Squall‐line initiation over the northern coast of Brazil in March: Observational features

Abstract

AbstractThe daily cycle of atmospheric conditions related to squall‐line (SL) initiation over the northern coast of Brazil (NCB) was obtained for a specific month (March) over the course of a decade (2004–2013). March falls during the NCB's rainy season. A comparison of atmospheric conditions between two categories, the most intense SL events (SLC+) and the weakest coastal convection events (CC–), was carried out. For both categories, the daily cycle of convective cluster properties (i.e. number and area) over the NCB showed that the transition from shallow to deep convection occurred from 1200 to 1500 UTC, and the development of multicell storms occurred from 1500 to 1800 UTC. From 1800 to 2100 UTC for the SLC+ events, the number of clusters decreased sharply, and the area slightly expanded, which indicated upscaling via merging; for the CC– events, merging was a secondary mechanism, and the clusters did not experience upscaling. Atmospheric conditions were computed from the ERA‐Interim reanalysis data. By focusing on the significant differences between the categories, it was found that SL initiation was related to a higher moisture content at approximately 700 hPa for all analysis times (0600, 1200, 1800 and 0000 UTC) and to a stronger low‐level moisture convergence from 0600 to 1800 UTC, mid‐level upward motion from 1200 to 0000 UTC, low‐level convergence at 1200 UTC, and upper‐level divergence from 1800 to 0000 UTC.