Hawai'i Physical Geography: Precipitation Patterns and Treelines: GPH 112: Intro to Phys Geography L

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Apr 3, 2024

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Hawai'i Physical Geography: Precipitation Patterns and Treelines Due Mar 31 at 11:59pm Points 8 Questions 2 Time Limit None Instructions BACKGROUND INFORMATION Over the ocean near Hawaii, rainfall averages between 25 and 30 inches a year. The islands receive as much as 15 times that amount in some places and less than one third of it in others. This is caused mainly by orographic or mountain rains, which form within the moist trade wind air as it moves from the sea over the steep and high terrain of the islands. Over the lower islands, the average rainfall distribution resembles closely the topographic contours. Amounts are greatest over upper slopes and crests and least in the leeward lowlands. On the higher mountains, the belt of maximum rainfall lies between 2,000 to 3,000 feet and amounts decrease rapidly with further elevation. As a result, the highest slopes are relatively dry. Another source of rainfall is the towering cumulus clouds that build up over the mountains and interiors on sunny calm afternoons. Although such convective showers may be intense, they are usually brief and localized. PRECIPIATION INFLUENCE 1 AND 2: TRADE WINDS AND RAIN SHADOWS Hawaii's mountains significantly influence every aspect of its weather and climate. The endless variety of peaks, valleys, ridges, and broad slopes, gives Hawaii a climate that is different from the surrounding ocean, as well as a climatic variety within the islands. These climatic differences would
not exist if the islands were flat and the same size. The mountains obstruct, deflect, and accelerate the flow of air. When warm, moist air rises over windward coasts and slopes, clouds and rainfall are much greater than over the open sea. Leeward areas, where the air descends, tend to be sunny and dry. In places sheltered by terrain, local air movements are significantly different from winds in exposed localities. Since temperature decreases with elevation by about 3 degrees per thousand feet, Hawaii's mountains, which extend from sea level to nearly 14,000 feet, contain a climatic range from the tropic to the sub-Arctic. The left image below is famous for its portrayal of the dramatic differences in rainfall on the eastern (right) and western (left) sides of Kohala volcano on the Big Island. Moist trade winds encounter Kohala’s north-east facing side and are forced to rise. Rising air expands and cools due to adiabatic processes. The cooling results in condensation, cloud formation, and lots of rain. However, when this air starts descending on the southwestern side, it warms. The opposite happens. Warming leads to cloud evaporation and much less rainfall. The effect is clearly dramatic in this image taken from the Space Shuttle. The image to the right is a diagram cross section looking north, where the eastern side is on the right. The trade winds are forced up and over a topographical barrier. The windward side will be cloudy and wet as air ascends, cools, and reaches the dew point (cloud formation occurs) The lee side will be warmer and drier as the air descends – and this is called the rainshadow.
PRECIPITATION INFLUENCE 3: SEA BREEZES The figure to the right shows you a mean monthly precipitation at two weather stations on the windward side of Mauna Loa and on the leeward side. Please focus on the vertical scale. The amount of precipitation is a lot lower on the leeward side due to the rainshadow effect. However, in this diagram, the weather station on the leeward side is actually in one of the wetter locations on the western side of the Big island. The conditions that promote this area of more rainfall occur in summer. The summer precipitation maximum on the Kona Coast on the western side of Mauna Loa and Hualalai volcanoes (North and South Kona Districts on the Island of Hawai ʻ i) have a unique rainfall pattern. The west-facing slopes of Hualalai and Mauna Loa are sheltered from the trade winds. But, as air flows around the large mountains, it curves back on the leeward side and flows up these slopes, producing a belt of persistent clouds and rain. This area is home to the farms that produce world-famous Kona coffee. Uplift is enhanced in the afternoons when the sun warms these slopes. Strong trade winds and intense heating during the summer also increase lifting, clouds, and rainfall on the Kona slopes. As a result, this is the only area in Hawai ʻ i with an afternoon rainfall peak, and with more rain in the summer than other seasons (see mean monthly rainfall at Kona station Honaunau, below and the map of Big Island winds below).
PRECIPITATION INFLUENCE 4 A fourth pattern to the precipitation involves the trade wind inversion. The image to the right shows the latitudes between the equator and just north of Hawaii at the subtropical high (on the right). Hawaii is between. All of the basic presentations about the Earth’s general circulation systems show this circulation cell (trade winds converge on the equator as the red lines and then return as the dark blue line to the subtropical high) called the Hadley Cell. However, reality is more complicated. The air starts to descend in the latitudes of Hawaii, but it just down not reach the surface. It typically reaches an elevation that ranges from 1800 to 2400 meters (6000 to 8000 feet). Then, this descending air creates a TRADE WIND INVERSION. What is the significance of the trade wind inversion? An inversion is where temperature begins to increase with elevation. The normal condition is the reverse, and that’s why its called an “inversion”. Increases in temperature with height is not at all conducive to rainfall. The moist-warm trade winds reach this inversion, and the clouds evaporate as the air warms up (as the air is pushed up slope). Thus, forests stop suddenly, and the vegetation comes scrub and then quickly desert-like, because of the great reduction in rainfall.
The upper right image shows the Trade Wind Inversion's influence on the temperature with height, in this diagram over volcanoes like Mauna Loa (& Haleakala(=). What this means is that the orographic effect of cloud formation and the associated rainfall is often STOPPED at the Trade Wind Inversion, capping any clouds or precipitation that would occur. This is the reason why the upper treeline can be so straight on the Big Island - like the lower right Landsat Image of Mauna Loa. The Trade Wind Inversion caps rainfall and the rainforest. So in total, there are four explanations of the pattern you see in precipitation across the Big Island of Hawai'i. They are: 1. Orographic effects of the northeasterly trade winds 2. Rainshadow effects on the lee sides of the volcanoes 3. Land-sea interaction (afternoon sea breeze) and complex wind patterns (curving of winds around volcanoes) above Kona on the west side of the island. 4. Trade wind inversion capping the orographic effect at a certain height EXAMPLE QUESTION 1: You are given four geographic locations on the Big Island that display a different aspect of precipitation variations. Match the cause of the precipitation pattern that you see in the geovisualization to the location. 20.1543 -155.7373 (precipitation layer) 19.8309 -155.4117 (dew point data layer)
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