Browsing by Subject "VAD"
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Item An investigation into the contamination of WSR-88D VAD wind profile output by migrating birds(Texas A&M University, 2004-09-30) Schulze, Karl WernerThe VAD Wind Profile (VWP), a time-height display of winds computed by the National Weather Service's WSR-88D radar, is known on occasion to have errors at night during the fall and spring seasons. Several studies, such Haro and Gauthreaux (1998), confirm that migrating birds often contaminate the VWP output. By means of telescopic observations of a full moon, birds were observed flying on two nights when VWP contamination was suspected. The nature of the VWP errors is consistent with migrating birds due to the seasonality, nocturnal nature, and the magnitude of the errors found (greater than 10 knots). With careful selection of data, two clusters of points on the Velocity-Azimuth Display (VAD) are found to exist at certain altitudes when birds begin migrating. One cluster of points is due to radar sample volumes containing birds, and the other cluster is from radar sample volumes without birds. Being able to determine which cluster represents the wind could allow the wind to be calculated by the VWP. Present limitations with the Radar Product Generator's processor and memory prohibit a very advanced detection algorithm. Two simple objective techniques to determine the existence of the two clusters, and determine the wind, were tested. While they show some promise, these methods require further operational testing to determine their usefulness for real-time warning of bird contamination and the reporting of the true wind.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.