Aaron Hill | Texas Tech University (original) (raw)

Papers by Aaron Hill

Two cases of dryline convection initiation (CI) over north Texas have been simulated (3 April 201... more Two cases of dryline convection initiation (CI) over north Texas have been simulated (3 April 2012 and 15 May 2013) from a 50-member WRF-DART EAKF ensemble. In this study, ensemble sensitivity analysis (ESA) is applied to a convective forecast metric, maximum composite reflectivity (referred to as the response function), as a simple proxy for CI to analyze dynamic mesoscale sensitivities at the surface and aloft. Analysis reveals positional and magnitude sensitivities related to the strength and placement of important dynamic features. CI is sensitive to the evolving temperature and dewpoint fields upstream of the forecast response region in the near-CI timeframe (0-12 hours), prior to initiation. The sensitivity to thermodynamics is also manifest in the magnitude of dewpoint gradients along the dryline that triggers the convection. ESA additionally highlights the importance of antecedent precipitation and cold pool generation that modifies the pre-CI environment. Aloft, sensitivity of CI to a weak shortwave trough and capping inversion-level temperature is coherent, consistent and traceable through the entire forecast period. Notwithstanding the (often) non-Gaussian distribution of ensemble member forecasts of convection, which violate the underpinnings of ESA theory, ESA is demonstrated to sufficiently identify regions that influence dryline CI. These results indicate an application of ESA for severe storm forecasting at operational centers and forecast offices as well as other mesoscale forecasting applications.

Ensemble sensitivity analysis (ESA) uses the covariance relationships between a chosen forecast a... more Ensemble sensitivity analysis (ESA) uses the covariance relationships between a chosen forecast aspect and initial condition variables as well as the variance in those initial variables to assess where small initial condition errors can lead to large forecast error. In a convective environment, accurate forecasts of vertical wind shear, convective available potential energy (CAPE), and composite reflectivity (MDBZ) are critical to the issuance of severe weather outlooks. ESA provides the forecaster with an understanding of how observations overlap with regions of large sensitivity in order to make informed decisions about potential errors in the forecast. An additional application of ESA is identifying observation targeting locations where assimilated observations will most reduce the variance of the chosen forecast aspect. Estimations of variance reduction can be made via mathematical relationships of sensitivity, observation variance, and initial condition variance. These estimati...

Prediction challenges still exist to correctly model and forecast severe convection along the dry... more Prediction challenges still exist to correctly model and forecast severe convection along the dryline. Deterministic mesoscale models have sufficient grid resolution, along with sophisticated data assimilation systems, to model severe storms, but the timing, location, and severity of these storms remains a challenge to reproduce. Ensembles can be used to characterize these errors, which may yield critical information to forecasters about the predictability of convective initiation (CI). Additional predictability information is gathered by utilizing the statistics of the ensemble to assess sensitivity of dryline CI to environmental influences. For this presentation, we have simulated two cases of dryline CI, 3 April 2012 and 15 May 2013, from a 50-member WRF-DART EAKF ensemble. Ensemble sensitivity analysis is applied to convective response functions to analyze dynamical sensitivities at the surface and aloft. Analysis reveals that convection is sensitive to 2-meter temperature and d...

A multiscale investigation into three years of anomalous floods in Pakistan provides insight into... more A multiscale investigation into three years of anomalous floods in Pakistan provides insight into their formation, unifying meteorological characteristics, mesoscale storm structures and predictability. Striking similarities between all three floods exist, from planetary and large-scale synoptic conditions down to the mesoscale storm structures, and these patterns were generally well-captured with the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). Atmospheric blocking events associated with high geopotential heights and surface temperatures over Eastern Europe were present during all three floods. Quasi-stationary synoptic conditions over the Tibetan plateau allowed for the formation of anomalous easterly midlevel flow across central India into Pakistan that advected deep tropospheric moisture from the Bay of Bengal into Pakistan, enabling flooding in the region. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar observations show that the flood-producing storms exhibited climatologically unusual structures during all three floods in Pakistan. These departures from the climatology consisted of westward-propagating precipitating systems with embedded wide convective cores, rarely seen in this region, that likely occurred when convection was organized upscale by the easterly midlevel jet across the subcontinent. Similar mesoscale structures in storms in other regions of the world contribute significantly to climatological precipitation and produce flash floods because of their combination of longevity and intensity. Predictability analysis using the ECMWF EPS system shows the ability to accurately forecast the conditions favouring storms of this type and hence floods in Pakistan over a week in advance with significant confidence.