Observations of the marine boundary layer under a cutoff low over the southeast Pacific Ocean
Stratocumulus is often present offshore of Peru and northern Chile and exists at the top of a cool, moist and well-mixed marine boundary layer (MBL) under a marked temperature inversion maintained by large-scale subsidence. The subtropical MBL and stratocumulus has been the focus of many recent studies, but mid-latitude systems can exert a strong influence. However, this connection is not well established due to debatable model results and few in situ measurements south of 20°S. During a 2-week field campaign in August 2011 at Robinson Crusoe Island (~700 km offshore at 33.6°S), radiosondes were launched to observe the response of the MBL to mid-latitude synoptic forcing. During the observation period a broad, slow-moving cutoff low (COL) passed over the region. Other observations include COSMIC GPS, infrared satellite imagery, TRMM radar reflectivity, and operational radiosondes from the Chilean weather service. A numerical simulation is included to diagnose the synoptic features. The inversion prior to the COL was maintained and lifted above 5 km as the COL passed over the island. Soon after the COL center passed the island, the MBL top did not descend or reform near the surface and then deepen, but rather an inversion reformed at ~2.7 km. Using a variety of datasets, the height of the reformation of the inversion is related to the cloud top height of the scattered shallow cumulus convection under the COL, which coincides with the level of maximum convergence of the vertical velocity.
KeywordsGlobal Position System Brightness Temperature Temperature Inversion Vertical Wind Shear Marine Boundary Layer
This project was supported by the Chilean Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT) 3110100 and New Faculty Startup funds at KU. Thanks to José Rutllant, Carlos Parra of the Dirección Meteorológica de Chile, and Rosa Zamora for their help during the field campaign on RCI. Roberto Rondanelli helped with processing the satellite data. I also thank two anonymous reviewers for their comments.
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