Browsing by Subject "MM5"
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Item Effects of baroclinicity on storm divergence and stratiform rain in a precipitating subtropical region(2009-05-15) Hopper, Jr., Larry JohnDivergence structures associated with the spectrum of precipitating systems in the subtropics and midlatitudes are not well documented. A mesoscale model (MM5) is employed to quantify the relative importance different baroclinic environments have on divergence profiles for common storm types in southeast Texas, a subtropical region. Divergence profiles averaged over a 100 x 100 nested grid with 3-km grid spacing are calculated from the model-derived wind fields for each storm. The divergence profiles simulated for selected storms are consistent with those calculated from an S-band radar using the velocity-azimuth display (VAD) technique. Divergence profiles from well-modeled storms vary in magnitude and structure across the spectrum of baroclinicities and storm types common in southeast Texas. Barotropic storms more characteristic of the Tropics generate the most elevated divergence (and thus diabatic heating) structures with the largest magnitudes. As the degree of baroclinicity increases, stratiform area fractions increase while the levels of non-divergence (LNDs) decrease. However, some weakly baroclinic storms contain stratiform area fractions and divergence profiles with magnitudes and LNDs that are similar to barotropic storms, despite having lower tropopause heights and less deep convection. Additional convection forms after the passage of some of the modeled barotropic and weakly baroclinic storms that contain elevated divergence signatures, circumstantially suggesting that heating at upper-levels may cause diabatic feedbacks that help drive regions of persistent convection in the subtropics. Applying a two-dimensional stratiform-convective separation algorithm to MM5 reflectivity data generates realistic stratiform and convective divergence signals. Stratiform regions in barotropic storms contain thicker, more elevated mid-level convergence structures with larger magnitudes than strongly baroclinic storms, while weakly baroclinic storms have LNDs that fall somewhere in between with magnitudes similar to barotropic storms. Divergence profiles within convective regions typically become more elevated as baroclinicity decreases, although variations in magnitude are less coherent. These simulations suggest that MM5 adequately captures mass field perturbations within convective and stratiform regions, the latter of which produces diabatic feedbacks capable of generating additional convection. As a result, future research determining the climatological dynamic response caused by divergence profiles in MM5 may be feasible.Item Investigation of surface inhomogeneity and estimation of the GOES skin temperature assimilation errors of the MM5 implied by the inhomogeneity over Houston metropolitan area(Texas A&M University, 2005-11-01) Han, Sang-OkThis study developed a parameterization method to investigate the impacts of inhomogeneous land surfaces on mesoscale model simulations using a high-resolution 1-d PBL model. Then, the 1-d PBL model was used to investigate the inhomogeneity-caused model errors in applying the GOES satellite skin temperature assimilation technique into the MM5 over the Houston metropolitan area (HOU). In order to investigate the surface inhomogeneity impacts on the surface fluxes and PBL variables over HOU, homo- and inhomogeneous 1-d PBL model simulations were performed over HOU and compared to each other. The 1-d PBL model was constructed so that the surface inhomogeneities were able to be represented within model grid elements using a methodology similar to Avissar and Pielke (1989). The surface inhomogeneities over HOU were defined using 30-m resolution land cover data produced by Global Environment Management (GEM), Inc. The inhomogeneity parameterization method developed in the 1-d model was applied to a standard MM5 simulation to test the applicability of the parameterization to 3-d mesoscale model simulations. From the 1-d simulations it was inferred that the surface inhomogeneities would enhance the sensible heat flux by about 36 % and reduce the latent heat flux by about 25 %, thereby inducing the warmer (0.7 %) and drier (-1.0 %) PBL and the colder and moister PBL top induced by greater turbulent diffusivities. The 3-d application of the inhomogeneity parameterization indicated consistent results with the 1-d in general, with additional effects of advection and differential local circulation. The original GOES simulation was warmer compared to observations over HOU than over surrounding areas. The satellite data assimilation itself would lead to a warm bias due to erroneous estimation of gridpoint-mean skin temperature by the satellite, but 1-d simulations indicate that the impact of this error should be much weaker than what was observed. It seems that, unless the already existing warm and dry bias of the MM5 is corrected, the inhomogeneity parameterization in the MM5 would adversely affect the MM5 performance. Therefore, consideration of the surface inhomogeneities in the urban area needs to be confined to the GOES skin temperature retrieval errors at the moment.