Antecedent Atmospheric Conditions Related to Squall-Line Initiation over the Northern Coast of Brazil in July

Abstract

The antecedent atmospheric conditions at midmorning (1200 UTC or 0900 LST) related to subsequent squall-line (SL) initiation in the late afternoon or early evening over the northern coast of Brazil (NCB) were obtained for a specific month (July) when the SL initiation is independent of the strong synoptic forcing related to the intertropical convergence zone. The cases of SL and sparse deep convection in the late afternoon or early evening over the NCB were identified both objectively and subjectively, and grouped into either the SL category (SLC) or the no-convection category (NOC). For the central area of the NCB, the vertical profiles at midmorning for SLC and NOC, computed from radiosonde and ERA-Interim data spanning 9 years (2004–12), were compared. By focusing on the significant differences for both datasets, it was found that two midmorning conditions are favorable to SL initiation in July: a moister layer between 850 and 350 hPa, and a northeasterly flow at 350 hPa. These regional conditions are part of a larger-scale pattern: moister (drier) conditions over the whole NCB (southeastern South America) at 700 hPa and more intense anticyclonic circulation over the Atlantic Ocean close to northeastern Brazil at 350 hPa. The paper’s findings have the potential to aid weather forecasting activities, such as those focused on the prediction of SL-related precipitation 6–12 h ahead.

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

“…At 1800 and 0000 UTC, there are also significant differences in specific layers (775 to 650 and 1,000 to 975 hPa, respectively). The presence of significant differences in upper levels at 1200 UTC indicates a precursor condition that agrees with Oliveira and Oyama (2015).…”

“…The higher moisture content at approximately 700 hPa at 1200 UTC is a precursor condition that agrees with Oliveira and Oyama (2015); at 1800 UTC, this precursor favours the transition from shallow to deep convection (Derbyshire et al, 2004;Kuang and Bretherton, 2006;Zhang and Klein, 2010). The notable increase in the number of convective clusters between 1200 and 1500 UTC for the SLC+ events ( Figure 5) is associated with the higher moisture content at approximately 700 hPa from 1200 to 1800 UTC.…”

“…The choice of a single month (instead of the whole quarter or longer periods) is a temporal delimitation in line with other climate studies (e.g. Chen and Huang, 2012;Ahmad et al, 2014;Oliveira and Oyama, 2015).…”

“…Following Oliveira and Oyama (2015), according to the deep convection and cloudiness features over the NCB in the late afternoon or early evening, each day in March is classified subjectively into one of the following three categories:…”

“…In the mid-morning profile over the NCB, a higher midlevel moisture content and an upper-level northerly flow comprise the precursor conditions that favour SL initiation in July (Oliveira and Oyama, 2015), while the presence of a deep and intense low-level jet favours SL propagation inland (Alcântara et al, 2011). These results are important for predicting the occurrence and propagation of SLs 6-12 hr in advance (e.g.…”