Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1999"

Construction of correlation functions in two and three dimensions

Abstract: This article focuses on the construction, directly in physical space, of simply parametrized covariance functions for data-assimilation applications. A self-contained, rigorous mathematical summary of relevant topics from correlation theory is provided as a foundation for this construction. Covariance and correlation functions are defined, and common notions of homogeneity and isotropy are clarified. Classical results are stated, and proven where instructive. Included are smoothness properties relevant to multivariate statistical-analysis algorithms where wind/wind and wind/mass correlation models are obtained by differentiating the correlation model of a mass variable. the Convolution Theorem is introduced as the primary tool used to construct classes of covariance and cross-covariance functions on three-dimensional Euclidean space R3. Among these are classes of compactly supported functions that restrict to covariance and cross-covariance functions on the unit sphere S2, and that vanish identically on subsets of positive measure on S2. It is shown that these covariance and cross-covariance functions on S2, referred to as being space-limited, cannot be obtained using truncated spectral expansions. Compactly supported and space-limited covariance functions determine sparse covariance matrices when evaluated on a grid, thereby easing computational burdens in atmospheric data-analysis algorithms. Convolution integrals leading to practical examples of compactly supported covariance and cross-covariance functions on R3 are reduced and evaluated. More specifically, suppose that gi and gj are radially symmetric functions defined on R3 such that gi(x) = 0 for |x| > di and gj(x) = 0 for |xv > dj, O di + dj and |x - y| > 2di, respectively, Additional covariance functions on R3 are constructed using convolutions over the real numbers R, rather than R3. Families of compactly supported approximants to standard second- and third-order autoregressive functions are constructed as illustrative examples. Compactly supported covariance functions of the form C(x,y) := Co(|x - y|), x,y ∈ R3, where the functions Co(r) for r ∈ R are 5th-order piecewise rational functions, are also constructed. These functions are used to develop space-limited product covariance functions B(x, y) C(x, y), x, y ∈ S2, approximating given covariance functions B(x, y) supported on all of S2 × S2.

A microphysically based precipitation scheme for the UK Meteorological Office Unified Model

Abstract: Many numerical weather prediction models are moving towards more prognostic schemes for the prediction of ice and liquid water contents within clouds. This paper describes a large-scale cloud and precipitation scheme developed for the UK Meteorological Office's Unified Model. It uses physically based transfer equations to predict ice as a prognostic variable. We review similar schemes and then describe our new scheme, giving examples of its performance in mesoscale forecasts compared with the current operational scheme and with observations. the microphysical processes occurring in a frontal cloud are well modelled. the prediction of supercooled stratocumulus cloud is much improved.

An improved fast radiative transfer model for assimilation of satellite radiance observations

Abstract: To assimilate atmospheric and surface radiance measurements from satellites in a numerical weather prediction model, a fast radiative transfer model is required to compute radiances from the model first guess fields at every observation point. Such a model for satellite infrared and microwave radiance measurements is used operationally for the assimilation of TIROS operational vertical sounder radiances at the European Centre for Medium-Range Weather Forecasts. An improved version of this model has been developed which requires ozone, in addition to temperature and water vapour, in the input profile and it has been generalized to compute radiances for other satellite radiometers using the same code. Instruments such as the high resolution infrared radiation sounder and the advanced microwave sounding unit on the National Oceanic and Atmospheric Administration polar orbiters, the METEOSAT water vapour imager and the Geostationary Operational Environmental Satellite infrared sounder have been simulated. It is demonstrated, by comparisons with line-by-line model computed radiances, that the fast model can reproduce the line-by-line model radiances for the TIROS operational vertical sounder stratospheric temperature sounding channels to an accuracy below the instrumental noise. The tropospheric, surface sensing, water vapour and ozone channel radiances cannot be predicted to such an accuracy, but still accurately enough for numerical weather prediction assimilation. A comparison of measured TIROS operational vertical sounder radiances with predicted values from numerical weather prediction model analyses gives larger differences than would be expected from the combination of the fast model and instrument related errors for most channels. The validity of the tangent linear approximation of the model gradient for typical radiance departures is also explored, with several examples, for the high resolution infrared radiation sounder/advanced microwave sounding unit instrument combination. The tangent-linear approximation is valid for temperature but significant departures from linearity about the first guess profile are observed for water vapour and ozone. Cloud affected infrared radiances have a highly non-linear response.

A broad-scale circulation index for the interannual variability of the Indian summer monsoon

Abstract: A broad-scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain-gauge data with satellite estimates of precipitation for the period 1979-96, it is shown that the variability of precipitation on seasonal to interannual time-scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70 degrees E-110 degrees E, 10 degrees N-30 degrees N. The EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. The June-to-September climatological mean EIMR is found to be larger than that of the LMR even though the former is averaged over a larger area. The dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north-western Pacific region (110 degrees E-160 degrees E, 10 degrees N-30 degrees N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70 degrees E-110 degrees E, 10 degrees S-5 degrees N). It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. An index of the monsoon Hadley (MH) circulation is defined as the meridional wind-shear anomaly (between 850 hPa and 200 hPa) averaged over the same domain as the EIMR. Using circulation data from two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium-Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi-biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Nino and Southern Oscillation during the latest eight years.

The representation of soil moisture freezing and its impact on the stable boundary layer

Abstract: The 1993 to 1996 version of the European Centre for Medium-Range Weather Forecasts model had a pronounced near-surface cold bias in winter over continental areas. the problem is illustrated in detail with help of tower observations. It is shown that a positive feedback exists in the land surface boundary-layer coupling that has the potential to amplify model biases. If the surface is cooled too much the boundary layer becomes too stable, reducing the downward heat flux and making the surface even colder. This positive feedback is believed to be stronger in the model than in the real atmosphere, resulting in diurnal temperature cycles that are too large and in excessive soil cooling on a seasonal time-scale in winter. An important contributor to the excessive winter cooling turns out to be the lack of soil moisture freezing in the model. the importance of this process is obvious from soil temperature observations. the seasonal soil temperature evolution shows a clear ‘barrier’ at 0°C due to the thermal inertia of freezing and thawing. A more quantitative illustration is the result of a simple calculation. This shows that the amount of energy necessary to freeze/thaw 1 m3 of wet soil, would cool/warm this soil by about 50 K if the phase transition was not taken into account. To reduce the winter cold bias in the model, three model changes have been tested and are described: (i) the introduction of the process of soil moisture freezing; (ii) revised stability functions to increase the turbulent diffusion of heat in stable situations; and (iii) an increase of the skin-layer conductivity. the effect of these changes on the seasonal evolution of soil and 2 m temperatures is investigated with long runs that have terms that relax towards the operational analysis above the boundary layer. In this way the impact can be studied on the temperature forecasts for the winter of 1995/1996, during which the operational model showed considerable soil temperature drift over Europe. Also, short periods of data assimilation (including 10-day forecasts) have been carried out to study the diurnal time-scales and the impact on model performance. The model changes eliminate to a large extent the systematic 2 m temperature biases for the winter of 1995/1996 over Europe and make the soil temperature evolution much more realistic. the soil moisture freezing, in particular, plays a crucial role by introducing thermal inertia near the freezing point, thereby reducing the annual temperature cycle in the soil. the process of soil moisture freezing leads to a considerable warming of the model's near-surface winter climate over continental areas.

Maintenance of the African easterly jet

Abstract: The maintenance of the African easterly jet (AEJ) has been examined using a zonally symmetric general circulation model with simple parametrizations. It is shown that the AEJ is maintained in association with two diabatically forced meridional circulations: one associated with surface fluxes and dry convection in the Saharan heat-low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this. the heat-low heating, which reaches the height of the AEJ around 700 mb, is particularly important in maintaining the AEJ and its associated meridional gradients in potential vorticity. It is concluded that the mean observed AEJ results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it.

On the predictability of the interannual behaviour of the Madden‐Julian oscillation and its relationship with el Nin̄o

Abstract: The Madden-Julian Oscillation (MJO) is the dominant mode of tropical variability at intraseasonal time-scales. It displays substantial interannual variability in intensity which may have important implications for the predictability of the coupled system. the reasons for this interannual variability are not understood. the aim of this paper is to investigate whether the interannual behaviour of the MJO is related to tropical sea surface temperature (SST) anomalies, particularly El Nino, and hence whether it is predictable. The interannual behaviour of the MJO has been diagnosed initially in the 40-year National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. the results suggest that prior to the mid-1970s the activity of the MJO was consistently lower than during the latter part of the record. This may be related either to inadequacies in the data coverage, particularly over the tropical Indian Ocean prior to the introduction of satellite observations. or to the real effects of a decadal time-scale warming in the tropical SSTs. the teleconnection patterns between interannual variations in MJO activity and SST show only a weak, barely significant, influence of El Nino in which the MJO is more active during the cold phase. As well as the NCEP/NCAR Reanalysis, a 4-member ensemble of 45-year integrations with the Hadley Centre climate model (HadAM2a), forced by observed SSTs for 1949-93, has been used to investigate the relationship between MJO activity and SST. HadAM2a is known to give a reasonable simulation of the MJO, and the extended record provided by this ensemble of integrations allows a more robust investigation of the predictability of MJO activity than was possible with the 40-year NCEP/NCAR Reanalysis. the results have shown that, for the uncoupled system, with the atmosphere being driven by imposed SSTs, there is no reproducibility of the activity of the MJO from year-to-year. the interannual behaviour of the MJO is not controlled by the phase of El Nino and would appear to be mainly chaotic in character. However, the model results have confirmed the low-frequency, decadal time-scale variability of MJO activity seen in the NCEP/NCAR Reanalysis. the activity of the MJO is consistently lower in all realizations prior to the mid-1970s, suggesting that the MJO may become more active as tropical SSTs increase. This result may have implications for the effects of global warming on the coupled tropical atmosphere-ocean system. Since the observed and simulated MJOs display clear seasonality in their occurrence, the relationship with interannual changes in the mean seasonal cycle of the tropical circulation has also been investigated. In contrast to the MJO, the interannual variability in the mean seasonal cycle is reproducible and influenced by the phase of El Nino. the implications of these results for the predictability of the tropical atmosphere-ocean system are discussed, particularly with reference to the strong El Nino event of 1997 which developed in association with a period of intense MJO activity.

Variational quality control

Abstract: We describe the implementation of variational quality control in the operational four-dimensional variational data assimilation scheme at the European Centre for Medium-Range Weather Forecasts (ECMWF) the quality control takes place during the iterative solution of the variational analysis problem itself, rather than in a separate step prior to the main analysis This ensures that the quality control is consistent with the analysis in terms of error statistics, background and model constraints, in particular All observational data are used and quality-controlled simultaneously the probability of gross error is computed for each observation, given the preliminary analysis at each iteration the weight of each observation is smoothly decreased with increased likelihood for gross error the formulation, the implementation details and results from data assimilation and forecast experiments are presented

A GCM study of climate change in response to the 11‐year solar cycle

Abstract: A general-circulation model (GCM) is used to investigate the impact of the 11-year solar-activity cycle on the climate of the lower atmosphere. Solar forcing is represented by changes in both incident irradiance and stratospheric ozone concentrations. Experiments are carried out with realistic and factor-ten enhanced solar irradiance changes and with ozone changes based on two-dimensional photochemical model results and loosely based on Total Ozone Mapping Spectrometer ozone column data. A pattern of response, consistent across all the experiments, is found in which the tropical Hadley cells weaken and broaden, and the sub-tropical jets and mid-latitude Ferrel cells move polewards. This causes sub-tropical warming and a characteristic vertical banding of mid-latitudes temperature changes in both the summer and winter hemispheres. the GCM results suggest that the precise response of the atmosphere depends on the magnitude and distribution of the ozone changes. This is confirmed by use of an equal-area model of the tropical Hadley cells in which the input radiative equilibrium-temperature anomalies are determined from the downward irradiance (both solar and long wave) at the tropopause as calculated in the GCM. As both the magnitude of the solar irradiance change and, more particularly, the latitude-height structure of solar-induced ozone changes over the 11-year cycle are not yet well established, the GCM study may well underestimate the solar effects.

Lightning activity as an indicator of climate change

Abstract: Data from the Optical Transient Detector lightning sensor are analysed to investigate the hypothesis that global lightning activity will increase should the average global temperature increase. It is shown that changes in global monthly land lightning activity are well correlated with changes in global monthly land wet-bulb temperatures. The correlation is strongest in the northern hemisphere and weak in the southern hemisphere. The conclusion is that a high land-area to sea-area ratio is necessary for a good correlation. Contrary to expectation, the tropics show no correlation. The results predict that a change in the average land wet-bulb temperature of the globe of just 1K would result in a change in lightning activity of about 40%.

A mechanism for moistening the lower stratosphere involving the Asian summer monsoon

Abstract: This study employs European Centre for Medium-Range Weather Forecasts (ECMWF) re-analysis data and the contour advection technique to investigate the water vapour distribution in the upper troposphere and lower stratosphere. Water vapour is the primary greenhouse gas and understanding the processes which determine its distribution and transport is crucial. Of special interest is the exchange of water vapour across the tropopause. This study considers how the Asian summer monsoon affects the moisture budget of the upper troposphere and lower stratosphere. The region of the Asian summer monsoon is identified as a significant moisture source for the upper troposphere outside the deep tropics. Monsoon convection moistens the region of the upper-level monsoon anticyclone which is located close to the dynamical tropopause, where isentropes cross from the troposphere into the stratosphere. An isentropic analysis reveals that transport from the troposphere into the stratosphere in this region is normally prevented by the strong potential-vorticity gradients around the tropopause. However, midlatitude synoptic disturbances occasionally interact with the monsoon anticyclone and pull filaments of tropospheric air from its northern flank. These filaments, characterized by high values of humidity and low values of potential vorticity, can extend far north and transport moisture irreversibly into the northern hemisphere lower stratosphere. MOZAIC (Measurement of OZone by Airbus In-service airCraft) data are used as an independent data source to validate the results obtained from the ECMWF analyses.

A dynamic chemical model of bi‐directional ammonia exchange between semi‐natural vegetation and the atmosphere

Abstract: A mechanistic, dynamic compensation point model to simulate the vegetation/atmosphere exchange of ammonia (NH3) is described. the model is applied to long-term micrometeorological measurements of NH3 exchange obtained over moorland in southern Scotland (1995-96). the model describes the gaseous bi-directional exchange between the atmosphere, leaf surface water films, plant stomata and apoplast. A simple chemistry module is included to simulate the exchange of water-soluble atmospheric pollutants at the air-water interface on wet plant surfaces. Initialization of the chemistry module is achieved during rain events using measured rain chemical composition. the exchange of NH3 with stomata is based on the concept of a stomatal compensation point parametrized by means of the apoplast ammonium/hydronium I ratio. the trans-cuticular transfer of ammonium may also constitute a sink for dissolved ammonia on plant cuticular water films, and is parametrized using a trans-cuticular resistance and the concentration difference between leaf surface water and the apoplast. the leaching of base cations from the inside of the plant to foliar surfaces is simulated in a similar fashion to ammonium transfer, and the heterogeneous oxidation of sulphur dioxide (SO2) in thin water films is also treated. Numerical iterative procedures at each time-step allow the calculation of pH and dissolved ion concentrations. Modelled NH3 fluxes were compared with 3259 half-hourly micrometeorological measurements over moorland, and with two existing modelling approaches, the static canopy compensation point model and the canopy resistance model. the dynamic and static canopy compensation point models both gave long-term estimates of the NH3 dry deposition flux to moorland within 10% of actual measurements, while the canopy resistance approach overestimated deposition by about 30%. the dynamic model performed best during wet conditions for which it was designed, and performed reasonably well during dry conditions using a more empirical resistance approach. The model was also capable of simulating SO2 dry deposition fluxes to within 20% of measured fluxes. the model provides a tool that may also be used to simulate scenarios, whereby NH3/SO2 concentration ratios in the atmosphere vary, and examine co-deposition interactions of these two species.

The horizontal and vertical structure of east Asian winter monsoon pressure surges

Abstract: East Asian cold-air outbreaks, accompanied by increasing surface pressure (‘pressure surges’), are shown to be an important aspect of the subseasonal variability of the winter monsoon system. In this study the statistical linear relationship between pressure surges, tropical convection, and tropospheric circulation is assessed using a ten-year data set (1985/86-1994/95) of the European Centre for Medium-Range Weather Forecasts gridded operational analyses. From spectral analysis the pressure, wind, and temperature fluctuations indicative of strong pressure surges are found to have statistically-significant spectral peaks at submonthly periods (6 to 30 days). Linear regression analysis is used to detail the time evolution of the dominant horizontal and vertical structure of east Asian pressure surges. Surges are shown to relate significantly to circulation anomalies in both the meridional and zonal components of the lower-tropospheric wind. Relationships between east Asian pressure surges in the submonthly band and tropical circulation anomalies are found over the Bay of Bengal, the eastern Indian Ocean, Indonesia, and the western Pacific regions. Submonthly surges over the South China Sea are related to strong surges and convective activity south of Indonesia, over the South China Sea, the eastern Indian Ocean, and the Philippine regions. Surge-enhanced convective activity is found to precede an enhancement of the local east Asian Hadley cell. Submonthly surges over the Philippine Sea are related to periods of westerly-wind anomalies and convective activity in the western Pacific. Upper-level wave activity over western Asia precedes submonthly surges. the wave activity amplifies in the region of the Pacific jet stream, and can be traced dispersing equatoward through the region of upper-level westerlies over the eastern tropical Pacific.

The sensitivity of domain‐averaged solar fluxes to assumptions about cloud geometry

Abstract: The effects on domain-averaged broad-band solar fluxes due to assumptions about geometry of convective clouds are explored using a Monte Carlo algorithm and 3D distributions of water generated by a cloud-resolving model (CRM). Domains are (400 km) 2 with 2 km horizontal grid-spacing, Δx, and 35 layers of varying thickness. Optical properties are computed based on single-scattering parametrizations for hydrometeors and the correlated k-distribution method for gaseous absorption. Benchmark fluxes are established using the CRM fields at Δx = 2 km. Four plane-parallel versions of these fields (affected by letting Δx → ∞) are considered which mimic 1D algorithms: the independent column approximation (ICA) uses the full CRM fields; for the others, mixing ratios of cloudy cells are reset to associated layer-mean values thus conserving water mass and cloud fraction in each layer. For the ICA, errors in reflected flux to space and surface irradiance rarely exceed 20 W m -2 . Total atmospheric absorption and heating rates are almost always within 5 W m -2 and ∼3%, respectively. This demonstrates that cloud sides and horizontal fluxes are unimportant for averages over large domains. However, when clouds are homogenized horizontally yet exact overlap is retained, errors increase by almost an order of magnitude. This demonstrates the importance of horizontal variability. When the same clouds are randomly overlapped, errors in boundary fluxes can exceed 250 W m -2 at high sun, and heating rates can be off by 50% to 100%. When these clouds follow maximal/random overlap, albedo is often underestimated because overlap of CRM liquid clouds falls between maximal and random. This demonstrates the importance of cloud overlap and ultimately the need for 1D models to account equally well for both subgrid-scale variability in cloud extinction and overlap.

3D‐Var Hessian singular vectors and their potential use in the ECMWF ensemble prediction system

Abstract: Singular vectors are computed which are consistent with 3D-Var (three-dimensional variational) estimates of analysis error statistics. This is achieved by defining the norm at initial time in terms of the full Hessian of the 3D-Var cost function. At final time the total energy norm is used. the properties of these Hessian singular vectors (HSVs) differ considerably from total energy singular vectors (TESVs) in such aspects as energy spectrum and growth rate. Despite these differences, the leading 25 TESVs and HSVs explain nearly the same part of the 2-day forecast error. Two experimental ensemble configurations are studied. One configuration uses perturbations based on HSVs in the computation of initial perturbation, the other uses TESVs and 2-day linearly evolved singular vectors (ESVs) of two days before. the latter approach provides a way to include more stable and large-scale structures in the perturbations. Ten pairs of ensembles are compared to the operational European Centre for Medium-Range Weather Forecasts Ensemble Prediction System. the ensembles using ESVs perform slightly better. the ensembles based on HSVs show a slightly worse performance and are lacking some spread in the medium range. Possible directions to improve the computation of HSVs are discussed.

Stratospheric water vapour and tropical tropopause temperatures in ECMWF analyses and multi-year simulations

Abstract: Stratospheric humidity analyses produced operationally by the European Centre for Medium-Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned. Near-tropopause analyses are in reasonable overall agreement with independent observations. Very low humidities occur in conjunction with deep convection and a particularly cold tropopause over the equatorial western Pacific during the northern winter. Drying occurs also in the cold core of the Antarctic polar-night vortex. the lower stratosphere is moistened in the outer tropics and subtropics in summer and autumn, predominantly in the northern hemisphere. Changes associated with the latest occurrence of El Nino are illustrated. Analysed temperatures near the tropical tropopause are generally in good agreement with corresponding radiosonde measurements, with standard-level biases of the order of 0.5 degC or less. the past two years are the coldest by about 1 degC in a series of tropical mean 100 hPa analyses extending back to 1979. A cooling trend of about 0.6 degC per decade is seen in the global means of the 100 hPa analyses. Moisture is spread zonally and upward from the tropical tropopause as the data assimilation proceeds, but the rate of upward transfer is much faster than observed. Substantial lateral mixing can occur within the stratosphere over the course of a season. Moistening at middle and high latitudes due to mixing with more humid tropospheric air is confined, realistically, to a shallow layer at the base of the stratosphere. The rate of upward transfer of tropical stratospheric moisture is much more realistic in a multi-year simulation using a version of the model that has finer stratospheric resolution than the version used for the operational data assimilation. Temperatures at the tropical tropopause and in the Antarctic polar night are accurately simulated, apart from excessive persistence of cold south-polar temperatures in late winter and early spring. the latter is conducive to drying the model stratosphere; lack of a parametrization of moistening due to methane oxidation is an obvious deficiency in this regard.

The relative importance of non‐sea‐salt sulphate and sea‐salt aerosol to the marine cloud condensation nuclei population: An improved multi‐component aerosol‐cloud droplet parametrization

Abstract: The effect of sub-cloud aerosol on cloud droplet concentration was explored over the north Atlantic and east Pacific under a variety of low and high wind speed conditions. A relationship of the form of D = 197{1 - exp(-6.13 × 103*A)} was found to fit best the relationship between cloud droplet concentration (D; cm-3) and sub-cloud aerosol concentration (A; cm-3) under low to moderate wind conditions. A few noticeable deviations from this relationship were observed which occurred under moderate to high wind speed condition. Under these high wind conditions, sea-salt aerosol provided the primary source of cloud nuclei due to their higher nucleation activity and larger sizes, even under sulphate-rich conditions. Simple model simulations reveal that the activation of sea-salt nuclei suppresses the peak supersaturation reached in cloud, and thus inhibits the activation of smaller sulphate nuclei into cloud droplets. A multi-component aerosol-droplet parametrization for use in general circulation models is developed to allow prediction of cloud droplet concentration as a function of sea-salt and non-sea-salt-(nss) sulphate nuclei. The effects of enhancing an existing nss-sulphate cloud condensation nuclei (CCN) population with sea-salt nuclei are to reduce the number of cloud droplets activated under high (polluted) sulphate conditions and to increase the cloud droplet concentration under low (clean) sulphate conditions. The presence of sea-salt CCN reduces the influence of nss-sulphate CCN on cloud droplet concentrations, and thus is likely to reduce the predicted effect of nss-sulphate indirect radiative forcing.

On the possible mechanisms of the evolution of a mini‐warm pool during the pre‐summer monsoon season and the genesis of onset vortex in the South‐Eastern Arabian Sea

Abstract: During the pre-summer monsoon season (February-May), the near-surface waters in the Arabian Sea progressively warm up and a mini-warm pool with a core >30°C is manifested in the south-eastem region. the possible mechanisms for the observed seasonal build-up of this mini-warm pool are examined, utilizing all the available monthly mean climatologies of surface wind field, surface heat fluxes, near-surface thermohaline fields, near-surface circulation, and mean sea level as monitored by satellites and by some of the recent model solutions on the Arabian Sea circulation. During winter (November-February), the equatorward-flowing East India Coastal Current in the western Bay of Bengal and westward-flowing North Equatorial Current in the southern Bay bring low-saline waters into the south-eastern Arabian Sea, causing a haline stratification within the near-surface isothermal layer. During December-April, the positive surface-wind-stress curl and the associated Ekman divergence shoals the pycnocline. A south-westward propagating mode-2 Rossby wave from off south-west India seen in satellite-derived mean sea level and model solutions also modulates the underlying pycnocline. During the pre-summer monsoon season, under clear skies and light wind conditions, the radiative heat input overwhelms turbulent heat losses at the air-sea interface, and the net surplus heat energy is absorbed in a shallow haline stratified near-surface layer, resulting in the formation of the observed mini-warm pool. An examination of historical data on the genesis of monsoon onset vortices reveals that on most occasions the genesis has occurred over this mini-warm pool region. Evidence for the geographic coincidence in the occurrence of the genesis of onset vortex and the sea surface temperature maxima during individual years of a three decade period (1961-90) is presented.

Accounting for subgrid‐scale cloud variability in a multi‐layer 1d solar radiative transfer algorithm

Abstract: A multi-layer, 1D solar radiative transfer algorithm that accounts for subgrid-scale cloud variability is presented. This algorithm is efficient and suitable for use in large-scale models such as global climate and weather prediction models. While it is built on the same principles as standard multi-layer 1D codes, there are two major differences. First, it is assumed that for all cloudy layers all the time, frequency distributions of optical depth r are described by gamma probability density functions p Γ (τ) and characterized by mean optical depth τ and a variance-related parameter ν. Albedos and transmittances for individual layers are estimated by integrals over all τ of the plane-parallel, homogeneous two-stream approximation equations weighted by p Γ (τ). Thus, the model is referred to as the gamma-weighted two-stream approximation. Second, in an attempt to counteract the use of horizontally homogeneous fluxes, a method was devised that often reduces layer values of τ. The gamma-weighted two-stream approximation was implemented in a well known broadband column model and the parametrizations upon which it is built were tested using 2D and 3D inhomogeneous cloud fields generated by a bounded cascade model and cloud-resolving models. All fields resolved the lowest 20 km of the atmosphere into at least 30 layers. Reference calculations were obtained by: (i) applying the 1D-plane-parallel, homogeneous model to each column and averaging (the independent column approximation); and (ii) a 3D Monte Carlo algorithm. The gamma-weighted two-stream approximation. the regular plane-parallel, homogeneous, and two other ID models operated on horizontally-averaged versions of the fields (i.e. ID vectors of cloud fraction, τ, and ν). For several demanding cases, the gamma-weighted two-stream approximation reduced plane-parallel, homogeneous-biases for TOA albedo and surface irradiance by typically more than 85%. Moreover, its estimates of atmospheric heating rates usually differed from the independent column approximation and Monte Carlo values by less than 10%. This translates into heating rate errors that are four to eight times smaller than those associated with conventional 1D plane-parallel, homogeneous algorithms. In a large-scale model, a multi-layer solar code with the gamma-weighted two-stream approximation should require about twice as much CPU time as its plane-parallel, homogeneous counterpart.

Correlation functions for atmospheric data analysis

Abstract: Atmospheric data assimilation techniques rely on parametric models for spatial correlation functions. This article proposes and discusses various families of homogeneous and isotropic correlation models on Euclidean spaces and on the sphere. In particular, three simply parametrized classes of compactly supported, smooth, and analytically simple correlation functions are proposed. the first two classes approximate standard second- and third-order autoregressive functions, and a member of the third family approximates the Gaussian function within a maximal error of 0.0056. Furthermore, correlation models suggested previously for meteorological applications are checked for permissibility, with both positive and negative results.

A similarity hypothesis for shallow-cumulus transports

Abstract: A similarity theory for cumulus convection is proposed, and applied to the problem of cumulus transports, using data from several large-eddy simulations to test the theory. the parameters in the similarity hypothesis include the cloud-base mass flux, the buoyancy of a parcel undergoing reversible ascent, through the cloud layer and the depth of the cloud layer. Using arguments based on the turbulence kinetic-energy budget a velocity-scale is derived, in addition to the conventional Convective Available Potential Energy scale. This new scale is analogous to the convective velocity-scale used in boundary-layer theories, but incorporates the effects of latent-heat release. the ratio between the cloud-base mass flux and the velocity-scale is found to be a key parameter in describing cumulus convection. It is shown that the similarity hypothesis can be applied to mass-flux schemes to determine the fractional entrainment rate. For this the entrainment rate is assumed to be related to the rate of generation of turbulent kinetic energy in the cloud ensemble. the resulting scaling is tested against fractional entrainment rates diagnosed from the large-eddy simulations.

The dynamics of heat lows

Abstract: A numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea. of particular interest is the evolution of the distributions of relative vorticity and potential vorticity. While the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land, the relative vorticity is strongest in the early morning hours following a prolonged period of low-level convergence. Thus the heat low is not approximately in quasi-geostrophic balance. the low-level convergence is associated with the sea-breeze and later with the nocturnal low-level jet. the effects of differing sea area, land area and Coriolis parameter on various aspects of the heat low are investigated. Although a cyclonic vortex, the heat low is characterized by an anticyclonic potential-vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well-mixed boundary layer; however, the surface temperature maximum over land corresponds with a cyclonic potential-vorticity anomaly at the surface. the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential-temperature gradient make a non-negligible contribution and of opposite sign to the potential vorticity in certain flow regions. Two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low-level atmospheric phenomena over the arid interior of Australia: these are the deep convective mixing over land during the daytime and the development of a nocturnal low-level jet, which leads to convergence in the trough. Such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores. It is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere.

Overview of the field phase of the fronts and Atlantic Storm-Track EXperiment (FASTEX) project

Abstract: Summary The eld phase of the FASTEX project took place between 5 January and 27 February 1997 with the deployment of a unique set of observing facilities across the North-Atlantic. The major objective was to document the life-cycle of a representative set of mid-latitude cyclones. Other objectives were to test the practical feasibility of iadaptivei observations with a view to improving the prediction of these same cyclones and to document the internal structure of the associated cloud systems using combined airborne Doppler radars and dropsondes. Another goal of FASTEX was to measure air-sea exchange parameters under conditions of strong winds with high seas. These objectives were successfully achieved. Intensive Observation Periods were conducted on 19 occasions. High-resolution vertical proles through the same cyclones at three dierent stages of their life-cycle have been obtained on more than 10 occasions. Calculation of areas where observations were needed to keep the growth of forecast error under control was undertaken using dierent techniques, and ights were planned and executed in these areas on time. Combined dropsonde and Doppler radar observations of cloud systems are available for 10 cases. A unique air-sea turbulent exchange dataset has been obtained.

As assessment of the singular-vector approach to targeted observing using the FASTEX dataset

Abstract: In this study, we investigate whether the results of assimilating special targeted observations from the Fronts and Atlantic Storm-Track EXperiment (FASTEX) in an operational forecast model support the underlying principles of the singular-vector (SV) approach to targeted observing. A simple framework is presented that allows explicit examination of the changes made to the analysis in the subspace of the leading SVs from assimilation of the observations. the impact of this component on the forecast provides a key measure of the effectiveness of SV-based targeting. Results confirm that the impact of the additional observations occurs primarily as a result of changes to the analysis in the subspace of the leading SVs. These changes account for a small fraction of the total targeting increment at initial time, but explain a large fraction of the response of the forecast at the verification time. the results also confirm that analysis errors in the middle and lower troposphere are an important source of error in forecasts of extratropical cyclones. While moist processes can play an important role in the forecast-error evolution, SVs that exclude these processes can remain an effective targeting tool. This is because the location of maximum sensitivity will not necessarily differ from that identified by the dry SVs. It is also shown that the locations of the leading (target) SVs can be computed accurately with lead times of up to 48 hours, allowing ample time for the deployment of observational resources.

An examination of the accuracy of the linearization of a mesoscale model with moist physics

Abstract: The accuracy of tangent linear and adjoint versions of a primitive-equation model with moist physics is examined with respect to growing perturbations having significant initial magnitudes. The Jacobians for the convective parametrizations are approximated using a perturbation method. These Jacobians are then quality controlled to ensure that the approximations are suitable. Results show that: (I) linearization of the diabatic moist physics can have a significant impact; (2) even where such impacts are large, the linearized versions of the model can yield good approximations to the nonlinear behaviour for significant perturbations, especially if there is sufficient dynamical influence: (3) poor approximations can be obtained when convection dominates the results; and (4) a straightforward linearization of some parametrization schemes may be inadequate. The results are encouraging for quantitative applications of some moist adjoint models to extratropical cyclones in the winter, but suggest some tangent linear approximations may be unsuitable in the tropics or over continents in the summer, except if qualitative agreements with nonlinear results are sufficient. Detailed comparisons of linear and nonlinear results should be made. particularly using optimal perturbations, prior to any applications of tangent linear or adjoint models.

Surface-flux aggregation in heterogeneous terrain

Abstract: Aggregation of surface fluxes in non-homogeneous terrain is a difficult task due to the highly nonlinear processes governing especially the momentum flux. Results of the current work are high-resolution maps of land-surface momentum flux in a strongly heterogeneous mid-latitude area in Europe, and aggregated values of the aerodynamic roughness lengths. This paper describes the development of a flux aggregation model based on a linearized set of the atmospheric flow equations solved by fast Fourier transforms. This is computationally efficient, therefore a large amount of data can be handled quickly. the input data to the model are high-resolution (30 m x 30 m) roughness maps derived from classified satellite images and assigned Meteorological field values within a region (100 km x 100 km). the model deals with neutral flow only. the momentum-flux results from this objective (physically based) method compare well with other models and experimental field data.

The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model

Abstract: Summary Global Circulation Models (GCMs) have generally treated only the radiative impacts of vertically varying cloud fraction by use of a cloud overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a sub-grid scale precipitation model which resolves the vertical variation of cloud fraction. This sub-grid model subdivides the grid boxes into homogeneous columns which are either completely clear or cloudy. By comparing the columnaveraged microphysical quantities from the sub-grid scale precipitation model to the parametrization in the ECMWF model, the ability of the ECMWF model to account for the sub-grid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates precipitation evaporation in the tropical mid-troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box. In addition to assessing the ability of the ECMWF model to parametrize the sub-grid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrization in GCMs, more attention should be paid to the impact of vertical cloud fraction variations on the parametrized microphysics.

Targeted observations in FASTEX: Adjoint-based targeting procedures and data impact experiments in IOP17 and IOP18

Abstract: The Fronts and Atlantic Storm-Track EXperiment (FASTEX) provided an opportunity for testing targeted-observing procedures in a real-time framework during January and February 1997. This study describes the use of singular vectors (SVs) for objective targeting during FASTEX, and the evaluation of the impact obtained from targeted dropsonde data, satellite wind data, and other observations on 1-2 day forecast skill in intensive observation periods (IOPs) 17 and 18. In IOP17, targeted dropsondes improve a 42 h forecast of L41 (Low 41; cyclones were numbered in sequence throughout FASTEX) in terms of sea-level pressure, but the forecast skill is degraded in the upper troposphere. It is suggested that the degraded forecast may be caused by an incomplete survey of the SV target area, that improved the analysis in one region, but made the analysis less accurate in an adjacent part of the target area where no dropsonde data were provided. In a series of experiments, the best 42 h forecast of L41 is obtained by the addition of a few radiosonde profiles provided specially for FASTEX at off-times, that provide observational data in the most sensitive part of the SV target area. the analysis differences introduced by the radiosonde profiles are much smaller in magnitude than those from the dropsonde data, but have a larger forecast impact, because they occur in an area that has larger error growth rates in this forecast. In a series of experiments for IOP18, the best 24 h forecast of L44 is obtained using a combination of targeted-dropsonde data and satellite wind data. Both data types can also be used separately to improve this forecast. the assimilation of satellite wind data and ship-based soundings in areas of weak initial-condition sensitivity (‘null’ areas) is shown to have minimal impact on the forecast error. the target areas identified by SVs in these two IOPs occur in strongly baroclinic regions, tending to favour the right-entrance and left-exit regions of the upper-level jet, but with greatest sensitivity near 600 hPa.

A tropospheric chemical‐transport model: Development and validation of the model transport schemes

Abstract: We describe a 3D off-line chemical-transport model (CTM) for studies of tropospheric chemistry. the tropospheric CTM is a development of the existing TOMCAT stratospheric CTM to which we have added a parametrized convection scheme and treatment of vertical diffusion. the CTM diagnoses the occurrence of convection during the model simulations from the forcing analysis fields of winds, temperature and humidity. The spatial distributions of the vertical mass fluxes calculated by these schemes appear realistic. the convective mass fluxes agree well with values derived from global analyses, having maximum values at low latitudes and at the intertropical convergence zone, although the TOMCAT cloud-top height may be too low. We have used the model's convection scheme to diagnose the convective precipitation when forced by European Centre for Medium-Range Weather Forecasts (ECMWF) initialized analyses. Despite the off-line approach, the CTM-diagnosed convective precipitation is in very good spatial agreement with global observations and output from a general-circulation model (GCM), showing that not only does convective venting of tracers occur in the correct locations, but also that the convection scheme can be coupled to wet-deposition schemes in full chemistry simulations. TOMCAT exhibits a low-latitude bias in cloud amount, due to the model's neglect of mid-level convection, and underestimates convective precipitation and cloud amount, the latter by at least 20% compared with GCM results. This underestimate may be due to excessive stability of the ECMWF analyses used in the off-line model or to differences in the formulation of convection in the ECMWF forecast model and in TOMCAT. We have evaluated the CTM's transport processes by performing a range of simulations with a surface-emitted radon (Rn) tracer. the agreement with radon observations improves with an increase in horizontal resolution from 7.5° × 7.5° to 2.8° × 2.8° and with the inclusion of vertical diffusion and convection in the model formulation. When run at a horizontal resolution of 2.8° × 2.8° TOMCAT captures the tracer transport and seasonal evolution in tracer transport associated with major meteorological features and with convective processes. TOMCAT reproduces the observed seasonality in radon at surface sites, but overestimates the surface radon concentration, partly due to insufficient vertical diffusion in the model. However, transient systems and their tracer transport within the boundary layer are simulated realistically. Compared with observations, the modelled radon profiles are too ‘C-shaped’ (the modelled profile has a mid-tropospheric minimum which is not present in the observations) which may be caused by the model's neglect of convective downdraughts and/or entrainment and detrainment rates that are too low in the cloud column, resulting in insufficient mixing within the cloud column. the radon concentration in the upper troposphere is underestimated by TOMCAT; this may be due to a number of factors: the underestimate in convective-cloud amount, the way in which the base mass flux is calculated, the limitation of organized entrainment to subcloud levels, and a possible static stability of the analyses.

Analysis and model dependencies in medium-range ensembles: Two transplant case-studies

Abstract: There are numerous occasions on which operational forecasts from different meteorological centres diverge in the medium range These differing evolutions may be due to the different analyses used by the centres or may be a result of the different models used to produce the forecasts; or a combination of both factors may be involved Ensemble forecasts in routine production at the European Centre for Medium-Range Weather Forecasts (ECMWF) are designed to address the problem of the impact of small analysis differences on forecast evolution the initial perturbations, generated using the singular-vector technique, are designed to maximise spread within the ensemble while being consistent with possible analysis errors the aim of the current work is to examine, in the context of ensemble forecasting, the effects of model and analysis differences on forecast evolution Two detailed case-studies have been performed using the ECMWF model at T63 resolution and the UK Meteorological Office (UKMO) Unified Model run at a comparable resolution In each case four 33-member ensembles were produced using all possible combinations of ECMWF and UKMO models and analyses the same initial perturbations, derived using the singular-vector technique with the ECMWF model, were used for all four ensemble sets Significant differences between all four ensemble sets are found in each case-study the relative impact of model and analysis dependencies varies with geographical region and forecast range; however the greater overall impact is, for both region and range, from the model differences the varying evolutions of the four ensemble sets are shown to have a substantial impact on synoptic features in the predicted 500 hPa height fields, such that a forecaster would have issued different predictions depending on which ensemble data were available the synoptic differences are consistent with the statistically significant model and analysis dependencies found using analysis of variance applied to anomaly correlations and to the distribution of ensemble members in phase space It is demonstrated that the calculation of the initial perturbations is relatively insensitive to differences between the initial analyses of the two centres, but that the dispersion of initial conditions resulting from the 16 singular vectors used to generate the perturbations does not cover the dispersion arising from the analysis fields In both of the case-studies each of the four ensemble sets contains information not available in any of the other ensembles These results raise the possibility that, to maximise the information available to the forecaster, both models and both analyses should be used in the generation of ensemble predictions