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Haldun Karan, Patrick J. Fitzpatrick, Christopher M. Hill, Yongzuo Li, Qingnong Xiao, and Eunha Lim

Abstract

A detailed observational and Weather Research and Forecasting (WRF) model analysis utilizing Weather Surveillance Radar-1988 Doppler (WSR-88D), surface, and upper-air observations, as well as Geostationary Operational Environmental Satellite (GOES) images, shows a chain of events that leads to the formation of two prefrontal squall lines along the western Gulf coast on 29–30 April 2005. An approaching surface cold front (CF) generated an atmospheric bore that propagated along an inversion layer and excited high-frequency, low-level tropospheric gravity waves, initiating a squall line 60 km east of the cold front. This sequence of events manifested itself as low-level convergence ahead of the CF, which was detected by nearby WSR-88D radars. Two WRF model experiments were conducted in which one assimilated conventional observations (CTRL), and another included radar radial winds from nine WSR-88D locations (denoted as RADAR). Better representation of the low-level kinematics in RADAR yielded a distinct convergence line associated with the primary squall line.

The RADAR experiment, as well as observations (such as an 0600 UTC Slidell, Louisiana, sounding), show that the secondary squall line formed ahead of the primary squall line due to high water vapor and warm temperature advection from the Gulf of Mexico that, when combined with a deep dry layer above the atmospheric boundary layer (ABL), destabilized the atmosphere. Concurrently, a lower-tropospheric trough, propagating faster than the surface front, enhanced lifting in the region and instigated the formation of new convection. RADAR forecasted the secondary convection not only in the right place but also at about the right time, while the CTRL experiment completely missed this secondary convection.

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Haldun Karan, Patrick J. Fitzpatrick, Christopher M. Hill, Yongzuo Li, Qingnong Xiao, and Eunha Lim
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Juanzhen Sun, Stanley B. Trier, Qingnong Xiao, Morris L. Weisman, Hongli Wang, Zhuming Ying, Mei Xu, and Ying Zhang

Abstract

Sensitivity of 0–12-h warm-season precipitation forecasts to atmospheric initial conditions, including those from different large-scale model analyses and from rapid cycled (RC) three-dimensional variational data assimilations (3DVAR) with and without radar data, is investigated for a 6-day period during the International H2O Project. Neighborhood-based precipitation verification is used to compare forecasts made with the Advanced Research core of the Weather Research and Forecasting Model (ARW-WRF). Three significant convective episodes are examined by comparing the precipitation patterns and locations from different forecast experiments. From two of these three case studies, causes for the success and failure of the RC data assimilation in improving forecast skill are shown. Results indicate that the use of higher-resolution analysis in the initialization, rapid update cycling via WRF 3DVAR data assimilation, and the additional assimilation of radar observations each play a role in shortening the period of the initial precipitation spinup as well as in placing storms closer to observations, thus improving precipitation forecast skill by up to 8–9 h. Impacts of data assimilation differ for forecasts initialized at 0000 and 1200 UTC. The case studies show that the pattern and location of the forecasted precipitation were noticeably improved with radar data assimilation for the two late afternoon cases that featured lines of convection driven by surface-based cold pools. In contrast, the RC 3DVAR, both with and without radar data, had negative impacts on convective forecasts for a case of morning elevated convection associated with a midlatitude short-wave trough.

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