Quantification of the Impact of Nauru Island on ARM Measurements

Abstract

auru Island at times generates low clouds that impact low-level cloud statistics and downwelling shortwave radiation measurements made at the Atmospheric Radiation Measurement Program (ARM) site. This study uses five years of Nauru data to quantify the island impact on the site measurements. The results indicate that the solar-heating-produced Nauru island effect occurs about 11% of the time during daylight hours. The island effect increases the 500?1000-m cloud base occurrence by 15%?20% when clouds occur, but because the island effect only occurs 11% of the time the overall increase in daylight low-cloud statistics is 2%, or 1% for 24-h statistics. In a similar way, the island effect produces a reduction of about 17% in the downwelling shortwave (SW) radiation across the daylight hours during the 11% of the time it occurs, an overall 2% daylight (or 1% for 24 h) average reduction. The island effect produces frequent positive downwelling SW cloud effects, in particular during the morning, which tend to somewhat mitigate the overall decrease in downwelling SW radiation that is due to clouds. This produces 17 W m?2 less daylight average SW cloud effect relative to non-island-effect times, in particular for the convectively suppressed regime that typifies island-effect-producing conditions. For long-term overall statistical studies such as model and satellite comparisons, the 2% daylight (or 1% per 24 h) average increase in low-level cloud occurrence and decrease in downwelling SW are not of large concern as long as researchers are aware of them. For shorter-term studies, however, or those that separate data by conditions such as convectively active/suppressed regimes, the Nauru island effect can have significant impacts.