25th Conference on Hurricanes and Tropical Meteorology

Monday, 29 April 2002: 2:15 PM
The effects of buoyancy, shear and humidity on convection in the eastern tropical Pacific
Graciela B. Raga, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico; and D. J. Raymond and D. Baumgardner
Poster PDF (28.3 kB)
This paper reports the results of a study of the factors governing the existence of deep convection over the east Pacific warm pool near 95 W, 10 N. The study is a part of the EPIC2001 project (east Pacific investigation of climate). Two aircraft, the NCAR C-130 and a NOAA P-3, and two ships, NOAA's Ron Brown and NSF's New Horizon were deployed for this project. The present results come primarily from in situ and dropsonde observations made by the C-130.

A companion study to be presented at this conference by López, Fuchs, and Raymond, using results from the P-3, documents the effects of convective inhibition on the existence of convection. Here we look only at situations in which the convective inhibition is small enough that parcels experience no difficulty in reaching the level of free convection.

Assuming sufficiently small convective inhibition, mid-tropospheric parcel buoyancy, shear, and humidity remain as possible governing factors for convection. Our results are as follows:

1. As long as the buoyancy is positive, its magnitude appears to exert little further control over the existence of deep convection. Some of the most intense rainfall days in the experimental area (as measured by satellite microwave radiometers) exhibited the smallest parcel buoyancy.

2. The magnitudes of shear which typically occur in the experimental area don't appear to inhibit deep convection to any signficant degree. However, convection tends to organize in shear-parallel lines when the shear is strong.

3. The humidity of the environment exerts a strong control over the existence of deep convection. A thin layer of dry air in the mid-troposphere is capable of inhibiting the upward passage of convective parcels, even when the parcel buoyancy is relatively large and the environment is otherwise moist. General dryness, which often occurs when the wind blows off the land, inhibits even shallow convection.

These results were obtained by observing the detailed response of convection to its immediate environment. Dry air was the single strongest factor controlling convection in the experimental area during the project.

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