the monsoon system.

A comprehensive two‐moment warm microphysical bulk scheme. I: Description and tests

Abstract: A bulk microphysical scheme which predicts the concentrations and mixing ratios of cloud droplets and raindrops is presented. The scheme draws its originality from the use of generalized gamma law as basis functions for the drop size distributions and also from the attention paid to performing analytical integrations of most of the microphysical transfer rates. The numerical representation of each process has been reviewed throughout and specific tests have been made to evaluate separately the accuracy of the scheme compared with a bin-size model. The scheme, which depends on the specification of a few input parameters shaping the activation spectrum, is incorporated in a three-dimensional non-hydrostatic model with some applications given in a companion paper (Part II).

The parametrization of drag induced by stratified flow over anisotropic orography

Abstract: A new parametrization of drag arising from the flow over unresolved topography (UT) in a general-circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low-level wave breaking, and a parametrization of low-level drag associated with upstream blocking and lee-vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low-level flow so that it is more parallel to unresolved two-dimensional topographic ridges. The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five-year seasonal means of present-day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity-wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity-wave field with two waves rather than just one allows 30-50% more gravity-wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere.

Retrieving temperature, water vapour and surface pressure information from refractive-index profiles derived by radio occultation: A simulation study

Abstract: A one-dimensional variational retrieval for the assimilation of refractive-index profiles derived from radio occultation (RO) measurements has been developed. The method is tested by using simulated data to assess the retrieval accuracy and information content of the measurements, using various realistic estimates for the assumed error distributions. Theoretical retrieval-error estimates given by a solution covariance matrix are in good qualitative agreement with those derived statistically from the comparison of the solution profiles with the precisely defined ‘true’ values. It is demonstrated that the ‘water vapour ambiguity’ inherent in more conventional RO inversion methods is resolved with this approach. It is found that the solution x2values, quantifying the fit to the observed refractivities and a priori profile estimates, are in good agreement with the theoretical distribution, suggesting that they could be used for quality-control purposes. Furthermore, it is shown that the measurements contain significant surface pressure information. This arises through the hydrostatic relationship, as a result of mapping the state-vector information on pressure levels to height coordinates. The simulations indicate that the measurements in the tropics contain the greatest surface pressure information, with a reduction of the background error of ∼45%. This new result has significance for the design of future observing systems.

The use of TOVS level-1b radiances in the NCEP SSI analysis system

Abstract: A methodology is described for the direct assimilation of TOVS (TIROS-N (Television Infra-Red Observation Satellite) Operational Vertical Sounder) radiance data that have not undergone the pre-processing (such as limb adjustment and cloud clearing) that is usually applied to make radiances suitable for use in linear retrieval algorithms Inherent nonlinearities in the observations can be treated successfully within a variational analysis scheme such as Spectral Statistical Interpolation (SSI) thus making radiance pre-processing unnecessary The benefit of using unprocessed (or level-1b) data is demonstrated in terms of an improved ability to quality control and bias correct the observations within the analysis The assimilation of level-lb radiance data in the SSI has shown a significant positive impact on the quality of the analysis compared with the use of pre-processed radiances, and a significant improvement in forecast skill for the tropics and southern hemisphere

Thermodynamic control of tropical rainfall

Abstract: SUMMARY In 1987, Neelin and Held showed how, in the tropics, surface heat fluxes and infrared radiation control atmospheric convergence, and hence rainfall, by means of their joint effect on the supply of moist static energy to the troposphere. They also showed that for a given rate of supply of moist static energy, the strength of convergence is inversely proportional to a ‘gross moist stability’, which is related to the humidity of the troposphere and to the difference between the height of the environmental minimum in moist static energy and the elevation of maximum vertical mass-flux. The present paper extends NeeIin and Held’s analysis to the non-equilibrium case by invoking the somewhat speculative hypothesis that rainfall is primarily controlled by the mean saturationdeficit of the troposphere. The relaxation time of the atmosphere to the Neelin-Held equilibrium is found to be a strong function of the existing saturation-deficit under this hypothesis. The means by which the large-scale circulation of the atmosphere controls convection and precipitation is still a matter of controversy. The present paper attempts to illuminate this matter by extending the ideas of Neelin and Held (1987) (hereafter NH87). NH87 related the change in the moist static energy of air passing through convective systems to the net input of energy into the troposphere by surface fluxes and radiation. This allowed them to infer how horizontal divergence is related to forcing by radiation and surface heat flux. Their analysis, however, is useful only in the steady state. The present paper extends the analysis to the non-equilibrium case. Moist static energy and moist entropy have quite similar properties in the atmosphere-the main difference being that the former is approximately conserved between hypothetical static states with zero kinetic energy before and after a dynamic event, whereas the latter is conserved during dynamic events to the extent that they operate in reversible adiabatic fashion. Neither quantity is perfectly conserved between real atmospheric states, however, so that the choice of which to use is somewhat a matter of taste. The present paper recasts the arguments of NH87 in terms of entropy and the closely related quantity equivalent potential temperature. Figure 1 shows the entropy currents pertinent to the analysis of NH87, who note that, in a steady situation, 1,s - led = le out - le in.

Multi-model spread and probabilistic seasonal forecasts in PROVOST

Abstract: The skill of the PROVOST (PRediction Of climate Variations On Seasonal to interannual Time-scales) long-range multi-model ensemble integrations is analysed. The ensemble PROVOST forecasts result from integrating three different models over the period 1979–93 using analysed sea surface temperatures. For each model, a set of nine-member ensembles have been run from consecutive European Centre for Medium-Range Weather Forecasts re-analyses. Using the full set of models, a large multi-model ensemble has been constructed and verified. Positive skill is found for forecasts of geopotential at 500 hPa, temperature at 850 hPa and precipitation. Skilful forecasts tend to occur at the same time in most of the models when skill is computed over large areas; the European region is poorly forecast. The skill commonality may be due to the use of either similar initial conditions or boundary conditions. Skill is shown to be at a maximum in late winter and early spring in mid latitudes. No means have been found for linearly predicting the skill of the ensemble mean using the ensemble spread. The multi-model ensemble improves the skill of the individual models only marginally when verifying the ensemble mean. However, when using the full ensemble in a probabilistic formulation, the multi-model approach offers a systematic improvement. The improvement arises both from the use of different models in the ensemble and from the higher ensemble size obtained by combining all of the models for building the multi-model ensemble. It is shown that a part of the skill improvement in the tropics is due to the multi-model approach, mainly in spring and summer. On the other hand, most of the gain in the extratropics comes from the increase in ensemble size.

Ice particle habits in stratiform clouds

Abstract: Ice crystals in clouds in the atmosphere have shapes that relate to their density, terminal fall velocity, growth rate and radiative properties. In calculations for climate-change predictions, forecasting of precipitation, and remote-sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are often assumed. The objective of this work is to study the frequency of occurrence of different habits of ice particles in natural clouds from aircraft observations. Images of cloud particles were measured by a PMS Optical Array Probe-2DC at 25 μm resolution installed on the National Research Council Convair-580. The processing of particle images was conducted with a newly developed algorithm for pattern recognition. Data were collected during four field projects in the Canadian and US Arctic over the North Atlantic near Newfoundland, and over the Great Lakes. Approximately 5 × 106 images of cloud particles having a size larger than 125 μm were analysed. The cloud particles were classified into four categories; spheres, irregulars, needles/columns and dendrites. The habit classification of particles was done for three different size ranges: > 125 μm, >250 μm. and >500 μm. The frequency of occurrence of different habits was found for each 5 degC temperature interval in the range −45°C < T < 0°C. It was concluded that the majority of ice particles in natural clouds were of irregular shape. The frequency of occurrence of irregular ice decreases with increasing particle size. On average, the concentration of particles larger than 125 μm was approximately constant down to −35°C, whereas the concentration of particles larger than 500 μm decreased at temperatures below −15°C. Since the data were collected in different climatic zones within many cloud lypes. and covered a significant cloud path length (3.6 × 104 km), the conclusions are applicable to most stratiform clouds containing ice.

A comparison of the impact of TOVS arid ATOVS satellite sounding data on the accuracy of numerical weather forecasts

Abstract: The Advanced TIROS Operational Vertical Sounder (ATOVS) was launched on the NOAA-15 satellite in May 1998. This provided a very significant improvement in the information available from meteorological polar-orbiting satellites compared with the previous TIROS Operational Vertical Sounder system, particularly for humidity and vertical resolution of temperature in cloudy areas. In preparation for assimilation of the observations into a three-dimensional analysis of atmospheric temperature and humidity, the observations have been compared with calculated top-of-atmosphere brightness temperatures computed from numerical weather prediction model profiles of temperature and humidity. Differences between observed and modelled brightness temperature are small. In some parts of the tropics and northern hemisphere the standard deviation of these differences for the tropospheric Advanced Microwave Sounding Unit sounding channels is only marginally higher than the radiometric noise of the observations. Early in 1999 a series of observation-system experiments were completed in which ATOVS observations were assimilated using a one-dimensional variational analysis. No use of the new humidity information could be made because of interference problems experienced by the microwave humidity sounder on ATOVS. Nonetheless, these experiments showed that the assimilation of the new temperature information provided by the radiance observations reduces forecast errors by as much as 20% in the southern hemisphere and 5% in the northern hemisphere. Further improvements have been found by assimilating more data over land. The major impact arises from the microwave channels. Whilst forward-model errors may be slightly lower for the microwave channels than the infrared channels the primary reason is the provision of sounding information in active weather systems, which are usually cloudy.

Influence of the Indian summer monsoon on ENSO

Abstract: Historical records (approximately 100 years) of Indian summer monsoon rainfall and El Nino/Southern Oscillation (ENSO) indices show a strong negative correlation. This negative correlation is strongest for east Pacific sea surface temperature anomalies (SSTA) that occur during the months of December through to March, which is about three to six months immediately following the monsoon season (June to September). Based on this correlation, one is tempted to speculate that monsoon variability affects ENSO variability. However, it is well known that ENSO is phase locked to the annual cycle in that the largest SSTA occur at the end of the calendar year. In other words, an ENSO which originated well before the summer monsoon season will have its peak amplitude at the end of the calendar year. The purpose of this study is to explain the impact of the monsoon which has a strong seasonal preference on ENSO which has a life cycle of about 4 years. First, a 50-year atmospheric general-circulation model simulation with climatological SST is examined to determine the tropical Pacific wind-stress anomalies that are associated with a variable monsoon but that are also independent of SST variability in the tropical Pacific. Using simple statistical techniques, it is found that a weak (strong) monsoon results in a weakening (strengthening) of the trade winds over the tropical Pacific. To examine how these 'monsoon-forced wind-stress anomalies' in the tropical Pacific affect ENSO, simulations were made with a simple coupled model that does not include the effects of a variable monsoon. The effects of the monsoon are then added in the coupled model by either specifying the strength of the monsoon or by parametrizing the strength of the monsoon in terms of the coupled-model simulated SSTA in the east Pacific. Based on these coupled simulations, a variable monsoon enhances the ENSO variability, particularly three to six months after the monsoon ends, and can also serve as a trigger mechanism for ENSO. It is found that an ongoing warm (cold) ENSO event is made even warmer (colder) by a weak (strong) monsoon. Similarly, warm (cold) events are weakened by a strong (weak) monsoon. These results also reproduce the observed lag/lead ENSO-monsoon relation where the maximum negative correlation between the monsoon and the SSTA in the east Pacific occurs 3-6 months after the monsoon season.

A gcss model intercomparison for a tropical squall line observed during toga-coare. I: Cloud-resolving models

Abstract: Results from eight cloud-resolving models are compared for the first time for the case of an oceanic tropical squall line observed during the Tropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment There is broad agreement between all the models in describing the overall structure and propagation of the squall line and some quantitative agreement in the evolution of rainfall There is also a more qualitative agreement between the models in describing the vertical structure of the apparent heat and moisture sources The three-dimensional (3D) experiments with an active ice phase and open lateral boundary conditions along the direction of the system propagation show good agreement for all parameters The comparison of 3D simulated fields with those obtained from two different analyses of airborne Doppler radar data indicates that the 3D models are able to simulate the dynamical structure of the squall line, including the observed double-peaked updraughts However, the second updraught peak at around 10 km in height is obtained only when the ice phase is represented The 2D simulations with an ice-phase parametrization also exhibit this structure, although with a larger temporal variability In the 3D simulations, the evolution of the mean wind profile is in the sense of decreasing the shear, but the 2D simulations are unable to reproduce this behaviour

The evolution of the tropical western Pacific atmosphere-ocean system following the arrival of a dry intrusion

Abstract: Recent studies using TOGA COARE data have found that extremely dry air from middle-latitude waves frequently intrudes into the equatorial troposphere over the western Pacific. Using sounding data taken during the COARE, the magnitude of the advection of water vapour for one event is calculated, and it is estimated that the lime for the atmosphere to recover to moist conditions was ∼ 10-20 days. From the magnitude of the drying and from the frequency of these events, it is proposed that dry intrusions must be a major contributor to the tropospheric moisture budget over the region during the COARE, making it difficult for the atmosphere to reach a radiative-convective equilibrium, intrusions, instead, can help to recharge the tropical atmosphere by decreasing convective activity and, thus, driving the atmosphere toward unusually large values of convective available potential energy. A variety of atmospheric and oceanic measurements are also used to study the recovery process in detail. A conceptual model is proposed based on this work and previous investigations. As in past studies, the recovery of the atmosphere to moist conditions is accomplished through detrainment from convective clouds that began to form soon after the arrival of the dry air mass and slowly deepen in height as the recovery progresses. Previous investigators concluded that the entrainment of dry air into convective ceils is generally the factor that tends to suppress convective activity and limits the height of any convection that does develop under these adverse conditions. The idea that entrainment limits convective activity is consistent with the commonly held perception that the western Pacific is a region where there is little inhibition to deep convection and, when inhibition does occur, it can be removed by surface fluxes within hours. In contrast, it is found that convective inhibition can be large enough to suppress convection following dry intrusions, and that the diurnal variation in rainfall is due partly to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical profiles of radiation, in surface fluxes, and perhaps in large-scale subsidence, leading to a minimum in convective inhibition during the late afternoon. In contrast, studies of this type of convection have generally emphasized diurnal variations in the surface fluxes, and often ignored convective inhibition and diurnal variations in atmospheric radiative heating.

Observed winter cyclone tracks in the northern hemisphere in re-analysed ECMWF data

Abstract: The observed cyclone activity in the northern-hemisphere winter (DJF) is investigated in 1979-97 European Centre for Medium-Range Weather Forecasts re-analyses. The cyclone trajectories are determined automatically by a next-neighbour search of minima identified in the 1000 hPa geopotential-height field (z1000). These are compared with the traditional storm track, defined by the root mean square of the band-pass filtered 500 hPa geopotential-height anomaly. The analysis covers the North Atlantic and North Pacific basins. The trend of the cyclone density (distribution) is similar to that of the storm track in the North Atlantic, but opposite in the North Pacific. In the Atlantic, the total number of cyclones with low central z1000 increases, due to the northward shift towards lower regional climatological pressure, whereas the number of cyclones with large gradients decreases. In the Pacific, the number of intense (weak) cyclones with large (low) gradients increases (decreases). The cyclone-track variability is investigated by a cluster analysts of the relative cyclone trajectories. The three centroids corresponding to north-eastward and zonally propagating cyclones and nearly stationary ones are very similar in both ocean basins. The winter-mean cluster occupation numbers of the two propagating cyclone clusters are positively correlated with one another in the Atlantic, and negatively in the Pacific. The three Atlantic cyclone clusters can be linked to particular aspects of the central European climate. The cyclonic activity can be related to teleconnection patterns which are dominant during northern hemisphere winter. During high North Atlantic Oscillation index winters (deeper Icelandic lows), the Atlantic cyclone density shifts northward and is associated with more stationary cyclones. During El Nino warm-event winters, the Pacific cyclone density is shifted northward and their propagation is oriented more zonally as in high Pacific/North American index phases (deeper Aleutian lows). In the North Atlantic propagating cyclones occur less and stationary ones more frequently in El Nino winters. The correlations between the north-east ward propagating cyclones in the Pacific and the north-eastward and zonally propagating ones in the Atlantic suggest an interaction between the two storm tracks.

Seasonal skill and predictability of ECMWF PROVOST ensembles

Abstract: Variations in seasonal-forecasting skill and predictability during the 15 years (1979–93) of the European Centre for Medium-Range Weather Forecasts (ECMWF) re-analysis (ERA), have been studied using 120-day ensemble integrations of the ECMWF model. These integrations form part of the European Union PROVOST (PRediction Of climate Variations On Seasonal to interannual Time-scales) project. Observed sea surface temperatures (SSTs) were updated daily at the model's lower boundary. Two major and three moderate El Nino Southern Oscillation (ENSO) events occurred during the ERA period. Results are interpreted as giving an upper bound on the predictive skill of a coupled ocean-atmosphere system as a function of season, location, and state of ENSO. The model systematic error was found to be comparable with a typical amplitude of interannual variation. When standardized by the corresponding ERA anomaly variance, systematic error appears to be largest in boreal spring in the northern extratropics, and in boreal summer in the tropics. Ensemble-mean skill scores were found to be positive overall. Apart from the northern winter season, the ensemble-mean skill for months 2–4 drops significantly when compared with months 1–3. The interannual variation of skill scores is much larger for the European region than for the hemispheric domain. Over the northern hemisphere, skill is much higher when only ENSO years are considered; for Europe, the enhancement in skill for ENSO years is much weaker. Estimates of intrinsic predictability were made for each year of the dataset. These estimates, defined both by a t-test and variance ratio, indicate generally high predictability in years when ENSO was strong. Apart from northern winter, the predictability estimates also showed a systematic drop between months 1–3 and months 2–4. It is therefore concluded that the fall in skill scores between months 1–3 and 2–4 indicates more a weakening of the impact of initial conditions (ICs) than, say, an increase in the effects of model error. In order to study this further, the relative impacts of SSTs and ICs, including land surface ICs, on interannual variation of precipitation have been examined in an additional set of experiments. Overall, SSTs have a dominant role, though the impact of ICs is not negligible. The predictability of tropical and extratropical precipitation is also discussed. The level of skill for precipitation in the extratropics is generally lower than in the tropics. However, within the tropics there are regions where the precipitation exhibits chaotic behaviour and is correspondingly less predictable.

The formation of vertical Vortices in the convective boundary layer

Abstract: A study of the dynamical formation mechanisms of atmospheric boundary-layer vortices is being conducted. These vortices are typically manifest as dust devils although there is evidence that they exist with some frequency in the absence of visible flow tracers. For example, in 1997 MacPherson and Belts pointed out instrument observations of invisible boundary-layer vertical vortices over the boreal forest. Several possible mechanisms for vortex formation under a variety of different dynamical regimes are described. Most observational investigators have reported that dust devils form in environments characterized by low wind speeds. The most intriguing unknown is the source of vorticity for the formation of vortices for convection in the absence (or near absence) of mean winds. Vertical vortex formation in convection without a mean shear has not often been documented in laboratory or numerical simulations. A Large-Eddy Simulation of the convective boundary layer is performed, without mean winds, with the purpose of examining vertical vortex-format ion mechanisms. The current work emphasizes the conjectured larger (convective) scale vorticity-generating mechanisms. The results indicate that vertical vortices form at some, but not all of the vertices of the simulated pattern of open convective cells. The columnar vortices may be essentially vertical or tilt with height, and may or may not extend to the surface. It is suggested that the vertical vorticity initially available to the vortices comes from the tilting of horizontal vorticity associated with local unidirectional shears due to the convective-cell circulations, with convective-cell asymmetries being important to the process. Subsequent evolution apparently results in the co-location of vertical vorticity centres with local updraught maxima.

An extended similarity theory for the stably stratified atmospheric surface layer

Abstract: An advanced similarity-theory formulation for the wind and temperature profiles in the stably stratified atmospheric surface layer (ASL) is developed with due regard to the effect of the free-flow static stability on the ASL. In the revised log-linear profiles, empirical coefficients traditionally considered as universal constants, namely the slope factors in the z-less stratification layer (beyond the logarithmic sub-layer), become functions of the dimensionless number S = N L/u * . Here, N is the Brunt-Vaisala frequency in the free flow, L is the Monin-Obukhov length, and u * is the friction velocity. The number S indicates how strongly the ASL is affected by the free-flow stability. This new formulation leaves room for the occurrence of well developed turbulence at much higher Richardson numbers, Ri, than had been suspected. Moreover, it results in a pronounced dependence of the turbulent Prandtl number on Ri in a wide range of Ri, including the z-less stratification layer, in correspondence with long-standing empirical evidence. The traditional Monin-Obukhov similarity theory disregards the above essential features of the stably stratified ASL. New data from measurements over a slightly inclined plateau in West Greenland provide experimental support for the proposed theory.

Characteristics of aerosols over a remote island, Minicoy in the Arabian Sea: Optical properties and retrieved size characteristics

Abstract: Changes in the characteristics of atmospheric aerosol over a marine environment are investigated by making regular spectral extinction measurements in the visible and near-infrared region from a tiny island location. Minicoy (8.3°N, 73.04°E), situated in the Arabian Sea about 400 km due west of the southern tip of the Indian peninsula. The role of seasonally changing air-mass type in causing a regular annual variation in the spectral optical depths is delineated. The association between aerosol optical depths, surface wind speed and rainfall is examined. An increase in wind speed causes an increase in optical depths, the effect is predominant when a marine air mass prevails. The impact of changes in wind speed on optical depths (due to sea-spray production over the sea) is parametrized in the case that the island is influenced by a marine air mass. Columnar size distributions, retrieved from the spectral optical depths, in general, show a bimodal log-normal distribution in the optically active size range. The accumulation mode is more sensitive to continental air-mass types, while the coarse mode is influenced by the marine conditions. The coarse mode is sharper but its position is variable. Increase in wind speed leads to a remarkable enhancement in the concentration and relative abundance of coarse particles, particularly during the monsoon season. The mass loading and effective radius are well associated and depend on wind speed histories. The findings are discussed.

Use of a neural‐network‐based long‐wave radiative‐transfer scheme in the ECMWF atmospheric model

Abstract: SUMMARY The definition of an approach for radiative-transfer modelling that would enable computation times suitable for climate studies and a satisfactory accuracy, has proved to be a challenge for modellers. A fast radiative-transfer model is tested at ECMWF: NeuroFlux. It is based on an artificial neural-network technique used in conjunction with a classical cloud approximation (the multilayer grey-body model). The accuracy of the method is assessed through code-by-code comparisons, climate simulations and ten-day forecasts with the ECMWF model. The accuracy of NeuroFlux appears to be comparable to the accuracy of the ECMWF operational scheme, with a negligible impact on the simulations, while its computing time is seven times faster. KEY WORDS: Artificial neural networks General-circulation models Long-wave radiative transfer 1. INTRODUCTION Parametric representation, or parametrization, is used in the numerical modelling of various atmospheric processes. It usually involves a statistical analysis that enables the representation of the true processes by simpler parametric relations. Three purposes may motivate such an analysis: (i) getting a better understanding of the system (e.g. Bretherton

Dew variability within a small arid drainage basin in the Negev Highlands, Israel

Abstract: SUMMARY Dew variability during the autumn dewy season within a small arid drainage basin in the Negev Highlands, Israel, is studied Dew measurements were carried out at 18 stations on four exposures, using the Cloth-Plate Method (CPM) and Duvdevani dew gauges The study also included periodical wind and substrate temperature measurements Dew variability within the drainage basin was high Average daily dew values obtained by the CPM were between 007 and 031 mm, whereas dew duration ranged between 16 and 41 hours per dewy morning Dew amounts monitored by the Duvdevani gauges were lower, between 009 and 020 mm Both methods show, however, consistent variability and correspond to a similar pattern Whereas near-ground dew measurements were the highest at the hilltops and at the bottom of the sun-shaded northern and western exposures, wadi bed stations and, especially, the south facing midslope station obtained the lowest dew quantities The near-surface dew patterns are not in agreement with the classical model of both Geiger and Oke, which predicts high dew quantities at the wadi beds (due to nocturnal down-slope wind) and at the lee side of the prevailing wind, ie the south-facing midslope station The low quantities at the south-facing midslope station is explained by the paramount role of surface temperatures, whereas variability in radiational cooling is seen as responsible for the high near-ground dew quantities at the hilltops and the low quantities at the wadis This conclusion is supported by dew measurements at 40 cm above ground Dew measurements at 40 cm above ground at the south-facing midslope station and at both wadi beds were significantly higher (p -= 005) than at 07 cm above ground Facilitating an efficient radiational cooling, and beyond the impact of the surface temperatures, dew measurements at this height correspond to the classical model, highlighting the important impact of surface temperatures and ventilation upon near-ground dew condensation in an arid drainage basin

Properties of the arctic tropopause

Abstract: A climatology of Arctic tropopause properties is derived using European Centre for Medium-Range Weather Forecasts Re-analyses, and is shown to be consistent with that from the Historical Arctic Radiosonde Archive. It is demonstrated that the thermal tropopause is generally easy to define from the temperature profile, even in the polar night, in contrast with the situation in Antarctic winter. In winter, the tropopause tends to have higher pressure and temperature in the western hemisphere and lower values in the eastern hemisphere. In summer the situation is more zonally symmetric with the higher pressure and temperature tropopause region near the pole. Strong annual cycles are seen in both temperature and pressure. these cycles being out of phase temporally. It is hypothesized that the tropopause temperature is largely determined by the approximately isothermal lower-stratospheric temperature, while the pressure is more strongly influenced by changes in the tropospheric lapse rate due to dynamical mechanisms. The multi-decade radiosonde dataset (1965–1990) shows some evidence of a long-term trend in tropopause properties. The wintertime tropopause pressure has decreased by approximately 14 mb per decade, while that in other seasons has decreased by around 5 mb per decade. The tropopause temperature only shows a significant trend during winter, having decreased by 1.6 K per decade since 1965. These changes are discussed in relation to changes in ozone and greenhouse gases, and the strength of the stratospheric vortex. In particular, the winter tropopause properties appear to be strongly correlated with the Arctic Oscillation Index.

The evolution of vortices in vertical shear. III: Baroclinic vortices

Abstract: The evolution of baroclinic tropical-eye lone-like vortices in environmental vertical shear on an f-plane is investigated Idealized numerical calculations are performed using a primitive-equation model The vertical structure of the initial vortex is varied by changing the strength of the upper-level tangential wind The baroclinic vortices develop a vertical tilt and a cyclonic rotation of the mid-level centre about the surface centre occurs In most of the calculations the upper-level part of the vortex is advected away from the surface centre The motion of the upper-level part of the vortex can be attributed to advection by the flow associated with large-scale asymmetries in the potential vorticity and to the vertically penetrating flow associated with the potential vorticity of the lower portion of the tilted vortex As in the case of initially barotropic vortices the cyclonic rotation is found to depend on the parameters in the Rossby penetration depth The height at which the environmental flow is equal to the Speed of vortex motion is found to be higher for vortices with stronger flow at upper levels This is consistent with observations of tropical cyclones Significant changes in the potential-vorticity structure of the vortex occur, especially at upper levels The vortex becomes elliptic in shape and may extrude filaments of potential vorticily These filaments thin, and sometimes break away from the main vortex, which as a result becomes more symmetric The separation of the filaments from the main part of the vortex is often accompanied by a decrease in the vortex tilt A: some levels no filaments form and the elliptic vortex becomes more symmetric with time Calculations with a barotropic model show that both the potential-vorticity structure of the initial vortex, and the horizontally sheared flow associated with the vertical projection of the tilted potential-vorticity anomaly, play a role in the changes in the vortex Structure The development of potential-temperature asymmetries and an adiabatic vertical circulation are shown to be related to the direction of the vortex tilt This occurs even when the direction of tilt changes with height The changes in the low-level static stability associated with the vortex tilt are investigated and the implications for tropical-cyclone intensity change are discussed