Spatial Variability of Surface Irradiance and Clouds at the Manus Site

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Description

The location of the Manus site was chosen because it is very close to the coast, in a flat, near-sea level area of the island, hopefully minimizing the impact of local island effects on the meteorology of the measurements. In this study, we confirm that the Manus site is indeed less impacted by the island meteorology than slightly inland by comparing over a year of broadband surface irradiance and ceilometer measurements and derived quantities at the standard Manus site and a second location 7 km away as part of the AMIE-Manus campaign. The two sites show statistically similar distributions of irradiance and other derived quantities for all wind directions except easterly winds, when the inland site is downwind from the standard Manus site. Under easterly wind conditions, which occur 17% of the time, there is a higher occurrence of cloudiness at the downwind site likely do to land heating and orographic effects. This increased cloudiness is caused by shallow, broken clouds often with bases around 700 m in altitude. While the standard Manus site consistently measures a frequency of occurrence of low clouds (cloud base height less than 1200 m) about 25% of the time regardless of wind direction, the AMIE site has higher frequencies of low clouds (38%) when winds are from the east. This increase in low, locally produced clouds causes an additional -20 W/m2 shortwave surface cloud radiative effect at the AMIE site than the Manus site.