Quantifying Cloud Radiative Effects in the Tropical Western Pacific

Abstract:

Cloud radiative effects are examined using long-term datasets collected at the three DOE Atmospheric Radiation Measurement (ARM) sites in the tropical western Pacific. We developed a climatology of downwelling shortwave (SW) and longwave (LW) radiative fluxes, cloud populations, and cloud radiative effects by partitioning the data by cloud type, time of day, and as a function of large scale modes of variability such as ENSO phase and wet/dry seasons. The Nauru cloud populations are impacted by ENSO variability whereas the cloud populations over Manus only shift slightly in response to changes in ENSO phase. Most of the variability in SW and LW cloud radiative effects among the three sites and across ENSO/monsoon phases at a given site is driven by changes in cloud frequency rather than variations in cloud properties. Similarly, the contribution of each cloud type to the total SW cloud radiative effect is strongly conditioned on frequency. Low clouds have the largest impact on total SW cloud radiative effect. We performed a sensitivity analysis that showed that missing the absolute frequency of deep convective clouds by +10% can change the total SW cloud radiative effect by -20 W m^-2. Timing errors of the diurnal cycle of cloud frequency, such as having a particular cloud type peak too early or too late in model simulations, have a smaller impact on the total SW cloud radiative effect compared to biases in absolute frequency.