5B.3 SPECIAL OBSERVATIONS OF THE LOW LEVEL FLOW OVER EASTERN BOLIVIA DURING THE 1999 ATMOSPHERIC MESOSCALE CAMPAIGN

Michael W. Douglas, National Severe Storms Laboratory/NOAA and Malaquías Peña, CIMMS/Univ. of Oklahoma
Norman, Oklahoma, USA

William Ramiro Villarpando C., Universidad Mayor de San Simon, Cochabamba, Bolivia

1. INTRODUCTION

Global and regional analyses show the region over eastern Bolivia and Paraguay to be the locus of strong mean winds in the lower troposphere. In addition, limited research studies have also suggested that the region is the locus of strong winds associated with synoptic disturbances. The uncertainty over the spatial extent and intensity of the region of strong winds as depicted by both global and regional models, served as motivation to establish limited pilot balloon observations at Santa Cruz, Bolivia, during the period January-March 1998. These observations, which revealed a low-level jet (LLJ) with strong synoptic variability, have been reported elsewhere (Douglas et al, 1999); the 1998 observations served to motivate additional observations during 1999 that are described below.


Fig. 1. The pilot balloon observing network during Jan-April 1999. The station in Paraguay and in far northern Bolivia (Cobija) were not operational in 1999

2. THE 1999 OBSERVING PROGRAM

During late 1998 funding became available to establish atmospheric sounding sites in eastern Bolivia as part of the Brazil-based Tropical Rainfall Measuring Mission (TRMM) - Large-Scale Atmosphere-Biosphere (LBA)and Atmospheric Mesoscale Campaign (AMC) (see the web site at http://olympic.atmos. colostate.edu/lba_trmm for details). The network that finally evolved consisted of one radiosonde station in Santa Cruz and 4 pilot balloon sites elsewhere in Bolivia (Fig. 1). These sites made observations from late January until mid-April, with some interruptions due to unavailability of gas, radiosondes, or other technical problems.

3. PRELIMINARY RESULTS

The few results discussed below should serve to highlight some of the characteristics of the data set, and should not be taken as final results of research underway, as most observations have not been thoroughly checked for errors. Fuller exploitation of the data set by interested investigators is very much encouraged, and the pilot balloon data and radiosonde data can be accessed from the PACS-SONET homepage at http://www.nssl.noaa. gov/projects/pacs. Additional acknowledgments can also be found there.

3.1 Comparison with 1998 observations

The mean winds at Santa Cruz, Bolivia during the period January-April 1999 were substantially weaker than observations made the previous year. The difference at 850 mb was approximately 3-4 m/s; with the 1999 winds being about 50% weaker than those in 1998. Thus there is substantial interannual variability of the low-level flow in this region.

3.2 Mean diurnal cycle over the Bolivian lowlands

One objective of the 1999 observational program was the determination of the diurnal variation of the tropospheric winds over the region. Figure 2 shows both the AM and PM mean meridional and zonal wind profiles at a flat site, Trinidad (~15°S, 65°W).These figures indicate that the afternoon winds show an anomaly (with respect to the daily mean) from the southwest (v and u both greater than mean) and the morning winds are from the northeast (u and v less than mean) in the 2-6 km layer. In the surface to 2 km layer however, the pattern is less distinct. The surface conditions show very little difference in mean values. The results for Robore (not shown) are similar to those for Trinidad.

3.3 Synoptic variability and the April 15-18 cold surge

Although the AMC ended in late February 1999, the Bolivian observations, because of their late start, continued for several additional months. A strong cold frontal passage was documented by the network in mid-April. Strong northwesterly prefrontal flow is depicted by a GPS radiosonde ascent made at 12UTC April 15 (Fig. 3). NW winds of almost 30 m/s at 850 mb are evident, with strong shear between 800 and 600mb, and weak winds between 600-550mb. Above 550mb the winds increased again, but from a more westerly direction.


Fig. 2. Mean meridional and zonal winds during morning (~11 UTC) and afternoon (~21 UTC) at Trinidad, Bolivia.


Fig. 3. Strong northwesterly flow at Santa Cruz prior to cold frontal passage

The change in the winds and temperature associated with the passage of the front can be seen in Figs 4-5, where the wind and temperature data (every 50mb) for the afternoon (~22Z) soundings on April 15 and 16 are shown. The pre-frontal flow is typically deeper than post-frontal flow, with maximum winds of 21 m/s near 2-2.5 km ASL. The post-frontal flow 24 hr later exhibits a sharper wind speed maximum near 1.4 km ASL, and decays to near calm by 2.5km. The temperature changes (Fig 5) show the maximum drop in the first 2 km, with maximum drop in the first 2 km, with maximum changes of 12 C below 1.5 km ASL. A well defined post-frontal inversion near 2 km is present, even in the 50 mb resolution data shown here.


Fig. 4. Profiles of windspeed and direction before and after cold frontal passage at Santa Cruz.


Fig. 5. temperature profiles during afternoon of April 15 and 16, 1999 showing cold frontal passage.

Although not shown here, pilot balloon wind observations from Trinidad showed southeasterly winds of ~25 m/s at 1 km ASL on the morning of the 17th and 18th, and showed essentially all features evident in the Viru-Viru radiosonde wind profiles.
Acknowledgments: The observational program would not have been possible without financial support from NASA's Office of Hydrology (Eric Wood). Pedro Silva Dias of the University of Sao Paulo provided the initial impetus for the effort and helped arrange the financial support. Many other individuals assisted in the field activities; see the PACS-SONET homepage for details.
REFERENCE
1999: Douglas, M.W., M. Nicolini, and A. Celeste Saulo: Observational evidences of a Low-level jet east of the Andes during January-March 1998. Meteorologica, 23, pp 63-72.