The 23rd Conference on Hurricanes and Tropical Meteorology

10E.2
VARIATIONS IN SURFACE DOWNWELLING SOLAR RADIATION OVER THE TROPICAL PACIFIC OCEAN

Thomas P. Ackerman, Penn State University, University Park, PA; and J. H. Mather and D. Mazeroski


The dominant input of energy to the tropical ocean is downwelling solar radiation, which is strongly modulated by tropical cloudiness. Daily average (24-hour) radiation values can vary by an order of magnitude from as low as 30 W/m2 to more than 300 W/m2. Cloudiness variability is driven by intra-annual cycles such as the Madden-Julian oscillation and interannual cycles such as ENSO. Much of our knowledge of this variability has been derived from satellite measurements of reflected solar radiation at the top of the atmosphere. In this study, we examine a set of surface measurements of downwelling solar radiation that have been made from several of the TAO buoys in the equatorial Pacific Ocean. Measurements have been made for over five years, unfortunately not continuously, on three buoys located along the equator at 156 E, 165 E and 140 W Longitude. A second set of measurements made on the TAO buoys along 165 E Longitude between 8 N and 8 S Latitude is available for the past year. In addition, a detailed set of surface radiation observations is available from the Department of Energy Atmospheric Radiation Measurement (ARM) site located in Manus Province, Papua New Guinea, at 147 E Longitude. We analyze these measurements to determine the nature of the variability in the records and the spatial correlation along the 165 E line. These records show clear indication of intra-annual oscillations on timescales ranging from a few days to MJO periods, as well as interannual variability associated with ENSO events. Surface cloud forcing may be defined as the difference between the observed solar radiation and the solar radiation from a clear (cloudless) sky. We compute the latter using a radiative transfer model and observed values of water vapor column concentration. From the observations and model results, we then compute the magnitude of surface cloud forcing at these various locations and its variability in time.

The 23rd Conference on Hurricanes and Tropical Meteorology