Clouds and Shortwave Fluxes at Nauru. Part II: Shortwave Flux Closure
The datasets currently being collected at the Atmospheric Radiation Measurement (ARM) Program's sites on the islands of Nauru and Manus represent the longest time series of ground based cloud measurements available in the tropical western Pacific region. In this series of papers, we present a shortwave flux closure study using observations collected at the Nauru site between June 1999 and May 2000. The previous paper presented frequency of occurrence of non-precipitating liquid and ice clouds detected by the millimeter wavelength cloud radar (MMCR) at Nauru and statistics of the retrieved microphysical properties. This paper presents estimates of the cloud radiative effect over the study period and results from a closure study in which the retrieved cloud properties are input to a radiative transfer model and the modeled surface fluxes are compared to observations. The average shortwave cloud radiative forcing at the surface is 48.2 W/m{sup 2}, which is significantly smaller than the cloud radiative forcing estimates found during the TOGA-COARE field project. The difference in the cloud radiative forcing estimates during the two periods are due to the variability in cloud amount over Nauru during the convective and suppressed phases of the El Nino-Southern Oscillation (ENSO). In the closure study, the modeled and observed surface fluxes show large differences at short time scales, due to the temporal and spatial variability of the clouds observed at Nauru. Averaging over 60 minutes reduces the average root-mean-square error in total flux to 10% of the observed flux. The modeled total downwelling fluxes are unbiased with respect to the observed fluxes while the direct fluxes are underestimated and the diffuse fluxes are overestimated. Examination of the errors over the dataset indicates that the cloud amount derived from the ground based measurements is an overestimate of the radiatively important cloud amount due to the anisotropy of the cloud field at Nauru, the interpolation of the radar data, the uncertainty in the microwave brightness temperature measurements for thin clouds, and the uncertainty in relating the 6th moment of the droplet size distribution observed by the radar to the more radiatively important moments of the distribution.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15020844
- Report Number(s):
- PNNL-SA-40148; KP1201030; TRN: US200521%%291
- Journal Information:
- Journal of the Atmospheric Sciences, Vol. 61, Issue 21
- Country of Publication:
- United States
- Language:
- English
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