Tuesday, 25 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
The evolution of a dryline that initiated a line of thunderstorms on 19 June 2002 during the International H20 project (IHOP_2002) is investigated in order to improve the forecasting of warm season deep moist convection. The ability to accurately predict deep convection has remained a challenge for both forecasters and researchers, because summer time storms are often forced by surface heating and processes in the convective boundary layer, as oppose to winter storms, which are more likely large-scale baroclinic disturbances and therefore more predictable. Some progress has been made, as research has shown that low-level convergence boundaries, such as drylines and fronts, are preferred regions for deep convection to initiate. However, it is still difficult for forecasters to determine which boundaries will eventually initiate storms. Even more difficult is determining the precise locations and timing of the initiation. The objective of the Convection Initiation (CI) component of IHOP_2002 was to gain a better understanding of the boundary layer processes that lead to the initiation of deep moist convection during the warm season over the Southern Great Plains. The mobile nature of many of the platforms used during IHOP_2002 allowed for an unprecedented dataset both temporally and spatially. The main purpose of this study is to better understand mesocale boundary layer dynamics, which govern convergence boundaries and the convection initiation process, by examining the mean vertical structure and the along-line variability of a dryline, which formed on 19 June 2002 in Northwestern Kansas. The primary instruments in this study are a 3-cm airborne dual Doppler radar (ELDORA) and a horizontally pointing water vapor DIAL (LEANDRE II), both housed on-board a Navy P3 aircraft. The aircraft flew an elongated box-shaped pattern, in the boundary layer, enclosing the fine line with the line-parallel part of the box around 75 km. This allowed for the collection of data showing the along-line variability of the convergence boundaries, but also provided enough data to obtain the mean characteristics. The average distance to the line ~ 2 km, such that the water vapor DIAL could measure the moisture variability in the boundary. The mean vertical structure of the dryline, obtained by averaging ~100 individual cross sections perpendicular to the fine line from ELDORA, suggest that the initiation of deep moist convection occurs when the low-level convergence is enhanced in the late afternoon due to a diurnally-induced easterly flow in the relatively moist airmass east of the dryline. As a result, the updrafts strengthened and consequently allowed air parcels to reach the LFC. Misocyclones were observed in the radar data and increased in strength during the observational period. They accounted for large along-line variability of the dryline. Updrafts and the maximum vorticity were not collocated. Rather the strongest updrafts were located to the north of the misocylones. Moisture data, collected by LEANDRE II, revealed pockets of relatively large (small) mixing ratio to the north (south) of the misocyclone, as the distorted wind field, produced by the misocyclones, advected moist (dry) air from the east (west) across the line. These high moisture pockets were collocated with the strongest updrafts, indicating preferred locations for convection initiation. The locations of the developing storms from ELDORA correlate well with the preferred locations for CI.
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