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Thomas T. Warner

This paper summarizes a number of best practices associated with the use of numerical models of the atmosphere and is motivated by the rapid growth in the number of model users, who have a range of scientific and technical preparations. An underlying important message is that models are complex and imperfect tools, and model users must be aware of their strengths and weaknesses and be thorough in the process of model configuration and verification.

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Paul Schultz and Thomas T. Warner

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A cross-sectional numerical primitive-equation model is used to simulate the summertime airflow pattern in the Los Angeles basin for calm synoptic-scale wind conditions. The contributions of the sea breeze, the urban heat island effect and the mountain-valley wind are quantified. The mountain-valley and sea-breeze circulations are of the same sense (landward at the surface, toward water aloft) and strength (maximum of 5-10 m s−1 at surface), but the urban heat island effect is negligible. Correct specification of the land surface characteristics is found to be important to the quality of the simulation.

Model output is then used to calculate estimates of the space and time variation of boundary-layer ventilation. Ventilation, defined as the product of the height of the planetary boundary layer and the mean wind speed therein, is found to be enhanced in the vicinity of the sea breeze front, and generally increases with distance from the ocean. In the stable marine air layer behind the front, the ventilation is especially low.

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Wei Wang and Thomas T. Warner

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The Penn State/NCAR mesoscale model has been used in a study of special static- and dynamic-initialization techniques that improve a very-short-range forecast of the heavy convective rainfall that occurred in Texas, Oklahoma and Kansas during 9–10 May 1979, the SESAME IV study period. In this study, the model is initialized during the precipitation event. Two types of four-dimensional data assimilation (FDDA) procedures are used in the dynamic-initialization experiments in order to incorporate data during a 12-hour preforecast period. With the first type, FDDA by Newtonian relaxation is used to incorporate sounding data during the preforecast period. With the second FDDA procedure, radar-based precipitation-rate estimates and hourly raingage data are used to define a three-dimensional latent-heating rate field that contributes to the diabatic heating term in the model's thermodynamic equation during the preforecast period. This latent-heating specification procedure is also employed in static-initialization experiments, where the observed rainfall rate and radar echo pattern near the initial time of the forecast are used to infer a latent-heating rate that is specified in the mesoscale model's thermodynamic equation during the early part of the actual forecast. The precipitation forecasts from these static- and dynamic-initialization experiments are compared with the forecast produced when only operational radiosonde data are used in a conventional static initialization.

The conventional (control) initialization procedure that used only operational radiosonde data produced a precipitation prediction for the first three to four hours of the forecast period that would have been inadequate in an operational setting. Whereas at the initial time of the forecast there was substantial convective precipitation observed in a band near the edge of an elevated mixed layer, the model did not initiate the heavy rainfall until about the fourth hour of the forecast.

The use of the experimental static initialization with prescribed latent heating during the first forecast hour produced greatly improved rainfall rates during the first three to four hours. The success of the technique was shown to be not especially sensitive to moderate variations in the duration, intensity and vertical distribution of the imposed heating. Applications of the Newtonian-relaxation procedure during the preforecast period, that relaxed the model solution toward the initial large-scale analysis, produced a better precipitation forecast than did the control, with a maximum in approximately the correct position, but the intensities were too small. Combined use of either the preforecast or in-forecast latent-heat forcing with the Newtonian relaxation produced an improved forecast of rainfall intensity compared to use of the Newtonian relaxation alone. Even though both the experimental static- and dynamic-initialization procedures produced considerably improved very-short-range precipitation forecasts, compared to the control, the experimental static-initialization procedure that used latent-heat forcing during the first forecast hour did slightly better for this case.

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Michael Fiorino and Thomas T. Warner

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The initialization of a three-dimensional model with operational data for Hurricane Eloise (1975) was studied to assess the impact of using bogus storm data, surface winds, rainfall rates, and a high-resolution surface pressure analysis in the initialization of forecasts of hurricane track and intensity.

Because the track and intensity forecasts based on the unaugmented NMC analyses were unsatisfactory, various data improvement procedures were used. Boundary-layer flow was diagnosed from the surface pressure with a primitive equation PBL model, a climatological hurricane circulation was inserted into the NMC wind analysis above the boundary layer, and the three-dimensional moisture field was defined with the aid of visible-image satellite photographs. Model simulations with this improved data set were designed to test the effectiveness of dynamic initialization (DI) and the data enhancement procedures in improving the numerical hurricane forecasts. A 24 h time period, starting at 0000 GMT 21 September 1975, was considered. In procedure A, all data improvements were made and surface pressure was taken directly from a detailed analysis. Procedure B represented what might be done operationally—the only modification to the original NMC data was the insertion of a bogus storm based on composite data and the diagnosis of surface pressure from the 1000 mb heights and temperatures.

For each procedure, three model integrations were made to test the effect of DI by nudging on the forecast. Model results were evaluated in terms of track, the boundary-layer flow, surface pressure and rainfall rates. All forecasts with the improved data were much better than in the preliminary model experiments with the unmodified NMC analysis. Procedure B track predictions, which were based on initial conditions that contained the least amount of mesoscale information, were somewhat better than the others, with vector position errors of <80 km. Dynamic initialization had little effect on the path of the model storm. Intensity forecasts were best using procedure A, in which the greatest amount of hurricane scale information went into the initial conditions, and when DI was employed. However, large-scale mass-momentum adjustment and the proximity of the model storm to the lateral boundaries distorted the predictions of boundary-layer flow and rainfall rates.

A time composite of surface wind reports from land-based stations, buoys, and ships represented the type of data that might be available from future remote sensing satellites like Seasat-A. Because the data were valid at only one synoptic time, a DI could not be performed. The impact of the surface winds on the initialization could only be examined in terms of a 12 h forecast. Several methods of incorporating the surface wind observations into the initial conditions included direct insertion of the data into the NMC wind analysis and a diagnosis of surface pressure from the surface winds through a divergence equation. Although satellite winds improved the mesoscale realism of the initial boundary layer winds and the surface pressure, model forecasts were virtually unimproved. Forecast errors associated with the large-scale mass momentum adjustments, the limitations of the model physics, the data enhancement procedures, and the accuracy of the surface wind analysis, prevented our reaching any definite conclusion about the benefits of supplementary near-surface wind data.

A 12 h DI was performed in which the latent heat release due to convection was externally specified based upon satellite estimates of rainfall rate. A comparison of 12 h forecasts based on this DI and a static initialization showed that this type of DI produced forecasts of surface pressure and precipitation that were greatly improved and which were reflective of observed storm intensity. Track forecasts were not significantly changed.

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Mark T. Stoelinga and Thomas T. Warner

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Experiments are described that provide an example of the baseline skill level for the numerical prediction of cloud ceiling and visibility, where application to aviation-system safety and efficiency is emphasized. Model simulations of a light, mixed-phase, East Coast precipitation event are employed to assess ceiling and visibility predictive skill, and its sensitivity to the use of data assimilation and the use of simple versus complex microphysics schemes. To obtain ceiling and visibility from the model-simulated, state-of-the-atmosphere variables, a translation algorithm was developed based on empirical and theoretical relationships between hydrometeor characteristics and light extinction. The model-simulated ceilings were generally excessively high; however, the visibility simulations were reasonably accurate and comparable to the existing operational terminal forecasts. The benefit of data assimilation for such very short-range forecasts was demonstrated, as was the desirability of employing a reasonably sophisticated microphysics scheme.

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James D. Doyle and Thomas T. Warner

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A nonhydrostatic version of the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, with a horizontal resolution of 5 km, is used with measurements taken during intensive observation period 2 of the Genesis of Atlantic Lows Experiment to study the offshore mesobeta-scale coastal front structure. Results from the 24-h model simulation and Doppler radar data indicate that precipitation bands, with embedded convective elements, are present along the coastal front in the vicinity of the Gulf Stream. As the frontogenesis evolves, the simulated surface frontal zone becomes fractured, and discontinuous lines of confluence and mesoscale ascent become apparent. A collapse of the cross-frontal thermal gradient is driven by intense gradients of the surface fluxes in the vicinity of the Gulf Stream.

A mesoscale wave train, consisting of a series of shallow, weak vortices with horizontal scales between 50 and 100 km, forms along the front in agreement with the Doppler radar data and surface observations. Diagnostic analysis of the model simulation and a series of model sensitivity experiments indicate that shearing instability along the frontal zone focuses the lower-tropospheric convergence. Subsequently, stretching of cyclonic vorticity, modulated by latent heating associated with the banded precipitation, leads to the generation of the mesobeta-scale vortices along the coastal front. The formation mechanisms of these vortices may have important implications for the genesis of coastal cyclones and polar lows along shallow baroclinic zones.

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Sharon G. Douglas and Thomas T. Warner

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A series of experiments was performed to test various methods of incorporating sounding data from the visible and infrared spin-scan radiometer atmospheric sounder (VAS) into the initial conditions of the Penn State University/National Center for Atmospheric Research mesoscale model. The VAS data for this oceanic-cyclogenesis case consist of 110 irregularly distributed temperature and humidity soundings located over the North Pacific Ocean and apply at approximately 1200 UTC 10 November 1981.

The use of the VAS data produced relatively large changes to the National Meteorological Center's (NMC) analysis, which was the only source of meteorological data over the Pacific Ocean where the cyclone developed. Both static and dynamic initialization procedures were tested. When the model was statically initialized at an early stage of the cyclogenesis, the cyclone was only forecast well when VAS data were used to help define the initial conditions and when a reasonable distribution function was used for the latent heating associated with the parameterized precipitation. When the model was initialized 12 hours earlier with only large-scale data from the NMC analysis, a good cyclogenesis forecast was also produced. The use of dynamic-initialization and geostrophic correction procedures in order to provide mesoscale structure in the windfield based on the VAS-derived mass-field information, resulted in mixed success and proved to be unnecessary in this case.

In addition to showing that VAS data were a useful supplement, in this case at least, to the operational meteorological analysis over a data-sparse region of the ocean, these results illustrate two other points. First of all, data-impact studies with numerical models frequently assume that the veracity of the initial data is the factor most seriously impairing forecast quality. From an experimental design standpoint, this is convenient because it is not feasible to perform a complete predictability assessment for each case in order to determine other limitations imposed by the model/s numerics, physical parameterizations and boundary conditions. However, in this case we show that VAS data only had a positive impact when an apparently critical aspect of the precipitation parameterization was properly treated. A reasonable, but perhaps inconvenient, compromise is to perform a modest number of sensitivity tests on each case in order to identify any major “weak links” in the modeling system other than the initial data. Obviously the nature of these tests will depend on the meteorological setting.

Secondly, the need for mesoscale data in defining the model initial state for a forecast can depend on the development stage of the phenomenon (e.g., cyclone, mesoscale convective system) being forecast. In this case, VAS data were not needed in order to provide a good cyclogenesis forecast when the model was initialized in the precyclogenesis period with only a smooth analysis. This is because the model was able to provide the nonlinear interactions, response to surface fluxes, etc. that were necessary to define the precursor conditions for development. In contrast, initialization at a later stage in the development benefited from the additional information or structure provided by VAS because the model was not, in effect, used as a dynamic-initialization device. Thus, the impact of the VAS data was also dependent on the time of the initialization during the life cycle of the phenomenon.

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James D. Doyle and Thomas T. Warner

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The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of coastal frontogenesis and mesoscale cyclogenesis observed during intensive observation period 2 (IOP 2) of the Genesis of Atlantic lows Experiment (GALE). The model accurately simulates many of the observed mesoscale Features including cold-air damming to the cast of the Appalachian Mountains, a coastal trough, coastal frontogenesis, and mesoscale cyclogenesis.

The coastal front becomes apparent approximately 6 h after the formation of a coastal trough in the vicinity of the Gulf Stream. An analysis of the model results indicates that both latent beating from banded precipitation over the Gulf Stream and surface sensible heating contribute to trough development. The deformation resulting from the isallobaric accelerations, associated with the pressure changes that occur as the coastal trough forms, initiates the coastal frontogenesis. Numerical sensitivity tests reveal that the diabatic processes dominate the coastal trough and front development. Initially, the frontogenetic effects of the deformation over the Gulf Stream are opposed by the frontolytic differential diabatic effects. The frontogenctic effects of differential diabatic heating at the coastline promote the westward movement of the northern portion of the front. With this westward movement of the coastal front, the deformation and diabatic effects act in concert to significantly strengthen the baroclinic zone.

A small-scale weak cyclone develops along the coastal front as a result of the strong low-level diabatic forcing associated with intense marine atmospheric boundary layer sensible heating and latent heating from copious precipitation. The mesoscale cyclone is characterized by a warm-core structure, with areas of ascent, cyclonic vorticity, and convergence confined to the lowest 3 km of the atmosphere. As the coastal cyclone moves northward along the coastal front, the baroclinic zone weakens substantially to its rear due to diabatic heating of the postfrontal air mass and strengthening westerlies to the rear of the cyclone.

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John M. Lanicci and Thomas T. Warner

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This study documents the cycle of lid formation and dissipation over the central U.S. during the spring season(defined as April, May, and June). The primary area of interest is Kansas, Oklahoma, and Texas; however, thestudy encompasses the surrounding states and the source regions for the elevated mixed layer, such as thewestern U.S. and northern Mexico. The database includes conventional surface and rawinsonde observations,as well as derived parameters that define the lid structure. We examine the temporal and spatial variability oflid occurrence and the associated surface/500-mb synoptic patterns to determine the periodicity of lid occurrence,seasonal tendencies, and relationships between different slages of the lid cycle and specific synoptic flow types.

Our results indicate that the lid cycle has a mean period of about one week. Synoptic typing shows that thereare basically two types of lid cycles: one that begins with a surface high pressure incursion into the southernPlains, and one that begins with a weak southerly surface flow. The first type of lid cycle represents about 60%of the total occurrences and appears throughout the entire season. It is of longer duration than the second andis associated with the progression of strong baroclinie waves in the we~teriies over the study area. The secondtype appears around mid-May, and subsequently becomes as frequent as the first type. It is typically associatedwith weak low4evel flow and subtropical circulations that exis~ over the region in late spring and summer afterthe polar jet has relreated northward. We define a four-phase composite of the chronology of the lid cycle..Analyses of composited synoptic-flow types to represent the various stages in each type of lid cycle are presented,and we examine several of these composites to identify geographically favored zones for initiation of deep convection.

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Ellen M. Salmon and Thomas T. Warner

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Model initializations are frequently assessed in terms of noise statistics or long-range forecast skill and predictability limits. However, meant motion or the utility of very short-range forecasts or nowcasts has stimulated interest in the possibility of using mesoscale models for their production. Thus, very short-range forecast skill has become an important criterion for evaluating the adequacy of model initializations. For example, previously acceptable forecast products such as total-event precipitation amounts do not provide the “nowcamer” with sufficiently detailed guidance. Models must now be able to predict hourly rain amount. It is the relationship between the quality of the very short-range forecasts of hourly rainfall and the specification of the initial divergence field that is the focus of this study.

The mesoscale initialization discussed by Tarbell et. al., in which horizontal divergence is diagnosed from a diabatic omega equation, was tested on a heavy rainfall case. The procedure for diagnosing the divergent-wind component included effects of latent heating obtained from the observed rain rates. In the real-data tests, three forecast periods were used during the SESAME III (1979) study period. Six-hour rainfall predictions initialized with the diagnosed divergence were compared to observed precipitation and to rainfall forecasts based on initial conditions containing, no divergence and observed divergence obtained from the standard rawinsonde winds. The utilization of the mesoscale rainfall information in diagnosing the initial divergent component was found to be important in correctly predicting hourly rainfall patterns, especially for the first few hours. The use of the divergence field obtained from rawinsonde data was only marginally better the use of nondivergent initial conditions.

A data-simulation procedure was also used to test this initialization technique. Model-generated data, which were in dynamic balance, represented an internally consistent high-resolution dataset that was used to solve the omega equation and to define the rain rates used both for verification and as input to the diabatic term of the omega equation. This experimental setting tested the ability of the diagnosed-divergence initialization to improve the very short-range precipitation forecast under idealized conditions—when balanced, high-resolution man/momentum data and grid-box average precipitation data are available. Results from these experiments were consistent with those that used real data. Only the diagnosed-divergence initialization produced reasonable rain rates during the first 4–6 hours, and it did so only when the observed rain rates at the initial time were used to define the diabatic term in the omega equation.

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