Thermodynamic and kinematic characterization of the low-level troposphere during SALLJEX in different large-scale environments

NICOLINI, MATILDE; SALIO, PAOLA VERÓNICA; BORQUE, PALOMA

Foz do Iguaçu, Brazil

Conferencia; 8th International Conference on Southern Hemisphere Meteorology and Oceanography; 2006

American Meteorological Society

In the last decade a significant effort has been devoted to improve the observational resources over South America. Field experiments like the Large-Scale Biosphere Atmosphere Experiment in Amazonia (LBA) and the South American Low-Level Jet Experiment (SALLJEX) are prominent examples of this regional challenge. Particularly, this last experiment conducted under the CLIVAR´s VAMOS program during the warm season period Nov. 15, 2002 to Feb. 15, 2003 was performed in Bolivia, Paraguay, central and northern Argentina, western Brazil and Peru. SALLJEX aimed to monitor, quantify and analyze the low-level circulation over this region and its variability (Nicolini et al. 2004a, Vera et al 2006 and references therein). SALLJEX data set provides a quantitative improvement in both spatial and temporal resolution over that of the operational upper-air network (see Fig. 1 for the topographical map and upper-air network sites during SALLJEX). Early findings related to low-level circulation depicted in SALLJEX data have been documented by Nicolini et al. 2004a. They addressed the SALLJ diurnal cycle and three dimensional structure based in a preliminary upper-air sounding data set and in the research flight missions from the NOAA WP-3D aircraft. Their results illustrate the vertical and horizontal structure of the low-level flow during a particular P-3 mission given by the series of ascents and descents and indicate a spatial variability in mean summer behavior in low-level wind diurnal cycle both in the magnitude of the maximum speed in the vertical wind profile and in the rotation of the wind vector.  They also show the benefit of stratifying the data in subsamples characterized by different synoptic patterns dominated by the presence or absence of the SALLJ within the region. The first two samples (Chaco Jet Events (Nicolini and Saulo, 2000, denoted as CJEs), and no-CJEs) integrate the SALLJ sample (low-level jet bounded by the Andes immersed in low-level poleward flow that extends from equatorial latitudes toward subtropical Argentina) and the main difference between them resides in the poleward extension of the jet core. The third sample (no-SALLJ) environmental conditions include a variety of synoptic patterns. An interesting result is that while the jet core remains limited to tropical latitudes during both CJE or no-CJE SALLJs, the no-SALLJ vertical wind profile composite reveals the presence of a low-level jet that generates locally over subtropical northwestern Argentina (clearly depicted at Santiago del Estero). Further stratification of the wind data in order to isolate the synoptic patterns that foster the occurrence of this maximum was addressed by Nicolini et al., 2004b and the new low-level jet category was denoted as Low-Level Jet Argentina cases (LLJA). This LLJ is immersed in a northeasterly flow in the west sector of a postfrontal anticyclone to the east of the Andes. Figure 2 shows a schematic of the low-level flow for each subsample CJE, no-CJE and LLJA, respectively. A motivation for this stratification of the environmental conditions is the increasing evidence that these three excluding patterns have different (and still under study) impacts in the spatial distribution, intensity and time phasing of convection/precipitation and in the moisture convergence area downwind the corresponding wind speed maximum (Nicolini et al. 2004c, Salio et al. 2004, Nicolini and Saulo 2006, Salio and Nicolini, 2006, this Conference). Afterwards, a careful quality control of SALLJEX sounding and surface data has been performed to build up a reliable data set. Progress in this direction allows a further thermodynamic and kinematic structure characterization of the low-level troposphere during SALLJEX. Besides, this characterization during one warm season provides elements to identify the preconditioning and/or triggering processes for severe weather including boundary layer processes related to low-level jets (LLJs). This controlled data set is also currently used for implementing downscaling and data assimilation in operative analysis for impact studies in short range forecasts and, as mentioned before, for research in mechanisms controlling SALLJ and its related precipitation (see Borque et al. and Garcia Skabar and Nicolini, this same Conference). A first study using operational fields from NCEP (Global Data Assimilation System - GDAS) and the aforementioned SALLJEX upper-air controlled data set (both thermodynamic and kinematic variables) was done by Nicolini et al. 2005. This study focused in analyzing nocturnal and daytime vertical profiles of thermodynamic variables at Resistencia and Santiago del Estero (Argentina) and the vertical structure of the wind at 12 UTC over the Bolivia, Paraguay and Argentina domain of the SALLJEX observational network, under the two strong low-level jet events CJE and LLJA. The first characterization of the stratification in Santiago del Estero documented in Nicolini et al. 2005 reveals a deep convectively unstable layer during the afternoon up to 4000 m in the CJEs composite with surface equivalent potential temperatures cooler respect to Resistencia where higher specific humidity values dominate. Mixing layer is deeper during CJEs respect to LLJA in both stations. Despite the progress done in the aforementioned characterization, the limitation of the wind analysis to only 12 UTC does not allow a diurnal cycle description and its variability characterization over the region. Also, it is of interest to extend the thermodynamic analysis to other stations within SALLJEX network besides Resistencia and Santiago del Estero. The purpose of this work is to characterize the spatial variability of the main low-level vertical thermodynamic and poleward wind profile features as well as their diurnal cycle under different environmental conditions over the SALLJEX region.