Middle Tropospheric Cyclonic Vortex Formation

Abstract

The short-term forecast is based on atmospheric synoptic systems analysis such as the Middle Tropospheric Cyclonic Vortices (MTCV). This research was aimed at analyzing and determining the formation processes of these vortices. Reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) with spatial resolution of 0.25º × 0.25º for nine vertical levels were used to identify the main features of MTCVs over Northeast Brazil and the Equatorial Atlantic Ocean. During 2010, 62 MTCVs were identified over these regions. Most MTCVs were short-lived events compared to similar systems as the South American Upper Tropospheric Cyclonic Vortex (UTCVs) and MTCVs over the Indian continent; 19 MTCVs lasted from 12 to 20 hours, while 15 of them were active from 30 to 42 hours, the other MTCVs (28) lasted for more than 48 hours. More frequently MTCV centers (55% of events, 34 cases) were observed at the level of 700 hPa. Three types of formation processes were identified: 1) Type I, trough in the air current from the west, 2) Type II, trough in the air current from the east, and 3) Type III in the meridional current from both hemispheres. Ten subtypes were distinguished depending on the position of the trough axis before the MTCV formation: meridional, NW - SE and SW – N. The most frequent subtype of Type I (17 events) were formations on troughs with an axis from northwest to southeast. Type II MTCVs developed frequently (13 events) on troughs with an axis from southeast to northwest. Low vorticity (-3 and -4×10^-5 s^-1) dominated in all MTCVs events. Only weak convergence was present in 16 events, while weak divergence and convergence were observed in the rest of the MTCVs. Only 36 out of 62 MTCVs presented both lifting and sinking during the vortex core formation. Prior to the MTCV formation, only sinking motion was observed in half of the events, while in the other half were lifting and sinking. Thus, MTCV study is important for improving short-term weather forecasting.