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Tsengdar J. Lee and Roger A. Pielke

Abstract

Correction to Volume 31, Issue 5, page 480.

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Tsengdar J. Lee and Roger A. Pielke

Abstract

Based on the recent experiment results, a formula is proposed to be used in numerical weather-climate models to estimate the soil surface humidity. The formula has a very simple form and shows a smooth transition in the soil surface specific humidity between wet and dry soil states. The formula is recommended as a replacement to the Philip formula.

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Conrad L. Ziegler, Tsengdar J. Lee, and Roger A. Pielke Sr.

Abstract

A nonhydrostatic, three-dimensional version of the Colorado State University Regional Atmospheric Modeling System (CSU-RAMS) is used to deduce the processes responsible for the formation of drylines and the subsequent initiation of deep, moist dryline convection. A range of cumuliform cloud types are explicitly simulated along drylines on 15, 16, and 26 May 1991 in accordance with observations.

In the simulations, narrow convergence bands along the dryline provide the lift to initiate deep moist convection. The thermally direct secondary convective boundary layer (CBL) circulations along the dryline are frontogenetic and solenoidally forced. Maximum updrafts reach 5 m s−1 and the bands are 3–9 km wide and 10–100 km or more in length. The updrafts penetrate and are decelerated by the overlying stable air above the CBL, reaching depths of about 2000 m in the cases studied. Moisture convergence along the mesoscale updraft bands destabilizes the local sounding to deep convection, while simultaneously decreasing the CIN to zero where storms subsequently develop. The lapse rates of vapor mixing ratio and potential temperature in the mesoscale updrafts are rather small, indicating that increases of the lifted condensation level (LCL) and level of free convection (LFC) due to mixing following the parcel motion are also small. Simulated convective clouds of all modes, including shallow forced cumulus and storms, develop in regions where the CIN ranges from zero up to the order of the peak kinetic energy of the boundary layer updraft and moisture is sufficiently deep to permit water saturation to develop in the boundary layer.

The findings suggest that classic cloud models may not adequately simulate the early development of dryline storms due to their use of thermal bubbles to initiate convection and their assumption of a horizontally homogeneous environment. In contrast, cautious optimism may be warranted in regard to operational numerical prediction of drylines and the threat of attendant deep convection with mesoscale models.

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Tsengdar J. Lee, Roger A. Pielke, Robert C. Kessler, and John Weaver

Abstract

The influence of cold pools downstream of mesoscale mountain barriers on downslope winds and flushing is investigated in this study by means of a numerical mesoscale model. The model is compared with existing analytical and numerical solutions. It is found that the numerical model produces phases and amplitudes of nonlinear mountain waves reasonably well.

The solutions show that the structure of mountain waves can be modified greatly by the presence of the cold pool. When a cold pool is present downstream of the mountain, the development of a large amplitude mountain wave is inhibited. In the absence of surface heating, downslope winds associated with a mountain wave can be prevented from penetrating the cold pool to reach the surface and flush out the very stable cold air, particularly when the synoptic pressure gradient is oriented so as to continuously replenish the cold air. Results also suggest that shear-induced turbulent mixing at the top of the cold air has little effect on flushing. Based on the observations and the numerical results, in the absence of significant surface heating a favorable large-scale surface-pressure gradient force must be involved to remove the cold pool before the downslope winds can actually reach the surface.

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Dragutin T. Mihailović, Roger A. Pielke, Borivoj Rajković, Tsengdar J. Lee, and Milan Jeftić

Abstract

In the parameterization of land surface processes, attention must be devoted to surface evaporation, one of the main processes in the air-land energy exchange. One of the most used approaches is the resistance representation which requires the calculation of aerodynamic resistances. These resistances have been calculated using K theory for different morphologies of plant communities, then the performance of the evaporation schemes within the “α”, “β”, and their combination approaches that parameterize evaporation from bare and partly plant-covered soil surfaces are discussed. Additionally, a new “α” scheme is proposed based on an assumed power dependence α on volumetric soil moisture content and its saturated value.

Finally, the performance of the considered and the proposed schemes is tasted based on time integrations using real data. The first set was for 4 June 1982, and the second for 3 June 1981 at the experimental site in Rimski Šanševi, Yugoslavia, on chernozem soil, as representative for a bare, and partly plant-covered surface, respectively. The accuracy of the schemes was compared with the observations using a root-mean-square error and the factor-of-deviation analyses.

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Xubin Zeng, Steve Ackerman, Robert D. Ferraro, Tsengdar J. Lee, John J. Murray, Steven Pawson, Carolyn Reynolds, and Joao Teixeira
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Robert L. Walko, Larry E. Band, Jill Baron, Timothy G. F. Kittel, Richard Lammers, Tsengdar J. Lee, Dennis Ojima, Roger A. Pielke Sr., Chris Taylor, Christina Tague, Craig J. Tremback, and Pier Luigi Vidale

Abstract

The formulation and implementation of LEAF-2, the Land Ecosystem–Atmosphere Feedback model, which comprises the representation of land–surface processes in the Regional Atmospheric Modeling System (RAMS), is described. LEAF-2 is a prognostic model for the temperature and water content of soil, snow cover, vegetation, and canopy air, and includes turbulent and radiative exchanges between these components and with the atmosphere. Subdivision of a RAMS surface grid cell into multiple areas of distinct land-use types is allowed, with each subgrid area, or patch, containing its own LEAF-2 model, and each patch interacts with the overlying atmospheric column with a weight proportional to its fractional area in the grid cell. A description is also given of TOPMODEL, a land hydrology model that represents surface and subsurface downslope lateral transport of groundwater. Details of the incorporation of a modified form of TOPMODEL into LEAF-2 are presented. Sensitivity tests of the coupled system are presented that demonstrate the potential importance of the patch representation and of lateral water transport in idealized model simulations. Independent studies that have applied LEAF-2 and verified its performance against observational data are cited. Linkage of RAMS and TOPMODEL through LEAF-2 creates a modeling system that can be used to explore the coupled atmosphere–biophysical–hydrologic response to altered climate forcing at local watershed and regional basin scales.

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