ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Characteristics and Analysis of Downslope Changes of Squall Lines in the Northern Shanxi Region
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Abstract:
In order to systematically investigate the evolution characteristics and mechanisms of squall lines descending mountains in Northern Shanxi, this study utilized the following datasets from the warm season (May-September) of 2021-2023: North China regional mosaic composite reflectivity radar products, single-site radar data from Northern Shanxi, ERA5 reanalysis data, and surface observation data. Statistical analyses were conducted on the changing characteristics of squall lines descending mountains in this region, yielding the following main conclusions: (1) A total of 29 squall lines were identified. Based on their direction of movement, they were classified into four types: west-moving, northwest-moving, north-moving, and basin-originating. Based on their intensity change upon descending mountains, they were classified into three types: intensifying, weakening, and maintaining types. The weakening type was the most common, accounting for 67%. It was also found that all west-moving squall lines belonged to the weakening type upon descent, while all north-moving types were intensifying. The northwest-moving type included squall lines that intensified, weakened, or maintained their intensity upon descent. (2) Analysis of the environmental background ahead of the descending path for intensifying and weakening types within the northwest-moving squall lines revealed that, compared to weakening squall lines, intensifying ones exhibited slightly stronger dynamic conditions (850 hPa divergence), while conditions related to moisture (850 hPa specific humidity, atmospheric total column precipitable water), convective available potential energy (CAPE), and vertical wind shear were comparable or slightly poorer. This suggests that changes in squall line intensity upon descent may be driven more by their internal dynamics (such as the balance between the cold pool and inflow) rather than static environmental conditions. Consequently, accurately predicting whether a squall line will intensify or weaken upon descent based solely on the environmental conditions ahead of its path is difficult. (3) For short-term nowcasting, radar data can be used to predict whether a squall line will intensify or weaken upon descent.?Intensifying squall lines?upon descent typically exhibit stronger echo intensity, a movement speed around 17 m·s?1, large gradient zones of reflectivity factor concentrated at the leading edge, an overall bow-shaped morphology accompanied by a gust front. Radial velocity cross-sections show a distinct organized structure with forward inflow ascending slantwise along the rear inflow.?Weakening squall lines?upon descent typically exhibit weak to moderate echo intensity, a movement speed below 10 m·s?1, large gradient zones of reflectivity factor concentrated at the rear, a relatively straight overall morphology without an accompanying gust front, and radial velocity cross-sections lacking a distinct organized structure with forward inflow ascending slantwise along the rear inflow.
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