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- Author or Editor: George H. Bryan x
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Abstract
Spatial patterns of tropical cyclone tornadoes (TCTs), and their relationship to patterns of mesoscale predictors within United States landfalling tropical cyclones (LTCs) are investigated using multicase composites from 27 years of reanalysis data from 1995 through 2021. For 72 cases of LTCs with wide ranging TC intensites at landfall, daytime TCT frequency maxima are found in the northeast, right-front, and downshear-right quadrants when their composites are constructed in ground-relative, TC-heading relative, and environmental shear relative coordinates, respectively. TCT maxima are located near maxima of 10-m to 700-hPa bulk wind difference (BWD), which are enhanced by the TC circulation. This proxy for bulk vertical shear in roughly the lowest 3 km is among the best predictors of maximum TCT frequency. Relative to other times, the position of maximum TCT frequency during the afternoon shifts ∼100 km outward from the LTC center toward larger MLCAPE values. Composites containing the strongest LTCs have the strongest maximum 10-m to 700-hPa and 10-m to 500-hPa BWDs (∼20m s−1) with nearby maximum frequencies of TCTs. Corresponding composites containing weaker LTCs but still many TCTs, had bulk vertical shear values that were ∼20% smaller (∼16 m s−1). Additional composites of cases having similarly weak average LTC strength at landfall, but few or no TCTs, had both maximum bulk vertical shears that were an additional ∼20% lower (∼12 m s−1) and smaller MLCAPE. TCT environments occurring well inland are distinguished from others by having stronger westerly shear and a west-to-east oriented baroclinic zone (i.e., north-to-south temperature gradient) that enhances mesoscale ascent on the LTC’s east side.
Abstract
Spatial patterns of tropical cyclone tornadoes (TCTs), and their relationship to patterns of mesoscale predictors within United States landfalling tropical cyclones (LTCs) are investigated using multicase composites from 27 years of reanalysis data from 1995 through 2021. For 72 cases of LTCs with wide ranging TC intensites at landfall, daytime TCT frequency maxima are found in the northeast, right-front, and downshear-right quadrants when their composites are constructed in ground-relative, TC-heading relative, and environmental shear relative coordinates, respectively. TCT maxima are located near maxima of 10-m to 700-hPa bulk wind difference (BWD), which are enhanced by the TC circulation. This proxy for bulk vertical shear in roughly the lowest 3 km is among the best predictors of maximum TCT frequency. Relative to other times, the position of maximum TCT frequency during the afternoon shifts ∼100 km outward from the LTC center toward larger MLCAPE values. Composites containing the strongest LTCs have the strongest maximum 10-m to 700-hPa and 10-m to 500-hPa BWDs (∼20m s−1) with nearby maximum frequencies of TCTs. Corresponding composites containing weaker LTCs but still many TCTs, had bulk vertical shear values that were ∼20% smaller (∼16 m s−1). Additional composites of cases having similarly weak average LTC strength at landfall, but few or no TCTs, had both maximum bulk vertical shears that were an additional ∼20% lower (∼12 m s−1) and smaller MLCAPE. TCT environments occurring well inland are distinguished from others by having stronger westerly shear and a west-to-east oriented baroclinic zone (i.e., north-to-south temperature gradient) that enhances mesoscale ascent on the LTC’s east side.
Abstract
Accurately representing boundary layer turbulent processes in numerical models is critical to improve tropical cyclone forecasts. A new turbulence kinetic energy (TKE)-based moist eddy-diffusivity mass-flux (EDMF-TKE) planetary boundary layer scheme has been implemented in NOAA’s Hurricane Analysis and Forecast System (HAFS). This study evaluates EDMF-TKE in hurricane conditions based on a recently developed framework using large-eddy simulation (LES). Single-column modeling tests indicate that EDMF-TKE produces much greater TKE values below 500-m height than LES benchmark runs in different high-wind conditions. To improve these results, two parameters in the TKE scheme were modified to ensure a match between the PBL and surface-layer parameterizations. Additional improvements were made by reducing the maximum allowable mixing length to 40 m based on LES and observations, by adopting a different definition of boundary layer height, and by reducing nonlocal mass fluxes in high-wind conditions. With these modifications, the profiles of TKE, eddy viscosity, and winds compare much better with LES results. Three-dimensional idealized simulations and an ensemble of HAFS forecasts of Hurricane Michael (2018) consistently show that the modified EDMF-TKE tends to produce a stronger vortex with a smaller radius of maximum wind than the original EDMF-TKE, while the radius of gale-force wind is unaffected. The modified EDMF-TKE code produces smaller eddy viscosity within the boundary layer compared to the original code, which contributes to stronger inflow, especially within the annulus of 1–3 times the radius of maximum wind. The modified EDMF-TKE shows promise to improve forecast skill of rapid intensification in sheared environments.
Abstract
Accurately representing boundary layer turbulent processes in numerical models is critical to improve tropical cyclone forecasts. A new turbulence kinetic energy (TKE)-based moist eddy-diffusivity mass-flux (EDMF-TKE) planetary boundary layer scheme has been implemented in NOAA’s Hurricane Analysis and Forecast System (HAFS). This study evaluates EDMF-TKE in hurricane conditions based on a recently developed framework using large-eddy simulation (LES). Single-column modeling tests indicate that EDMF-TKE produces much greater TKE values below 500-m height than LES benchmark runs in different high-wind conditions. To improve these results, two parameters in the TKE scheme were modified to ensure a match between the PBL and surface-layer parameterizations. Additional improvements were made by reducing the maximum allowable mixing length to 40 m based on LES and observations, by adopting a different definition of boundary layer height, and by reducing nonlocal mass fluxes in high-wind conditions. With these modifications, the profiles of TKE, eddy viscosity, and winds compare much better with LES results. Three-dimensional idealized simulations and an ensemble of HAFS forecasts of Hurricane Michael (2018) consistently show that the modified EDMF-TKE tends to produce a stronger vortex with a smaller radius of maximum wind than the original EDMF-TKE, while the radius of gale-force wind is unaffected. The modified EDMF-TKE code produces smaller eddy viscosity within the boundary layer compared to the original code, which contributes to stronger inflow, especially within the annulus of 1–3 times the radius of maximum wind. The modified EDMF-TKE shows promise to improve forecast skill of rapid intensification in sheared environments.
Abstract
An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land–lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence.
Analysis of the Weather Surveillance Radar-1988 Doppler radar data from Buffalo, New York, and Cleveland, Ohio, revealed that most of the radar-derived precipitation estimates for the region were overdone except for the region affected by the quasi-stationary rainband, which was underestimated. Reconstruction of the conditions in the vicinity of the band indicate that cloud bases were considerably lower and equivalent potential temperatures higher than for the areas of precipitation farther east over northwestern Pennsylvania and southwestern New York State. It is postulated that, due to the long distance from the radar sites to the Erie area, the radar was unable to observe large amounts of cloud condensate produced by warm-rain processes below 4 km. Estimates of precipitation rates from a simple cloud model support this interpretation.
Abstract
An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land–lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence.
Analysis of the Weather Surveillance Radar-1988 Doppler radar data from Buffalo, New York, and Cleveland, Ohio, revealed that most of the radar-derived precipitation estimates for the region were overdone except for the region affected by the quasi-stationary rainband, which was underestimated. Reconstruction of the conditions in the vicinity of the band indicate that cloud bases were considerably lower and equivalent potential temperatures higher than for the areas of precipitation farther east over northwestern Pennsylvania and southwestern New York State. It is postulated that, due to the long distance from the radar sites to the Erie area, the radar was unable to observe large amounts of cloud condensate produced by warm-rain processes below 4 km. Estimates of precipitation rates from a simple cloud model support this interpretation.
