Showing papers in "Monthly Weather Review in 1997"

Ensemble Forecasting at NCEP and the Breeding Method

Abstract: The breeding method has been used to generate perturbations for ensemble forecasting at the National Centers for Environmental Prediction (formerly known as the National Meteorological Center) since December 1992. At that time a single breeding cycle with a pair of bred forecasts was implemented. In March 1994, the ensemble was expanded to seven independent breeding cycles on the Cray C90 supercomputer, and the forecasts were extended to 16 days. This provides 17 independent global forecasts valid for two weeks every day. For efficient ensemble forecasting, the initial perturbations to the control analysis should adequately sample the space of possible analysis errors. It is shown that the analysis cycle is like a breeding cycle: it acts as a nonlinear perturbation model upon the evolution of the real atmosphere. The perturbation (i.e., the analysis error), carried forward in the first-guess forecasts, is ‘‘scaled down’’ at regular intervals by the use of observations. Because of this, growing errors associated with the evolving state of the atmosphere develop within the analysis cycle and dominate subsequent forecast error growth. The breeding method simulates the development of growing errors in the analysis cycle. A difference field between two nonlinear forecasts is carried forward (and scaled down at regular intervals) upon the evolving atmospheric analysis fields. By construction, the bred vectors are superpositions of the leading local (timedependent) Lyapunov vectors (LLVs) of the atmosphere. An important property is that all random perturbations assume the structure of the leading LLVs after a transient period, which for large-scale atmospheric processes is about 3 days. When several independent breeding cycles are performed, the phases and amplitudes of individual (and regional) leading LLVs are random, which ensures quasi-orthogonality among the global bred vectors from independent breeding cycles. Experimental runs with a 10-member ensemble (five independent breeding cycles) show that the ensemble mean is superior to an optimally smoothed control and to randomly generated ensemble forecasts, and compares favorably with the medium-range double horizontal resolution control. Moreover, a potentially useful relationship between ensemble spread and forecast error is also found both in the spatial and time domain. The improvement in skill of 0.04‐0.11 in pattern anomaly correlation for forecasts at and beyond 7 days, together with the potential for estimation of the skill, indicate that this system is a useful operational forecast tool. The two methods used so far to produce operational ensemble forecasts—that is, breeding and the adjoint (or ‘‘optimal perturbations’’) technique applied at the European Centre for Medium-Range Weather Forecasts—have several significant differences, but they both attempt to estimate the subspace of fast growing perturbations. The bred vectors provide estimates of fastest sustainable growth and thus represent probable growing analysis errors. The optimal perturbations, on the other hand, estimate vectors with fastest transient growth in the future. A practical difference between the two methods for ensemble forecasting is that breeding is simpler and less expensive than the adjoint technique.

The Naval Research Laboratory’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS)

Abstract: The three-dimensional Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) has been developed by the Naval Research Laboratory COAMPS consists of an atmospheric data assimilation system comprising data quality control, analysis, initialization, and nonhydrostatic forecast model components, as well as a hydrostatic ocean model The models can be integrated simultaneously so that the surface fluxes of heat, momentum, and moisture are exchanged across the air–water interface every time step Optionally, either the atmospheric or ocean model can be used as a stand-alone system The atmospheric component of COAMPS was used for operational support for the America3 team in the 1995 America’s Cup races Results of these forecasts indicated the necessity of data assimilation to reduce model spinup in the first 6 h of the forecast Accurate forecasts of the low-level wind in the coastal race area was accomplished by utilizing triply nested grids to attain the necessary high resolution to resolve

The resolution dependence of explicitly modeled convective systems

Abstract: The representation of convective processes within mesoscale models with horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the authors investigate the effects of grid resolution on convective processes using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in mesoscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decreased is documented and the processes that are not properly represented with the coarser resolutions are identified. Results from quasi-three-dimensional squall-line simulations for midlatitude-type environments suggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolutions is characteristically slow...

Representation of Topography by Shaved Cells in a Height Coordinate Ocean Model

Abstract: Height coordinate ocean models commonly represent topography as a ‘‘staircase’’ of discontinuous steps that are fitted to the model grid. Here the ramifications of an alternative approach are studied in which ‘‘shaved cells’’ are used to represent irregular topography. The problem is formulated using the finite-volume method and care is taken to ensure that the discrete forms have appropriate conservation properties. Two representations of topography, ‘‘partial step’’ and ‘‘piecewise linear,’’ are considered and compared with the staircase approach in some standard problems such as the topographic b effect and flow over a Gaussian bump. It is shown that shaved cells are clearly more accurate than the conventional staircase representation. The use of partial steps, although not as accurate as the piecewise linear approach, is seen to be superior to the staircase approach. Moreover, partial steps can be readily implemented in existing height coordinate models.

Alternating Wet and Dry Conditions over South America during Summer

Abstract: Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin. This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry cond...

Climatology and Interannual Variation of the East Asian Winter Monsoon: Results from the 1979-95 NCEP/NCAR Reanalysis

Abstract: This paper presents the climatology and interannual variation of the East Asian winter monsoon based on the 1979–95 National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis. In addition to documenting the frequency, intensity, and preferred propagation tracks of cold surges and the evolution patterns of related fields, the authors discuss the temporal distribution of the Siberian high and cold surges. Further, the interannual variation of the cold surges and winter monsoon circulation and its relationship with ENSO were examined. There are on average 13 cold surges in each winter season (October–April), of which two are strong cases. The average intensity of cold surges, measured by maximum sea level pressure, is 1053 hPa. The cold surges originate from two source regions: 1) northwest of Lake Baikal, and 2) north of Lake Balkhash. The typical evolution of a cold surge occurs over a period of 5–14 days. Trajectory and correlation analyses indicate that, du...

Verification of Eta–RSM Short-Range Ensemble Forecasts

Abstract: Motivated by the success of ensemble forecasting at the medium range, the performance of a prototype shortrange ensemble forecast system is examined. The ensemble dataset consists of 15 case days from September 1995 through January 1996. There are 15 members of the ensemble, 10 from an 80-km version of the eta model and five from the regional spectral model. Initial conditions include various in-house analyses available at the National Centers for Environmental Prediction as well as bred initial conditions interpolated from the mediumrange forecast ensemble. Forecasts from the 29-km mesoeta model were archived as well for comparison. The performance of the ensemble is first evaluated by the criterion of ‘‘uniformity of verification rank.’’ Assuming a perfect forecast model, equally plausible initial conditions, and the verification is a plausible member of the ensemble, these imply the verification when pooled with the 15 ensemble forecasts and sorted is equally likely to occur in each of the 16 ranks. Hence, over many independent samples, a histogram of the rank distribution should be nearly uniform. Using data from the ensemble forecasts, rank distributions were populated and found to be nonuniform. This was determined to be largely a result of model and initial condition deficiencies and not problems with the verification data. The uniformity of rank distributions varied with atmospheric baroclinicity for midtropospheric forecast variables but not for precipitation forecasts. Examination of the error characteristics of individual ensemble members showed that the assumption of identical errors for each member is not met with this particular ensemble configuration, primarily because of the use of both bred and nonbred initial conditions in this test. Further, there were both differences in the accuracy of eta and regional spectral model bred member forecasts. The performance of various summary forecasts from the ensemble such as its mean showed that the ensemble can generate forecasts that have similar or lower error than forecasts from the 29-km mesoeta, which was approximately equivalent in computational expense. Also, by combining the ensemble forecasts with rank information from other cases, reliable ensemble precipitation forecasts could be created, indicating the potential for useful probabilistic forecasts of quantitative precipitation from the ensemble.

Transient Climate Change in the CSIRO Coupled Model with Dynamic Sea Ice

Abstract: The CSIRO coupled model has been used in a “transient” greenhouse experiment. This model contains atmospheric, oceanic, comprehensive sea-ice (dynamic/thermodynamic plus leads), and biospheric submodels. The model control run (over 100 years long) employed flux corrections and displayed only a small amount of cooling, mainly at high latitudes. The transient experiment (1% increase in CO2 compounding per annum) gave a 2°C warming at time of CO2 doubling. The model displayed a “cold start” effect with a (maximum) value estimated at 0.3°C. The warming in the transient run had an asymmetrical response as seen in other coupled models, with the Northern Hemisphere (NH) warming more than the Southern Hemisphere (SH). However, the land surface response in this model is different from some other transient experiments in that there is not a pronounced drying of the midlatitudes in the NH in summer. In the control run the ice model gave realistic ice distributions at both poles, with the NH ice in particula...

A Multiscale Numerical Study of Hurricane Andrew (1992). Part I: Explicit Simulation and Verification

Abstract: In this study, the inner-core structures of Hurricane Andrew (1992) are explicitly simulated using an improved version of the Penn State–NCAR nonhydrostatic, two-way interactive, movable, triply nested grid mesoscale model (MM5). A modified Betts–Miller cumulus parameterization scheme and an explicit microphysics scheme were used simultaneously to simulate the evolution of the larger-scale flows over the coarser-mesh domains. The intense storm itself is explicitly resolved over the finest-mesh domain using a grid size of 6 km and an explicit microphysics package containing prognostic equations for cloud water, ice, rainwater, snow, and graupel. The model is initialized with the National Centers for Environmental Prediction analysis enhanced by a modified moisture field. A model-generated tropical-storm-like vortex was also incorporated. A 72-h integration was made, which covers the stages from the storm’s initial deepening to a near–category 5 hurricane intensity and the landfall over Florida. As...

A Prognostic Cloud Scheme for Operational NWP Models

Abstract: An explicit cloud prediction model has been developed and incorporated into the Eta Model at the National Centers for Environmental Prediction. In this scheme, only one predictive variable, cloud mixing ratio, is added to the model’s prognostic equations to represent both cloud liquid water and cloud ice. Precipitation is diagnostically calculated from cloud mixing ratio. Extensive tests have been performed. The statistical results show a significant improvement in the model precipitation forecasts. Diagnostic studies suggest that the inclusion of cloud ice is important in transferring water vapor to precipitation and in the enhancement of latent heat release; the latter subsequently affects the vertical motion field significantly.

A Comparison Study of Convective Parameterization Schemes in a Mesoscale Model

Abstract: A comparison study of four cumulus parameterization schemes (CPSs), the Anthes‐Kuo, Betts‐Miller, Grell, and Kain‐Fritsch schemes, is conducted using The Pennsylvania State University‐National Center for Atmospheric Research mesoscale model. Performance of these CPSs is examined using six precipitation events over the continental United States for both cold and warm seasons. Grid resolutions of 36 and 12 km are chosen to represent current mesoscale research models and future operational models. The key parameters used to evaluate skill include precipitation, sea level pressure, wind, and temperature predictions. Precipitation is evaluated statistically using conventional skill scores (such as threat and bias scores) for different threshold values based on hourly rainfall observations. Rainfall and other mesoscale features are also evaluated by careful examination of analyzed and simulated fields, which are discussed in the context of timing, evolution, intensity, and structure of the precipitation systems. It is found that the general 6-h precipitation forecast skill for these schemes is fairly good in predicting four out of six cases examined in this study, even for higher thresholds. The forecast skill is generally higher for cold-season events than for warm-season events. There is an increase in the forecast skill in the 12-km model, and the gain is most obvious in predicting heavier rainfall amounts. The model’s precipitation forecast skill is better in rainfall volume than in either the areal coverage or the peak amount. The scheme with the convective available potential energy‐based closure assumption (Kain‐Fritsch scheme) appears to perform better. Some systematic behaviors associated with various schemes are also noted wherever possible. The partition of rainfall into subgrid scale and grid scale is sensitive to the particular parameterization scheme chosen, but relatively insensitive to either the model grid sizes or the convective environments. The prediction of mesoscale surface features in warm-season cases, such as mesoscale pressure centers, windshift lines (gust fronts), and temperature fields, strongly suggests that the CPSs with moist downdrafts are able to predict these surface features more accurately.

The Genesis of Hurricane Guillermo: TEXMEX Analyses and a Modeling Study

Abstract: The transformation of a mesoscale convective system into Hurricane Guillermo was captured by aircraft and Doppler wind data during the Tropical Experiment in Mexico. The early phase of the system evolves in a way very similar to previously documented mesoscale convective systems, with a midlevel mesocyclone developing in the stratiform precipitation region. More unusually, the cyclone extends to low altitudes: A weak cyclone is discernable even in the 300-m altitude wind field. After another day of evolution, a small, surface-based warm-core cyclone is observed to develop within the relatively cold air associated with the mesocyclone aloft. This mesocyclone develops into a hurricane over the subsequent day. A nonhydrostatic, axisymmetric numerical model is used to explore the evolution of the initially cold-core, midlevel vortex into a tropical cyclone. A mesoscale, midlevel “showerhead” is switched on in an initially quiescent, tropical atmosphere overlying a warm ocean surface. Evaporation of t...

The Role of High- and Low-Frequency Dynamics in Blocking Formation

Abstract: Time evolutions of prominent blocking flow configurations over the North Pacific and Europe are compared based upon composites for the 30 strongest events observed during 27 recent winter seasons. Fluctuations associated with synoptic-scale migratory eddies have been filtered out before the compositing. A quasi-stationary wave train across the Atlantic is evident during the blocking amplification over Europe, while no counterpart is found to the west of the amplifying blocking over the North Pacific. Correlation between the tropopause-level potential vorticity (PV) and meridional wind velocity associated with the amplifying blocking is found to be negative over Europe in association with the anticyclonic evolution of the low-PV center, but it is almost zero over the North Pacific. Feedback from the synoptic-scale eddies, as evaluated in the form of 250-mb geopotential height tendency due to the eddy vorticity flux convergence, accounts for more than 75% of the observed amplification for the Pacific blocking and less than 45% for the European blocking. This difference is highlighted in two types of ‘‘contour advection with surgery’’ experiments. In one of them PV contours observed four days before the peak blocking time were advected by composite time series of the low-pass-filtered observational wind, and in the other experiment they were advected by the low-pass-filtered wind from which the transient eddy feedback evaluated as above had been removed at every time step. Hence, the latter data should be dominated by low-frequency dynamics. For the European blocking both experiments can reproduce the anticyclonic evolution of low-PV air within a blocking ridge as observed. For the Pacific blocking, in contrast, the observed intrusion of low-PV air into the higher latitudes cannot be reproduced without the transient feedback. Furthermore, in a barotropic model initialized with the composite 250-mb flow observed three days before the peak time, a simulated blocking development over the North Pacific is more sensitive to the insertion of the observed transient feedback than that over Europe. These results suggest that the incoming wave activity flux associated with a quasi-stationary Rossby wave train is of primary importance in the blocking formation over Europe, whereas the forcing by the synoptic-scale transients is indispensable to that over the North Pacific.

The Canadian MC2: A Semi-Lagrangian, Semi-Implicit Wideband Atmospheric Model Suited for Finescale Process Studies and Simulation

Abstract: This paper attempts to document the developmental research and early mesoscale results of the new fully nonhydrostatic atmospheric model called MC2 (mesoscale compressible community). Its numerical scheme is the semi-implicit semi-Lagrangian approach conceived and demonstrated by Tanguay, Robert, and Laprise. The dominant effort required to become a full-fledged mesoscale model was to connect it properly to a full-scale and evolving physics package; the enlarged scope of a package previously dedicated to hydrostatic pressure coordinate-type models posed some new questions. The one-way nesting is reviewed and particularly the self-nesting or cascade mode; the potential implication of this mode is explored with a stand-alone forecast experiment and related to the other existing approach employing hemispheric or global variable meshes. One of the strong assets of MC2 is its growing community of users and developers. To demonstrate the wideband characteristic of MC2, that is, its applicability to a l...

Potential Forecast Skill of Ensemble Prediction and Spread and Skill Distributions of the ECMWF Ensemble Prediction System

Abstract: Ensemble forecasting is a feasible method to integrate a deterministic forecast with an estimate of the probability distribution of atmospheric states. At the European Centre for Medium-Range Weather Forecasts (ECMWF), the Ensemble Prediction System (EPS) comprises 32 perturbed and 1 unperturbed nonlinear integrations, at T63 spectral triangular truncation and with 19 vertical levels. The perturbed initial conditions are generated using the most unstable directions growing over a 48-h time period, computed at T42L19 resolution. This work describes the performance of the ECMWF EPS during the first 21 months of daily operation, from 1 May 1994 to 31 January 1996, focusing on the 500-hPa geopotential height fields. First, the EPS is described, and the validation approach followed throughout this work is discussed. In particular, spread and skill distribution functions are introduced to define a more integral validation methodology for ensemble prediction. Then, the potential forecast skill of ensemb...

Advanced Data Assimilation for Strongly Nonlinear Dynamics

Abstract: Advanced data assimilation methods become extremely complicated and challenging when used with strongly nonlinear models. Several previous works have reported various problems when applying existing popular data assimilation techniques with strongly nonlinear dynamics. Common for these techniques is that they can all be considered as extensions to methods that have proved to work well with linear dynamics. This paper examines the properties of three advanced data assimilation methods when used with the highly nonlinear Lorenz equations. The ensemble Kalman filter is used for sequential data assimilation and the recently proposed ensemble smoother method and a gradient descent method are used to minimize two different weak constraint formulations. The problems associated with the use of an approximate tangent linear model when solving the Euler‐Lagrange equations, or when the extended Kalman filter is used, are eliminated when using these methods. All three methods give reasonable consistent results with the data coverage and quality of measurements that are used here and overcome the traditional problems reported in many of the previous papers involving data assimilation with highly nonlinear dynamics.

Scale interactions during the formation of typhoon irving

Abstract: The development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data. The development process is treated as a dry-adiabatic vortex dynamics problem, and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation. Synoptic-scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development. Mesoscale interactions with the low-level, large-scale circulations and with other midlevel, mesoscale features result in development of vorticity in the midlevels and enhancement of the low-level vorticity associated with the developing surface cyclone. Multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low-level confluence and a tendency toward recurrent development of associated mesoscale convective vortices. This is observed in both aircraft data and satellite imagery where subsequent interactions, including mergers with the low-level, tropical depression vortex, are observed. A contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices. Results from a simple baroclinic experiment show that the midlevel vortices affect the large-scale, low-level circulation in two ways: 1) initially, interactions between midlevel vortices produce a combined vortex of greater depth; 2) interaction between midlevel vortices and the low-level circulation produces a development downward of the midlevel vorticity. This strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere.

Mesoscale Interactions in Tropical Cyclone Genesis

Abstract: With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient. A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system. The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during TOGA COARE (1993). In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.

Performance of Different Soil Model Configurations in Simulating Ground Surface Temperature and Surface Fluxes

Abstract: This study compares three modifications to the one-dimensional planetary boundary layer scheme that is implemented in the σ–θ hybrid-b version of the Mesoscale Analysis and Prediction System (MAPS) and the Rapid Update Cycle (RUC). All three modifications are based on the incorporation of a simple soil model into the basic version to more accurately calculate the moisture and heat fluxes across the ground surface. The presented schemes are of increasing sophistication: the first model combines the soil model with heat and moisture budget equations for the ground surface and uses an explicit numerical scheme to compute the surface fluxes; the second model uses a more energy-conservative implicit solution for the latent and sensible surface fluxes and heat and moisture soil fluxes; the third model further incorporates a simple parameterization of the evapotranspiration process. The comparison includes the effect of different schemes on diurnal changes of surface temperature and soil heat flux. The ...

Assessment of the Land Surface and Boundary Layer Models in Two Operational Versions of the NCEP Eta Model Using FIFE Data

Abstract: Data from the 1987 summer FIFE experiment for four pairs of days are compared with corresponding 48-h forecasts from two different versions of the Eta Model, both initialized from the NCEP–NCAR (National Centers for Environmental Prediction–National Center for Atmospheric Research) global reanalysis. One used the late 1995 operational Eta Model physics, the second, with a new soil and land surface scheme and revisions to the surface layer and boundary layer schemes, used the physics package that became operational on 31 January 1996. Improvements in the land surface parameterization and its interaction with the atmosphere are one key to improved summer precipitation forecasts. The new soil thermal model is an improvement over the earlier slab soil model, although the new skin temperature generally now has too large a diurnal cycle (whereas the old surface temperature had too small a diurnal cycle) and is more sensitive to net radiation errors. The nighttime temperature minima are often too low, b...

A General Weak Constraint Applicable to Operational 4DVAR Data Assimilation Systems

Abstract: A technique to apply the forecast model as a general weak constraint in a complex variational algorithm, such as NCEP’s regional 4DVAR data assimilation system, is presented. The proposed definition of the model error has a flexible time resolution for the random error term. It has a potential for operational application, because the coarse time resolution of the random error term and a diagonal in time random error covariance matrix, as used in this study, require less computational space. The results presented in this study strongly indicate the need for a weak constraint (as opposed to a strong constraint formulation) in order to get the full benefit of a 4DVAR method. The inclusion of the model error term, even only the systematic error part, gives a main contribution to the capability of the 4DVAR method to outperform the optimal interpolation method.

Large-Scale Influences on Tropical Cyclogenesis in the Western North Pacific

Abstract: Objectively analyzed data from the European Centre for Medium-Range Weather Forecasts are used to examine the large-scale aspects of the formation of tropical cyclones. It is hypothesized that tropical cyclogenesis occurs when external atmospheric forcing on the synoptic or larger scale provides uplift through a deep layer, enhancing convection, in a region with environmental conditions favorable for genesis. Emphasis is placed on the roles of upper-level troughs, low-level wind surges, preexisting tropical cyclones, and propagating wave disturbances in triggering tropical cyclogenesis. Composites of the 200-hPa and 850-hPa flows reveal the presence of both upper-level troughs and low-level wind surges, respectively, prior to genesis. In the composites, the wind surges also appear to be related to the presence of a prior circulation located approximately 2000 km to the west of the genesis location. An examination of the individual cases demonstrates that approximately 85% of all storms had either...

The Diurnal March of Convective Cloudiness over the Americas

Abstract: Based on nine years (1983–91) of infrared data from geostationary satellites (the B3 ISCCP product), several features of the diurnal march of the frequency of convective cloudiness over the tropical and subtropical Americas are documented with 3-h temporal resolution and 0.5° × 0.5° latitude–longitude spatial resolution. The frequency of convective cloudiness in each grid box is defined in terms of the fraction of temporal samples that exhibit cloud-top temperatures colder than 235 K. The effect of varying the threshold is explored and selected results are compared with rainfall estimates based on microwave (SSM/I) imagery. Convective cloudiness over most land areas exhibits a coherent diurnal march with relatively clear mornings, a rapid afternoon buildup, and a more gradual nighttime decay. The highest, coldest convective clouds peak a few hours earlier than those with lower tops. Morning to noontime maxima tend to be prevalent over offshore waters that experience significant convection such as...

Horizontal Convective Rolls: Determining the Environmental Conditions Supporting their Existence and Characteristics

Abstract: Data from the Convection and Precipitation/Electrification (CaPE) project, as well as results from numerical simulations, are used to study horizontal convective rolls. The environmental conditions necessary for sustaining rolls and for influencing the aspect ratio, ratio of roll wavelength to convective boundary layer (CBL) depth, and orientation are examined. Observations and numerical model simulations both suggest that a moderate surface sensible heat flux and some vertical wind shear are necessary for roll existence. Unlike some previous studies, however, it is shown that rolls occurred within very low CBL shear conditions (∼2 × 10−3 s−1). In addition, the low-level (i.e., ∼200 m) shear seems to be more important than the shear through the depth of the CBL in roll sustenance. The aspect ratio is shown to be proportional to the CBL instability, measured in terms of the Monin–Obukhov length. The roll orientation is similar to the wind direction at 10 m AGL, the CBL wind direction, the inversio...

Short-Range Ensemble Forecasting of Quantitative Precipitation

Abstract: The impact of initial condition uncertainty (ICU) on quantitative precipitation forecasts (QPFs) is examined for a case of explosive cyclogenesis that occurred over the contiguous United States and produced widespread, substantial rainfall. The Pennsylvania State University‐National Center for Atmospheric Research (NCAR) Mesoscale Model Version 4 (MM4), a limited-area model, is run at 80-km horizontal resolution and 15 layers to produce a 25-member, 36-h forecast ensemble. Lateral boundary conditions for MM4 are provided by ensemble forecasts from a global spectral model, the NCAR Community Climate Model Version 1 (CCM1). The initial perturbations of the ensemble members possess a magnitude and spatial decomposition that closely match estimates of global analysis error, but they are not dynamically conditioned. Results for the 80-km ensemble forecast are compared to forecasts from the then operational Nested Grid Model (NGM), a single 40-km/15layer MM4 forecast, a single 80-km/29-layer MM4 forecast, and a second 25-member MM4 ensemble based on a different cumulus parameterization and slightly different unperturbed initial conditions. Large sensitivity to ICU marks ensemble QPF. Extrema in 6-h accumulations at individual grid points vary by as much as 3.000. Ensemble averaging reduces the root-mean-square error (rmse) for QPF. Nearly 90% of the improvement is obtainable using ensemble sizes as small as 8‐10. Ensemble averaging can adversely affect the bias and equitable threat scores, however, because of its smoothing nature. Probabilistic forecasts for five mutually exclusive, completely exhaustive categories are found to be skillful relative to a climatological forecast. Ensemble sizes of approximately 10 can account for 90% of improvement in categorical forecasts relative to that for the average of individual forecasts. The improvements due to short-range ensemble forecasting (SREF) techniques exceed any due to doubling the resolution, and the error growth due to ICU greatly exceeds that due to different resolutions. If the authors’ results are representative, they indicate that SREF can now provide useful QPF guidance and increase the accuracy of QPF when used with current analysis‐forecast systems.

The Impact of Ocean Initial Conditions on ENSO Forecasting with a Coupled Model

Abstract: A coupled atmosphere–ocean GCM (general circulation model) has been developed for climate predictions on seasonal to interannual timescales. The atmosphere model is a global spectral GCM T30L18 and the ocean model is global on a 1° grid. Initial conditions for the atmosphere were obtained from National Meteorological Center (now known as the National Centers for Environmental Prediction) analyses, while those for the ocean came from three ocean data assimilation (DA) systems. One system is a four-dimensional DA scheme that uses conventional SST observations and vertical temperature profiles inserted into the ocean model and is forced from winds from an operational analysis. The other two initialization schemes are based on the coupled model, both nudging the surface temperature toward observed SSTs and one nudging surface winds from an operational analysis. All three systems were run from 1979 to 1988, saving the state of the ocean every month, thus initial conditions may be obtained for any mont...

Potential Vorticity, Easterly Waves, and Eastern Pacific Tropical Cyclogenesis

Abstract: A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean. During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

Distribution and statistics of African mesoscale convective weather systems based on the ISCCP Meteosat imagery

Abstract: This paper provides for the first time an objective short-term (8 yr) climatology of African convective weather systems based on satellite imagery Eight years of infrared International Satellite Cloud Climatology Project–European Space Agency’s Meteorological Satellite (ISCCP–Meteosat) satellite imagery has been analyzed using objective feature identification, tracking, and statistical techniques for the July, August, and September periods and the region of Africa and the adjacent Atlantic ocean This allows various diagnostics to be computed and used to study the distribution of mesoscale and synoptic-scale convective weather systems from mesoscale cloud clusters and squall lines to tropical cyclones An 8-yr seasonal climatology (1983–90) and the seasonal cycle of this convective activity are presented and discussed Also discussed is the dependence of organized convection for this region, on the orography, convective, and potential instability and vertical wind shear using European Centre for

Predictability of a Coupled Model of ENSO Using Singular Vector Analysis. Part I: Optimal Growth in Seasonal Background and ENSO Cycles

Abstract: The fastest initial error growth (optimal growth) in the Zebiak and Cane (ZC) forecast model for the El Nino- Southern Oscillation (ENSO) is analyzed by singular value decomposition of a forward tangent model along a trajectory in a reduced EOF space. In this paper (Part I of II), optimal growth about the seasonally varying background and ENSO cycles from a long model run are discussed. Among the many forms of nonlinearity in ZC, the discontinuity of the slope in subsurface temperature at zero thermocline depth and the nonlinear advection of SST are the most significant. That positive perturbations grow much faster than negative perturbations around the seasonally varying background is first attributable to the discontinuity and, second, attributable to nonlinear advection. About the seasonally varying background, 6-month optimal growth is largest for early (boreal) spring starts, which is related to the enhanced atmospheric heating due to equatorward movement of the ITCZ. One dominant growing structure is found, characterized by north-south and east-west SST dipoles, convergent winds on the equator in the eastern Pacific, and a deepened thermocline in the whole equatorial belt. This structure is insensitive to start month and optimization time. Optimal growth about ENSO cycles in a long model run is generally much smaller than that about the seasonally varying background. As before, one dominant growing structure, insensitive to start time and optimization time, is found. During the warm phase of ENSO, optimal growth is modulated by season as is that about the seasonal varying background. During the onset and mature phases of ENSO, the final pattern of the optimal structure in 6 months is confined to the eastern Pacific; during the decay phase of ENSO, it spreads to the western Pacific as well. During the cold phase of ENSO, optimal growth has two maxima in a year—early spring and fall; the optimal perturbation propagates westward associated with surface layer-wind interaction. The authors also compare the singular vector analysis in EOF space and the standard one in physical space. The importance of norm definition to optimal growth and optimal structure is discussed.

The Influence of Mesoscale Orography on a Coastal Jet and Rainband

Abstract: The role of mesoscale orography along the central California coast in the development and evolution of a coastal jet and rainband is investigated using a high-resolution, triply nested, nonhydrostatic numerical model. Comparison of the model simulations, which use horizontal grid increments of 5 and 2 km on the inner computational meshes, with a coastal mesoscale observation network indicates that the finescale structure of the jet and rainband dynamics are adequately simulated, although phase and orientation errors occur. The observed and simulated near-surface winds have maximum speeds that exceed 22 m s−1 and a direction nearly parallel to the coastline and topography. Force balance analysis indicates that blocking in the lowest 500 m and flow over the coastal range above this layer contribute to mesoscale pressure perturbations, including pressure ridging upstream of the coastal mountains, which forces the ageostrophic dynamics of the coastal jet. Pressure perturbations associated with the to...