Observational study of a rainy January day in the Northeast Brazil semi-arid region: synoptic and mesoscale characteristics

This is an analysis of the atmospheric structure of a particular rainy day in January 1985, a month of well-above-normal rainfall in the Northeast Brazil semi-arid region. Close attention is given to 24 January 1985, with a focus on the rain structures that are most indicative of the mesoscale convective systems (MCSs) within a radar coverage area centred at Petrolina (9 degrees 24'S, 40 degrees 30'W), situated in the semi-arid lower-middle portion of the Sao Francisco River Valley. The contribution of the synoptic environment to the development of these systems is also investigated. C-band meteorological radar, along with surface rain-gauge data (intensities and 24-hour totals), has made possible a diagnosis of the mesoscale structure of the rain systems. Surface data, upper-air data, NCEP/NCAR reanalyses and Meteosat satellite images are employed as well. On the synoptic scale, the results reveal that during the greater part of the month, the area of study was largely influenced not only by the western sector of the South Atlantic upper-tropospheric trough but also by a weak South Atlantic subtropical high, as indicated by surface winds in Petrolina over all quadrants of the wind rose, except for the typical southeasterly winds. On day 24, the upper trough showed high amplitude, whereas the South Atlantic high was weak, as demonstrated by less intense subsidence inversion, high moisture content, and light and variable winds in the mid-to-lower troposphere. Extensive (meso-beta-scale), long-lasting stratiform rain areas with embedded convection that became intensified in the mid-to-late afternoon indicated the presence of MCSs. These precipitation areas were dynamically supported by low-level moisture convergence and upward motion on the mesoscale and larger scales, while surface heating due to solar radiation may have been an additional factor for convection growth in the afternoon. A large number of rain-gauge stations reporting rain and surface rain rates (5-minute duration) higher than 60 mm h(-1) gave additional evidence of widespread light (stratiform) and localized intense (convective) rainfall, indicating the presence of MCSs. Copyright (C) 2007 Royal Meteorological Society.