51 research outputs found

    A meteorological overview of the Pacific Exploratory Mission (PEM) Tropics period

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    NASA's Pacific Exploratory Mission-Tropics (PEM-T) experiment investigated the atmospheric chemistry of a large portion of the tropical and subtropical Pacific Basin during August to October 1996. This paper summarizes meteorological conditions over the PEM-T domain. Mean flow patterns during PEM-T are described. Important circulation systems near the surface include subtropical anticyclones, the South Pacific Convergence Zone (SPCZ), the Intertropical Convergence Zone (ITCZ), and middle latitude transient cyclones. The SPCZ and ITCZ are areas of widespread ascent and deep convection; however, there is relatively little lightning in these oceanic regions. A large area of subsidence is associated with the subtropical anticyclone centered near Easter Island. PEM-T occurred during a period of near normal sea surface temperatures. When compared to an 11 year climatology (1986-1996), relatively minor circulation anomalies are observed during PEM-T. Some of these circulation anomalies are consistent with much stronger anomalies observed during previous La Nina events. In general, however, the 1996 PEM-T period appears to be climatologically representative. Meteorological conditions for specific flights from each major operations area are summarized. The vertical distribution of ozone along selected DC-8 flights is described using the DIAL remote sensing system. These ozone distributions are related to thermodynamic soundings obtained during aircraft maneuvers and to backward trajectories that arrived at locations along the flight tracks. Most locations in the deep tropics are found to have relatively small values of tropospheric ozone. Backward trajectories calculated from global gridded analyses show that much of this air originates from the east and has not passed over land within 10 days. The deep convection associated with the ITCZ and SPCZ also influences the atmospheric chemistry of these regions. Flights over portions of the subtropics and middle latitudes document layers of greatly enhanced tropospheric ozone, sometimes exceeding 80 ppbv. In situ carbon monoxide in these layers often exceeds 90 ppbv. These regions are located near, and especially south of Tahiti, Easter Island, and Fiji. The layers of enhanced ozone usually correspond to layers of dry air, associated with widespread subsiding air. The backward trajectories show that air parcels arriving in these regions originate from the west, passing over Australia and even extending back to southern Africa. These are regions of biomass burning. The in situ chemical measurements support the trajectory-derived origins of these ozone plumes. Thus the enhanced tropospheric ozone over the central Pacific Basin may be due to biomass burning many thousands of kilometers away. Middle-latitude portions of the PEM-T area are influenced by transient cyclones, and the DC-8 traversed tropopause folds during several flights. The flight area just west of Ecuador experiences outflow from South America. Thus the biomass burning that is prevalent over portions of Brazil influences this area. Copyright 1999 by the American Geophysical Union

    Aerosols from biomass burning over the tropical South Atlantic region: Distributions and impacts

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    The NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) expedition was conducted September 21 through October 26, 1992, to investigate factors responsible for creating the seasonal South Atlantic tropospheric ozone maximum. During these flights, fine aerosol (0.1-3.0 μm) number densities were observed to be enhanced roughly tenfold over remote regions of the tropical South Atlantic and greater over adjacent continental areas, relative to northern hemisphere observations and to measurements recorded in the same area during Ac wet season. Chemical and meteorological analyses as well as visual observations indicate that the primary source of these enhancements was biomass burning occurring within grassland regions of north central Brazil and southeastern Africa. These fires exhibited fine aerosol (N) emission ratios relative to CO (dN/dCO) of 22.5 ± 9.7 and 23.6 ± 15.1 cm-3 parts per billion by volume (ppbv)-1 over Brazil and Africa, respectively. Convection coupled with counterclockwise flow around the South Atlantic subtropical anticyclone subsequently distributed these aerosols throughout the remote South Atlantic troposphere. We calculate that dilute smoke from biomass burning produced an average tenfold enhancement in optical depth over the continental regions as well as a 50% increase in this parameter over the middle South Atlantic Ocean; these changes correspond to an estimated net cooling of up to 25 W m-2 and 2.4 W m-2 during clear-sky conditions over savannas and ocean respectively. Over the ocean our analyses suggest that modification of CCN concentrations within the persistent eastern Atlantic marine stratocumulus clouds by entrainment of subsiding haze layers could significantly increase cloud albedo resulting in an additional surface radiative cooling potentially greater in magnitude than that caused by direct extinction of solar radiation by the aerosol particles themselves
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