Circulations and Rainfall on the Lee Side of the Island of Hawaii during HaRP

Yang Yang Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii

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Yi-Leng Chen Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii

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Abstract

Using aircraft data and portable automated mesonet (PAM) data during the Hawaiian Rainband Project (HaRP; 11 July–24 August 1990), the circulation on the lee side of the island of Hawaii is analyzed. The largest temperature deviations (2–3 K) and negative mixing ratio deviations (−2 to −4 g kg−1) are found in the northwestern and southwestern leeside areas of the island because of the descending trade winds aloft in the lee of low (<2 km) mountain ridges and tops. Rainfall in these areas is much less compared to the central leeside coast (e.g., Kona coast). In these areas, the sea-breeze duration is short compared to the Kona coast because of the presence of opposing trade winds; the wind speed is greater because of larger land–sea thermal contrast. During the stronger trade wind days, adiabatic descent in the lee is more significant and the LCL is higher. As a result, in the northern and southern leeside areas, localized trade wind rain showers driven by local circulations occur mainly under weak trades.

In the Kona area, the mean surface winds are calm behind the massive mountains, with a daytime upslope (onshore) and nighttime downslope (offshore) flow. Along the axis of the return flow in the large wake off the Kona coast, the water vapor content is higher than the ocean upstream, possibly caused by the low-level convergence between the counterrotating eddies along the wake axis. Along the Kona coast, the rainfall in the afternoon hours continues in the early evening and reaches the daily maximum after the onset of the land breezes, possibly because of the lifting of stronger moist return flow by the land-breeze front. Under stronger trade winds, the return flow would be stronger because of stronger island blocking. More rainfall occurs there when trades are stronger.

Corresponding author address: Dr. Yi-Leng Chen, Dept. of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, HI 96822. Email: Yileng@hawaii.edu

Abstract

Using aircraft data and portable automated mesonet (PAM) data during the Hawaiian Rainband Project (HaRP; 11 July–24 August 1990), the circulation on the lee side of the island of Hawaii is analyzed. The largest temperature deviations (2–3 K) and negative mixing ratio deviations (−2 to −4 g kg−1) are found in the northwestern and southwestern leeside areas of the island because of the descending trade winds aloft in the lee of low (<2 km) mountain ridges and tops. Rainfall in these areas is much less compared to the central leeside coast (e.g., Kona coast). In these areas, the sea-breeze duration is short compared to the Kona coast because of the presence of opposing trade winds; the wind speed is greater because of larger land–sea thermal contrast. During the stronger trade wind days, adiabatic descent in the lee is more significant and the LCL is higher. As a result, in the northern and southern leeside areas, localized trade wind rain showers driven by local circulations occur mainly under weak trades.

In the Kona area, the mean surface winds are calm behind the massive mountains, with a daytime upslope (onshore) and nighttime downslope (offshore) flow. Along the axis of the return flow in the large wake off the Kona coast, the water vapor content is higher than the ocean upstream, possibly caused by the low-level convergence between the counterrotating eddies along the wake axis. Along the Kona coast, the rainfall in the afternoon hours continues in the early evening and reaches the daily maximum after the onset of the land breezes, possibly because of the lifting of stronger moist return flow by the land-breeze front. Under stronger trade winds, the return flow would be stronger because of stronger island blocking. More rainfall occurs there when trades are stronger.

Corresponding author address: Dr. Yi-Leng Chen, Dept. of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, HI 96822. Email: Yileng@hawaii.edu

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