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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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Mark T. Stoelinga
and
Thomas T. Warner

Abstract

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

Abstract

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of three low-level jets (LLJs) observed during the second intensive observation period of the Genesis of Atlantic Lows Experiment: 1) a Piedmont LLJ along the east slope of the Appalachians, 2) a coastal LLJ (the focus of this study) along the Carolina coastline, and 3) an LLJ to the rein of a cold-frontal system positioned over the Gulf Stream. Geostrophic forcing was important for the formation of the LLJs. Shallow local baroclinity near the top of the cold dome associated with the cold air dammed to the east of the Appalachian Mountains forced the Piedmont LLJ. An analysis of the model momentum tendencies reveals that the coastal LLJ developed and was maintained by strong geostrophic forcing associated with the coastal baroclinic zone, and its strength was modulated by strong, inertial accelerations. Significant horizontal structure in the coastal LLJ developed during the daytime as a result of the different vertical mixing properties associated with continental and maritime parcel source regions.

Model sensitivity experiments indicate that diabatic processes substantially influence the evolution of the coastal and cold-frontal LLJs, Latent heating associated with banded precipitation over the Gulf Strum to the rear of the front was the primary Forcing mechanism for the frontal LLJ. Sensible heating within the marine atmospheric boundary layer acted to enhance the coastal baroclinic zone and low-level geostrophic forcing, and to subsequently strengthen the coastal LLJ. Cold-air damming and strong lower-tropospheric sensible and latent heating in the vicinity of the Gulf Stream, which frequently occur during autumn and winter months along the East Coast, combine to produce a favorable mesoscale environment for LLJ formation with a wind direction parallel to the coastline.

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James D. Doyle
and
Thomas T. Warner

Abstract

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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Thomas T. Warner
and
Hsiao-Ming Hsu

Abstract

Future-generation, operational, weather prediction systems will likely include storm-scale, limited-area models that will explicitly resolve convective precipitation. However, the high-resolution convection-resolving grids will need to be embedded, or nested, within coarser-resolution grids that will provide lateral-boundary conditions. It is the purpose of this study to illustrate how the convective environment on a convection-resolving storm-scale model grid, and therefore the convection itself, can be significantly influenced by the treatment of convection on the coarser grids within which the fine grid is embedded.

It was confirmed that, as in the actual atmosphere, mass-field adjustments resulting from convection in one area (the outer grids, in this case) affect the convection in other areas (the inner, convection-resolving grid). That is, the errors in precipitation timing, precipitation intensity, and the vertical distribution of latent heating, associated with the treatment of convection on the outer grids, greatly affect the explicit convection on the inner grid. In this case, the different precipitation parameterizations on the outer two grids produce up to a factor of 3 difference in the 24-h amount of explicit rainfall simulated on the inner grid. Even when the parameterization is limited to only the outer grid, with explicit precipitation on both the middle and inner grids, over a factor of 2 difference in 24-h total explicit rainfall is produced on the inner grid. The different precipitation parameterizations on the outer grids appear to differently modulate the intensity and the timing of the explicit convection on the inner grid through induced subsidence.

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James D. Doyle
and
Thomas T. Warner

Abstract

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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James D. Doyle
and
Thomas T. Warner

Abstract

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the sensitivity of the structure and evolution of mesoscale coastal phenomena to the sea surface temperature (SST) distribution in the vicinity of the Gulf Stream during intensive observation period 2 (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE). Experiments are performed with three different SST analyses: A 1) a high-resolution 14-km analysis, 2) a medium-resolution 275-km analysis, and 3) a coarse-resolution 381-km analysis.

The results indicate that numerical simulations of mesoscale phenomena embedded in the marine atmospheric boundary layer (MABL) in the vicinity of the Gulf Stream are very sensitive to the SST distribution. The total (sensible and latent) average heat fluxes differ by less than 15% among the three experiments; however, the mesoscale distributions of the oceanic surface heat fluxes differ substantially. As a result of large differences in the lower-tropospheric diabatic heating, significant dissimilarities occur among the three experiments in terms of the intensity and movement of the north-wall MABL front, MABL structure, coastal front, cyclone, and precipitation. The maximum values of diagnosed quantities (e.g., vorticity, divergence, thermal gradients, and frontogenesis) in the vicinity of the Gulf Stream vary by as much as a factor of 8 among the three simulations. Also, the lower-tropospheric geostrophic forcing along the coast is relatively weak in the two simulations that used lower-resolution SST analyses. This weak geostrophic forcing in the lower-resolution SST experiments results in the development of a low-level jet that is weaker than observed and simulated in the experiment with the high-resolution analysis.

Among the three experiments, the high-resolution SST analysis simulation best captures the analyzed intensity, structure, and movement of the mesoscale coastal phenomena. Thus, the use of high-resolution SST analyses in research and operational mesoscale models may be essential in some cases for the accurate prediction of coastal cyclones and their associated mesoscale structures.

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Mercedes N. Lakhtakia
and
Thomas T. Warner

Abstract

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

Abstract

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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John M. Lanicci
and
Thomas T. Warner

Abstract

A case of lid formation and evolution over the southern Great Plains during early spring is analyzed using conventional data analysis and a 120-h simulation from the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model, Version 4 (MM4). The authors begin with a brief discussion of the evolution of the synoptic-scale circulation during the different phases of the lid development cycle in this case and follow with a description of MM4 as it was configured for this study. The model results are then used to supplement the data analyses in an examination of how the synoptic-scale circulation evolves and how it influences the physical and dynamical processes acting on the elevated mixed layer (EML) and low-level moist-layer source regions of the North American plateau and the Gulf of Mexico, respectively. The importance of choosing an appropriately large model domain and long simulation period in order to document properly the sequence of events, focusing primarily on the precursors to, and genesis of, the lid environment is stressed. The 120-h simulation allows one to focus on the ways in which synoptic and mesoscale processes interact to produce the observed mesoscale lid features such as inversions, moist layers, and EMLs over Texas, Oklahoma, and the adjacent coastal region.

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