A Climatology of Surface Cloud Radiative Effects at the ARM Tropical Western Pacific Sites

Abstract

loud radiative effects on surface downwelling fluxes are investigated using datasets from the Atmospheric Radiation Measurement Program (ARM) sites in the tropical western Pacific Ocean (TWP) region. The Nauru Island (Republic of Nauru) and Darwin, Australia, sites show large variability in sky cover, downwelling radiative fluxes, and surface cloud radiative effect (CRE) that is due to El Niño?Southern Oscillation (ENSO) and the Australian monsoon, respectively, whereas the Manus Island (Papua New Guinea) site shows little intraseasonal or interannual variability. At Nauru, the average shortwave (SW) surface CRE varies from ?38.2 W m?2 during La Niña conditions to ?90.6 W m?2 during El Niño conditions. The average longwave (LW) CRE ranges from 9.5 to 15.8 W m?2 during La Niña and El Niño conditions, respectively. At Manus, the average SW and LW CREs vary by less than 5 and 2 W m?2, respectively, between the ENSO phases. The variability at Darwin is even larger than at Nauru, with average SW (LW) CRE ranging from ?27.0 (8.6) W m?2 in the dry season to ?95.8 (17.0) W m?2 in the wet season. Cloud radar measurements of cloud-base and cloud-top heights are used to define cloud types to examine the effect of cloud type on the surface CRE. Clouds with low bases contribute 71%?75% of the surface SW CRE and 66%?74% of the surface LW CRE at the three TWP sites, clouds with midlevel bases contribute 8%?9% of the SW CRE and 12%?14% of the LW CRE, and clouds with high bases contribute 16%?19% of the SW CRE and 15%?21% of the LW CRE.