25th Conference on Hurricanes and Tropical Meteorology


Convection and easterly waves observed in the eastern Pacific ITCZ during EPIC2001

Walter A. Petersen, Colorado State University, Fort Collins, CO; and R. Cifelli, D. J. Boccippio, and S. A. Rutledge

During the last three weeks of September 2001, the East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System (EPIC2001) intensive field campaign focused on studies of deep convection in the ITCZ-cold tongue complex over the Mexican warm-pool region (10 N 95 W) of the eastern Pacific Ocean. Major observational platforms deployed during this phase of EPIC2001 included two ships, the NOAA R/V Ronald H. Brown and the NSF R/V Horizon, and two research aircraft including a NOAA P-3 and the NCAR C-130. This study utilizes new C-band Doppler radar and sounding observations collected aboard the R/V Ronald Brown to describe the 4-D structure of ITCZ convection as a function of the environmental forcing and phase of 3-5 day easterly wave passages.

Three distinct easterly wave passages occurred during EPIC2001. Each wave originated in the eastern Atlantic Ocean and after moving over Central America and into the eastern Pacific, were easily identified in time-height profiles of wind and thermodynamic data collected at the position of the R/V Brown. In all cases, the wave trough axes (as defined by changes in the meridional and zonal wind direction and changes in pressure altitude) exhibited relatively weak shear at low to mid-levels and tilted westward with height. The humidity profile in each wave did not exhibit as great a tilt in the vertical as the trough axes. Consistent with previous studies of westward tilting waves over the western Pacific Ocean, peaks in radar-diagnosed rainfall tended to lead the passage of the surface wave trough by 0-2 days.

The vertical structure and coverage of convection were strongly modulated by the passage of each wave. Time-height series of 30 dBZ (a proxy for convective radar echo) and 15 dBZ echo top height frequencies exhibited broader distributions 0-2 days prior to wave trough passages. Similarly, a time series of 40 dBZ echo coverage also peaked prior to wave trough passage. These trends point to the presence of deeper and more intense convection just ahead of the wave trough. Upon passage of the trough axes, 10 dBZ echo coverage peaked as did the vertical extent of relative humidities exceeding 80%. However, 40 dBZ echo coverage decreased and the 30 dBZ height distributions narrowed considerably. These trends suggest that while cloud coverage was broad and convection was still deep in the wave trough, it was less vertically developed and more stratiform in nature. Concomitant time series of boundary layer theta-e and cloud-to-ground lightning flash counts also exhibited peaks 0-2 days prior to wave trough passage, consistent with the trends in radar-diagnosed convective vertical structure and rainfall.

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Session 13B, Climate Processes in the Americas and Eastern Pacific I (Parallel with Sessions 13A, 13C, and 13D)
Thursday, 2 May 2002, 11:00 AM-12:30 PM

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