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Catherine A. Finley, W. R. Cotton, and R. A. Pielke Sr.

Abstract

A nested grid primitive equation model (RAMS version 3b) was used to simulate a high-precipitation (HP) supercell, which produced two weak tornadoes. Six telescoping nested grids allowed atmospheric flows ranging from the synoptic scale down to the tornadic scale to be represented in the simulation. All convection in the simulation was initiated with resolved vertical motion and subsequent condensation–latent heating from the model microphysics; no warm bubbles or cumulus parameterizations were used.

Part I of this study focuses on the simulated storm evolution and its transition into a bow echo. The simulation initially produced a classic supercell that developed at the intersection between a stationary front and an outflow boundary. As the simulation progressed, additional storms developed and interacted with the main storm to produce a single supercell. This storm had many characteristics of an HP supercell and eventually evolved into a bow echo with a rotating comma-head structure. An analysis of the storm's transition into a bow echo revealed that the interaction between convective cells triggered a series of events that played a crucial role in the transition.

The simulated storm structure and evolution differed significantly from that of classic supercells produced by idealized simulations. Several vertical vorticity and condensate maxima along the flanking line moved northward and merged into the mesocyclone at the northern end of the convective line during the bow echo transition. Vorticity budget calculations in the mesocyclone showed that vorticity advection from the flanking line into the mesocyclone was the largest positive vorticity tendency term just prior to and during the early phase of the transition in both the low- and midlevel mesocyclone, and remained a significant positive tendency in the midlevel mesocyclone throughout the bow echo transition. This indicates that the flanking line was a source of vertical vorticity for the mesocyclone, and may explain how the mesocyclone was maintained in the HP supercell even though it was completely embedded in heavy precipitation.

The simulated supercell also produced two weak tornadoes. The evolution of the simulated tornadoes and an analysis of the tornadogenesis process will be presented in Part II.

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M. Baldi, G. A. Dalu, and R. A. Pielke Sr.

Abstract

It is shown that landscape variability decreases the temperature in the surface layer when, through mesoscale flow, cool air intrudes over warm patches, lifting warm air and weakening the static stability of the upper part of the planetary boundary layer. This mechanism generates regions of upward vertical motion and a sizable amount of available potential energy and can make the environment of the lower troposphere more favorable to cloud formation. This process is enhanced by light ambient wind through the generation of trapped propagating waves, which penetrate into the midtropospheric levels, transporting upward the thermal perturbations and weakening the static stability around the top of the boundary layer. At moderate ambient wind speeds, the presence of surface roughness changes strengthens the wave activity, further favoring the vertical transport of the thermal perturbations. When the intensity of the ambient wind is larger than 5 m s−1, the vertical velocities induced by the surface roughness changes prevail over those induced by the diabatic flux changes. The analysis is performed using a linear theory in which the mesoscale dynamics are forced by the diurnal diabatic sensible heat flux and by the surface stress. Results are shown as a function of ambient flow intensity and of the wavelength of a sinusoidal landscape variability.

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U. S. Nair, Mark R. Hjelmfelt, and Roger A. Pielke Sr.

Abstract

Strong easterly flow of low-level moist air over the eastern slopes of the Black Hills on 9–10 June 1972 generated a storm system that produced a flash flood, devastating the area. Based on observations from this storm event, and also from the similar Big Thompson 1976 storm event, conceptual models have been developed to explain the unusually high precipitation efficiency. In this study, the Black Hills storm is simulated using the Colorado State University Regional Atmospheric Modeling System. Simulations with homogeneous and inhomogeneous initializations and different grid structures are presented. The conceptual models of storm structure proposed by previous studies are examined in light of the present simulations.

Both homogeneous and inhomogeneous initialization results capture the intense nature of the storm, but the inhomogeneous simulation produced a precipitation pattern closer to the observed pattern. The simulations point to stationary tilted updrafts, with precipitation falling out to the rear as the preferred storm structure. Experiments with different grid structures point to the importance of removing the lateral boundaries far from the region of activity. Overall, simulation performance in capturing the observed behavior of the storm system was enhanced by use of inhomogeneous initialization.

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Alexandre A. Costa, William R. Cotton, Robert L. Walko, and Roger A. Pielke Sr.

Abstract

A cloud-resolving model coupled to an ocean model with high vertical resolution is used to investigate air–sea interactions in 10-day long simulations. Modeled fields showed good agreement with two different convective regimes during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Research Experiment (TOGA COARE) Intensive Observing Period. The model simulates the formation of precipitation-produced, stable freshwater lenses at the top of the ocean mixed layer, with a variety of horizontal dimensions and lifetimes. The simulated fresh anomalies show realistic features, such as a positive correlation between salinity and temperature, the development of a surface jet in the direction of the wind, and, as a consequence, downwelling (upwelling) on its downwind (upwind) edge. The dataset generated by the coupled model is used to evaluate the contribution from several factors (ocean currents, gustiness, and correlations between wind speed and air temperature, wind speed and water vapor mixing ratio, and wind speed and SST) to the surface heat fluxes. Gustiness was shown to be a major contribution to the simulated surface heat fluxes, especially when convection is active. In a multiday average, the contributions from the other effects (currents and wind speed–air temperature, wind speed–water vapor mixing ratio, and wind speed–SST correlations) are small; however, they cannot be neglected under certain circumstances.

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G. A. Dalu, M. Baldi, R. A. Pielke Sr., and G. Leoncini

Abstract

A theory is presented for the evaluation of the different terms of the pressure gradient force, when mesoscale flow is driven by a sensible heat source in the planetary boundary layer (PBL), or by an elevated confined heat source, such as the release of the latent heat of condensation in a cloud. The nonlinear and linear, and the nonhydrostatic and the hydrostatic pressure gradient contributions are evaluated. The validity of the different approximations is discussed as a function of time and space scales. In addition, the validity of this approach is explored as a function of atmospheric environmental parameters, such as static stability, large-scale flow, and dissipation.

By accessing the relative importance of each contribution, specific solution techniques for mesoscale atmospheric flows can be adopted. For example, when the linear contributions dominate, an exact analytic model could be used, rather than relying on numerical approximation solution techniques. When the hydrostatic contribution dominates, the spatial variation of the vertical temperature profile can be used to uniquely define the horizontal pressure gradient force.

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Alexandre A. Costa, William R. Cotton, Robert L. Walko, Roger A. Pielke Sr., and Hongli Jiang

Abstract

A two-dimensional cloud-resolving model (CRM) was used to simulate the evolution of convection over the western Pacific between 19 and 26 December 1992, during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. A control simulation (CONTROL) was performed in which observed, time-evolving, spatially homogeneous SSTs were used as a lower boundary condition. It showed that the CRM was able to properly represent the evolution of the cloud systems.

Sensitivity experiments were carried out, in which the sea surface temperature was increased (SST+) or decreased (SST−) by 1°C and the same evolving large-scale forcing used in CONTROL. The similarities among all simulations suggested that the large-scale forcing is the dominant mechanism controlling the statistics of the cloud systems, including the total precipitation. However, the convective–stratiform partition of the cloud systems was altered, the convective part being favored in SST+ and the stratiform part favored in SST−. In terms of the radiative budget, the reduced low-level cloud coverage in SST+ acted to compensate the enhancement of high-cloud coverage produced by more vigorous convection (the opposite occurred in SST−). As a consequence, the surface downward radiation was approximately the same in CONTROL, SST+, and SST−.

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R. A. Pielke Sr., R. L. Walko, L. T. Steyaert, P. L. Vidale, G. E. Liston, W. A. Lyons, and T. N. Chase

Abstract

Using identical observed meteorology for lateral boundary conditions, the Regional Atmospheric Modeling System was integrated for July–August 1973 for south Florida. Three experiments were performed—one using the observed 1973 landscape, another the 1993 landscape, and the third the 1900 landscape, when the region was close to its natural state. Over the 2-month period, there was a 9% decrease in rainfall averaged over south Florida with the 1973 landscape and an 11% decrease with the 1993 landscape, as compared with the model results when the 1900 landscape is used. The limited available observations of trends in summer rainfall over this region are consistent with these trends.

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Richard Rotunno, Leonard J. Pietrafesa, John S. Allen, Bradley R. Colman, Clive M. Dorman, Carl W. Kreitzberg, Stephen J. Lord, Miles G. McPhee, George L. Mellor, Christopher N. K. Mooers, Pearn P. Niiler, Roger A. Pielke Sr., Mark D. Powell, David P. Rogers, James D. Smith, and Lian Xie

U.S. Weather Research Program (USWRP) prospectus development teams (PDTs) are small groups of scientists that are convened by the USWRP lead scientist on a one-time basis to discuss critical issues and to provide advice related to future directions of the program. PDTs are a principal source of information for the Science Advisory Committee, which is a standing committee charged with the duty of making recommendations to the Program Office based upon overall program objectives. PDT-1 focused on theoretical issues, and PDT-2 on observational issues; PDT-3 is the first of several to focus on more specialized topics. PDT-3 was convened to identify forecasting problems related to U.S. coastal weather and oceanic conditions, and to suggest likely solution strategies.

There were several overriding themes that emerged from the discussion. First, the lack of data in and over critical regions of the ocean, particularly in the atmospheric boundary layer, and the upper-ocean mixed layer were identified as major impediments to coastal weather prediction. Strategies for data collection and dissemination, as well as new instrument implementation, were discussed. Second, fundamental knowledge of air–sea fluxes and boundary layer structure in situations where there is significant mesoscale variability in the atmosphere and ocean is needed. Companion field studies and numerical prediction experiments were discussed. Third, research prognostic models suggest that future operational forecast models pertaining to coastal weather will be high resolution and site specific, and will properly treat effects of local coastal geography, orography, and ocean state. The view was expressed that the exploration of coupled air-sea models of the coastal zone would be a particularly fruitful area of research. PDT-3 felt that forecasts of land-impacting tropical cyclones, Great Lakes-affected weather, and coastal cyclogenesis, in particular, would benefit from such coordinated modeling and field efforts. Fourth, forecasting for Arctic coastal zones is limited by our understanding of how sea ice forms. The importance of understanding air-sea fluxes and boundary layers in the presence of ice formation was discussed. Finally, coastal flash flood forecasting via hydrologic models is limited by the present accuracy of measured and predicted precipitation and storm surge events. Strategies for better ways to improve the latter were discussed.

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Rezaul Mahmood, Roger A. Pielke Sr., Kenneth G. Hubbard, Dev Niyogi, Gordon Bonan, Peter Lawrence, Richard McNider, Clive McAlpine, Andres Etter, Samuel Gameda, Budong Qian, Andrew Carleton, Adriana Beltran-Przekurat, Thomas Chase, Arturo I. Quintanar, Jimmy O. Adegoke, Sajith Vezhapparambu, Glen Conner, Salvi Asefi, Elif Sertel, David R. Legates, Yuling Wu, Robert Hale, Oliver W. Frauenfeld, Anthony Watts, Marshall Shepherd, Chandana Mitra, Valentine G. Anantharaj, Souleymane Fall, Robert Lund, Anna Treviño, Peter Blanken, Jinyang Du, Hsin-I Chang, Ronnie Leeper, Udaysankar S. Nair, Scott Dobler, Ravinesh Deo, and Jozef Syktus
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