Monday, 22 October 2018
Stowe & Atrium rooms (Stoweflake Mountain Resort )
The prediction of the most likely mode of severe convective weather is a challeging task for weather forecasters around the world. In this study, composite synoptic maps are produced for a moving domain of 30° latitude x 30° longitude centered at the occurrences of severe convective weather in southern Brazil and neighboring areas. These maps are generated for three classes of severe weather that are mutually exclusive: very heavy rainfall/flash floods (HVR); damaging straight-line winds (SLW); and tornadoes (TOR). Each class consists of a sample of 40 events. In any of these samples hail may also have occurred because, in this study, emphasis is given to the possible discrimination between typical weather conditions favorable to HVR and those conducive to SLW, and at the same time contrasting both with TOR conditions. Data from the Climate Forecast System Reanalysis and Climate Forecast System version 2 (CFSR-CFSv2) are obtained for the times nearest to the occurrence of the events and for the moving domain centered on the location of the weather events, producing, for each class, mean compositions of surface, low-level, mid-level and upper-level synoptic charts. For each composite a measure of dispersion (standard deviation) is also informed. In addition, composite maps of convective parameters that quantify moisture content, conditional instability and vertical wind shear (in distinct layers) are also produced, utilizing the SHARPpy algorithm (Blumberg et al., 2016) applied to the CFSR-CFSv2 data. Moreover, magnitudes of these convective parameters on the central gridpoint of the domain are extracted in order to determine their main percentiles for each mode of severe weather. Overall, the composite charts highlight the important role played by a NW-SE-oriented trough of low pressure that extends from northeastern Argentina/Paraguay into southern Brazil in conditioning the synoptic environment that is favorable to the occurrence of all three modes of severe convective weather. This is so because the trough induces a lower-tropospheric north-northwestely flow that promotes warm and moist advection in southern Brazil, often in the form of a low-level jet. However, in the HVR sample this low pressure system is weaker, with the trough being less pronounced, and is accompanied by less baroclinicity compared to SVR and TOR samples. Accordingly, the low-level northerly flow (and, thus, vertical wind shear) in HVR also is considerably weaker. Among the three modes, SLW is the one that exhibits the most intense surface frontogenetic function and sharpest surface trough of low pressure. In the TOR composite the region of strongest northerly winds at 850 hPa coincides with the central region of the domain (i.e., where the tornado episodes were reported) while in the SLW composite the maximum northerly winds are placed farther upstream from the central piont. Hence, the TOR composite is also the one with strongest low-level shear over the location of the severe weather occurrence. At mid-levels, the composite charts show a migratory trough to the S-SW of the severe weather occurrence in classes SLW and TOR, while in HVR a more zonal flow is in place, in average. In all three samples the central point of the domain is situated in the equatorial entrance of an upper-level jet streak, but with weaker and more zonal upper-level winds in the HVR composite. As expected, the HVR class exhibits the highest values of precipitable water and low and mid-level relative humidity. Additional results from the composite charts, as well as the comparison of the quantiles of the convective parameters among the different samples, and the identification of similarities and distinctions between the general synoptic pattern of South American and North American severe weather environment will all be discussed in the Extended Abstract.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner