Radar observations of convection in a large-scale disturbance during NAMMA

During the NASA Monsoon Multidisciplinary Analyses (NAMMA) experiment, the NASA TOGA C-band radar was deployed near Praia, Republic of Cape Verde (~300 km west of Dakar, Senegal) from 16 August to 16 September 2006. This location provided a unique opportunity to observe the transition of continental convection (sampled by the MIT and NPOL radars) into the maritime environment sampled by TOGA. The primary objective of TOGA was to document convection and accompanying mesoscale processes associated with passage of large-scale disturbances (i.e., easterly wave) over the region. The TOGA radar operated 24 hours a day and recorded about 750 hours of data during the experiment. A total of six major precipitation events were sampled, each lasting at least six hours and exhibiting significant mesoscale organization of precipitation echos. TOGA sampled a wide variety of mesoscale features in these events, including: leading line-trailing straiform, leading straiform-trailing convective, quasi linear convective bands, orographically forced convection, and large stratiform regions embedded within synoptic-scale regions of cyclonic circulation. The latter features were associated with easterly wave disturbances identified in satellite imagery. Two of these large-scale events crossed the Cape Verdes Island region and later developed into tropical cyclones (Debbie and Helene).

Quality Control (QC) procedures of the TOGA data are on-going. TRMM precipitation radar (PR) overpass data are being used to assess absolute calibration and uncertainty and several objective algorithms are being investigated to remove spurious echos from the data set (ground and sea clutter, 2nd trip echo and side lobe contamination).

On 3 September, a relatively long-lasting period of convective activity was sampled by TOGA. Satellite imagery showed that the activity was associated with a synoptic-scale disturbance that had moved off the African coast the previous day. The radar sampled a broad region of cyclonic circulation associated with the passage of the disturbance, which was centered within several hundred kilometers (south) of the Cape Verde Islands. Radiosonde data, launched at the nearby Praia airport at 4 hour intervals, documented a signficant increase in lower-mid troposphere relative humidity and CAPE during the passage of the disturbance. Convective echos embedded within the stratiform precipitation region produced significant flash flooding in the nearby city of Praia. The NASA DC-8 coordinated observations with TOGA for part of the day in order to sample the microphysical characteristics of precipitation within the large-scale disturbance. A spiral descent was performed within a broad area of stratiform precipitation near TOGA and dropsondes were launched to sample to environmental winds and thermodynamic structure.

This paper will present an overview of TOGA operations during NAMMA as well as preliminary results from the 3 September event. For this case study, the focus will be on placing the radar, sounding and aircraft insitu microphysical observations within the context of the large scale easterly wave structure identified in the satellite data. Moreover, comparisons for this event will be made between the structure of convection observed by the NPOL radar near the African coast and TOGA.