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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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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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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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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 field program in March 1982 obtained rewinsonde data over a mesoscale network that had resolution similar to that of the temperature and moisture data simultaneously obtained from VAS (Visible and infrared spin-span radiometer Atmospheric Sounder). This provides a unique opportunity to verify objective analysis procedures used to combine standard rawinsonde and VAS soundings of temperature and moisture.

In this study, various combinations of VAS data, conventional rawinsonde data, and gridded data from the National Weather Service's (NWS) global analysis, are used in successive-correction (SC) and variations objective analysis procedures. The analysis are objectively and subjectively compared with the AVE/VAS special-network rawinsonde data, where the major discernable mesoα-scale feature at this time was a cold-air pocket at 500 mb.

The objective three-dimensional verification statistics show that the use of VAS data to supplement the NWS rawinsonde data significantly decreased the mixing-ratio error, but also significantly increased the temperature error. The SC procedure used to analyse the VAS data reduced the mixing-ratio error more than did any of the variational procedure. Compared to the error associated with the basic NWS global analysis that has not been supplemented with rawinsonde or VAS data, the use of VAS temperature and mixing-ratio data had a positive impact when combined with these global fields. The positive impact on the moisture field was considerably greater however.

Subjective verification of the temperature fields at 500 mb produced additional insight. First, the VAS retrieval data were able to modify the very smooth global analysis to produce a fairly realistic temperature minimum in the verification-network region. Also, the variational procedures were able to successfully blend the rawinsonde and VAS data to that the bed subjective verification of the cold-pocket structure was produced when both data sources wore employed.

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George M. Modica and Thomas T. Warner

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This study utilizes a model-generated dataset to evaluate the errors associated with the diagnosis of geopotential height using three approximate forms of the divergence equation (DE). The DE was solved on the original sigma-coordinate surfaces of the model because the objective is to isolate the error associated with use of particular forms of the DE and to exclude any error produced in the process of interpolating the data from observation points to the coordinate surface of the DE. Note that it is not uncommon practice with research or operational models to define balancing relationships, such as this DE, on the model-coordinate surfaces.

An inviscid, nonlinear, divergent DE produced rms geopotential height errors as large as 23 m and rms temperature errors of over 26°C in the lower planetary boundary layer (PBL), with minimal errors above 800 mb. Elimination of the isobaric divergence in the velocity field caused additional height errors of 5–6 m and temperature errors of 0.4°–1.2°C in the free troposphere. Use of a further-degraded DE without nonlinear terms caused height errors of 10–15 m in the upper troposphere with modestly increased temperature errors. Error fields at all levels had synoptic-scale features, with some meso-alpha structure. No spatial noise was apparent. Use of a temporal spectral filter to eliminate high-frequency modes from the model-generated data did not significantly influence the error associated with use of various forms of the DE.

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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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Mercedes N. Lakhtakia and Thomas T. Warner

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Alternative treatments of the hydrologic and thermodynamic processes at the earth's surface within a mesoscale model are discussed in this study. Specifically, the question of under what circumstances it is necessary to use a complex surface parameterization scheme as opposed to simpler ones is addressed.

Three versions of a one-dimensional planetary boundary layer model were employed, where the primary differences among them are in their surface modules. One uses a simple treatment of the surface characteristics (time independent). In another, the surface processes are represented by a complex surface physics-soil hydrology scheme, while the third one is similar to the first one but the moisture-availability parameter has a specified temporal variation during and after a precipitation event.

Several numerical simulations were performed. They showed that the models’ solutions differ the most when the vegetation cover and the surface net radiative flux are large, and the soil-water content cannot satisfy the evapotranspiration demand. When a precipitation event is present during the simulation period, the largest differences are apparent when the preprecipitation surface evapotranspiration is restricted and the precipitation event occurs in the morning. The simulations also showed that the upgraded simple scheme can sometimes represent a satisfactory substitute for the simple scheme when a precipitation event is present during the simulation period.

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James D. Doyle and Thomas T. Warner

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During Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment, a number of mesoscale phenomena were observed with special and conventional observing systems over the land and coastal waters. This study involved analysis of these data for the period 24–26 January 1986 in order to define the structure and dynamics of three features: the coastal front; a shallow cyclone that propagated along the coastal front, modifying it as it moved northward; and a low-level jet that formed in the strong coastal pressure-gradient field.

The coastal front formed in an existing pressure trough over the Gulf Stream as a result of both ageostrophic deformation and differential diabatic heating. There existed considerable variability in the frontal strength and position on both the mesoalpha and mesobeta scales. The level of strongest frontogenesis was near the surface, with frontolysis calculated above 950 mb.

The marine atmospheric boundary layer (MABL) over the Gulf Stream was conducive to cyclone formation. Latent and sensible heat fluxes of up to 800 W m−2 and 400 W m−2 respectively, were calculated early in the study period, and a deep, moist conditionally unstable boundary layer was present. Calculation of the vorticity tendency associated with the sensible heating yielded a narrow band of positive values to the east of the coastline. As a weak midtropospheric wave reached this favorable region to the cut of Florida, a shallow cyclone formed along the coastal front. As the cyclone tracked northeastward along the front, geostrophic deformation ahead of it strengthened the front while strong cold-air advection to its rear displaced the coastal front to the east, leaving behind a dry, stable MABL with low-level, cold-air advection and weak descent. As the cyclone moved northward along the front, conditionally unstable, moist, low-level air ahead was forced by the southeasterly flow to rise along the coastal front and its extension over the cold air near the coastline, causing enhanced precipitation.

A low-level northeasterly jet was also observed over the Carolinas, and formed as a result of the strong low- level pressure gradient created by the proximity of the cold continental air over land and the warm air of the Gulf Stream MABL near the coast. This jet, with a maximum near 960 mb, showed a diurnal variation of up to 20 m s−1 which likely resulted from day/night variations in mixing at jet level, an inertial oscillation with the frictional decoupling of the low-level flow at sunset, and isallobaric accelerations.

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