Wei-Kuo Tao

A Study of Landscape-Generated Deep Moist Convection

Abstract A two-dimensional version of a cloud-resolving model was used to study the generation of deep moist convection over heterogeneous landscapes. Alternating patches of dry and wet soil were simulated for various profiles of background wind. Results suggested a significant, systematic impact of patch length and background wind on moist convection. Rainfall occurred most intensely along sea-breeze-like fronts, which formed at patch boundaries. Total accumulated rainfall—as the average over simulations with the same patch size but with different background wind profiles—was largest for a patch length of 128 km. This patch length was similar in size to a local radius of deformation (ro = HN/ω). The deposition of rainfall generated a much different distribution of soil moisture after one day of model simulation. This new distribution, however, was far from equilibrium, as the landscape still consisted of a number of wet and dry soil patches. The cloud structure of moist convection was also examined using...

Improved Simulation of Florida Summer Convection Using the PLACE Land Model and a 1.5-Order Turbulence Parameterization Coupled to the Penn State–NCAR Mesoscale Model

Three major modifications to the treatment of land surface processes in the Pennsylvania State University‐ National Center for Atmospheric Research mesoscale model MM5, are tested in a matrix of eight model experiments. Paired together in each dimension of the matrix are versions of the code with and without one of the changes. The three changes involve 1) a sophisticated land surface model [the Parameterization for Land‐ Atmosphere Convective Exchange (PLACE)], 2) the soil moisture and temperature initial conditions derived from running PLACE offline, and 3) a 1.5-order turbulent kinetic energy (TKE) turbulence boundary layer. The code without changes, defined as the control code, uses the most widely applied land surface, soil initialization, and boundary layer options found in the current MM5 community code. As an initial test of these modifications, a case was chosen in which they should have their greatest effect: conditions where heterogeneous surface forcing dominates over dynamic processes. The case chosen is one with widespread summertime moist convection, during the Convection and Precipitation Electrification Experiment (CaPE) in the middle of the Florida peninsula. Of the eight runs, the code with all three changes (labeled TKE-PLACE) demonstrates the best overall skill in terms of biases of the surface variables, rainfall, and percent and root-mean-square error of cloud cover fraction for this case. An early, isolated convective storm that formed near the east coast, at the downwind edge of a region of anomalous wet soil, and within the dense cluster of CaPE mesoscale observation stations, is correctly simulated only by TKE-PLACE. It does not develop in any of the other seven runs. A factor separation analysis shows that a successful simulation requires the inclusion of the more sophisticated land surface model, realistic initial soil moisture and temperature, and the higher-order closure of the planetary boundary layer (PBL) in order to better represent the effect of joint and synergistic (nonlinear) contributions from the land surface and PBL on the moist convection.

A Parameterization for the Triggering of Landscape-Generated Moist Convection. Part I: Analysis of High-Resolution Model Results

Abstract To develop a parameterization for the triggering of moist convection by landscape-generated mesoscale circulations, a set of relatively high-resolution three-dimensional (3D) simulations was produced. These simulations modeled the development of landscape generated mesoscale circulations that triggered moist convection over west-to-east dry patches. No clear relationship existed between average patch size and average rainfall. Rather, rainfall averaged over the area of individual patches varied linearly with the size of these patches. Thus, cumulus parameterization schemes need to account for a population of clouds (over individual patches) within each domain of a large-scale atmospheric model (i.e., numerical weather prediction and global circulation models). It is demonstrated that mesoscale perturbations in velocity, temperature, and moisture need to be included in triggering functions when evaluating whether moist convection will occur. Yet, the largest patches did not always produce the larg...

A Parameterization for the Triggering of Landscape-Generated Moist Convection. Part II: Zero-Order and First-Order Closure

Abstract To improve the triggering of clouds over landscape heterogeneity, it is suggested that the forcing by mesoscale circulations generated by landscape patches be included. For this purpose, it is suggested that a relatively simple zero-order closure be used to obtain a triggering parcel’s mesoscale perturbation vertical velocity, potential temperature, and specific humidity. In combination with a turbulent fluctuation averaged over a parcel area, one can obtain a parcel’s (total) velocity, temperature, and moisture. The authors used similarity theory to parameterize the mesoscale perturbations, using a dataset generated by a three-dimensional, high-resolution cumulus ensemble model with west-to-east land surface patches. Alternatively, the authors used one-dimensional budget equations that contain mesoscale and turbulent fluctuations (and source terms) to obtain the vertical profile of potential temperature and specific humidity within a triggering parcel. Here, it is suggested that first-order clos...