Tuesday, May 29, 2012

Hilo Upper-Air Soundings - Big Island, Hawaii

Since there's little to talk about locally, I'm going to discuss some of the weather we observed on the Big Island of Hawaii earlier this month. We were on the Big Island from 6 May until 9 May and we stayed in the small town of Volcano. See the map above. Volcano is south-southwest of Hilo and is at an elevation of almost 4,000 ft MSL. Each day the clouds would fill in and more or less steady precipitation would begin just before dark. The rains were quite heavy each night, but ended around sunrise. The exception was May 8th which had light rain continuing through most of the day.Volcano receives about 160 inches of rain each year and is definitely about as different from Tucson as one can imagine. I was curious about the diurnal cycle we observed during our days at, and near, Volcano and took a look at the upper-air soundings from Hilo. What I found was quite interesting.

Above and below are four 1200 UTC soundings from Hilo (obtained from the University of Wyoming uppper-air page). At the longitude of Hilo, 1200 UTC and 0000 UTC occur at 2 am and 2 pm (respectively) local time. Each of these soundings (taken during the time that substantial, nocturnal rains were falling in Volcano) exhibit layers, or levels, that have absolute moist instability. Such layers (MAULs) were identified by Bryan and Fritsch in 2000 (see  http://journals.ametsoc.org/doi/pdf/10.1175/1520-0477%282000%29081%3C1287%3AMAITSS%3E2.3.CO%3B2  ). They were studying deep convection over the continents; however similar, but more shallow, layers appear in the Hilo soundings. The instability in the Hilo soundings reaches only to about 700 mb. However, given persistent, upslope trade winds, these soundings were associated with nearly continuous light to moderate, nighttime rains. The rain was clearly produced by warm processes and the small-drop, high moisture contents of the clouds frequently cause sonde problems when the soundings penetrate into the warm and dry air above the clouds.

Photo below is from our flight departing Hilo at about noon on May 9th; view is looking south toward  Mauna Kea volcano (elevation 13,796 ft MSL). There appear to be at least two different boundary layers at this time - one is filled with shallow cumulus between the plane and the mountain. Over the mountain, cumulus, while still shallow, clearly have higher bases. So, that's the account of our brief encounter with the complex meteorology of the Hawaiian Islands.

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