The Significance of Boundary-Layer and Upper-Level Processes in Wintertime Extratropical Cyclogenesis
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Date
2006-11-27
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Abstract
Winter storms along the U.S. East Coast can significantly impact the nation's economy. Despite improvement in recent decades, numerical models still produce false alarms or underestimate storm development. Operational meteorologists often seek simple analysis methods to quickly condense the wealth of meteorological information into a format that can be easily interpreted. For this reason, the Atlantic Surface Cyclone Intensification Index (ASCII) and Improved ASCII (I-ASCII) were developed. ASCII relates the pre-storm low-level baroclinicity to the deepening rate of East Coast cyclones, while I-ASCII adds a measure of upper-level forcing. Such products are meant to supplement numerical models and help forecasters quickly gauge the likelihood of a rapidly deepening cyclone.
One purpose of this study was to evaluate the performance of I-ASCII at the National Weather Service (NWS) office in Wilmington, NC for the winter of 2005-2006. Unfortunately, major storms did not occur during the study period preventing a statistically significant analysis from being performed. However, some useful analysis was still performed. It was found that ASCII outperformed I-ASCII for all observed cases due to the conflicting grid resolutions used for the original I-ASCII development and its operational implementation. Three quick fixes are proposed: the first to interpolate the analysis to the grids from which the real-time 500-mb absolute vorticity was originally obtained; the second to correlate the values of 500-mb absolute vorticity maxima between the two conflicting grids; the third to employ the ijskip variable in GEMPAK to effectively reduce grid spacing. Only the third solution will allow I-ASCII to outperform ASCII consistently suggesting that it is the broad net effect of the upper-level divergence pattern which is important in storm development as opposed to the fine details of the pattern.
The second part of this research was to determine the source of bad I-ASCII forecasts. Two cases were chosen: December 25, 2002 in which I-ASCII performed well; and December 14, 2003 in which I-ASCII performed poorly. The Weather Research and Forecasting (WRF) model, operated with a 12-km outer domain and 4-km nested domain, was used to simulate the two cases using high resolution sea-surface temperatures (SSTs) and North American Regional Reanalysis data. The most likely source of I-ASCII error is the quantification of upper-level forcing using 500-mb absolute vorticity, which is highly dependant on grid resolution. In addition, the location, timing, and orientation of vorticity maxima are essential to the evolution of the surface low, but are not included in I-ASCII.
Lastly, two WRF simulations were performed for the February 12, 2006 winter storm. The first objective was to determine whether there are any fundamental differences in the development of Miller Type-A and Type-B snowstorms with respect to the performance of I-ASCII. While there were no significant differences in how well I-ASCII handles the two storm types, the comparison of these two cases revealed the importance of a southerly and easterly component to the surface flow prior to the development of surface lows to help destabilize the boundary layer. The second objective was to identify how sensitive WRF was to differences in the resolution of the input SST analysis. Slight differences in surface heat fluxes may result in more accurate simulations of weakly forced synoptic scale lows or meso-lows when using higher resolution SST analyses, but the impact on strongly forced events is not great enough to necessitate the usage of high resolution SST analyses in all model simulations and forecasts.
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Keywords
cyclogenesis, baroclinicity, model simulations, gulf stream, absolute vorticity, surface heat fluxes
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
