Tuesday, June 18, 2013

NASA's 2013 HS3 hurricane mission to delve into Saharan dust

NASA: NASA's 2013 Hurricane and Severe Storms Sentinel or HS3 mission will investigate whether Saharan dust and its associated warm and dry air, known as the Saharan Air Layer or SAL, favors or suppresses the development of tropical cyclones in the Atlantic Ocean. The effects of Saharan dust on tropical cyclones is a controversial area of science. During the 2012 campaign, NASA's Global Hawk unmanned aircraft gathered valuable data on the dust layer that swirled around Tropical Storm Nadine for several days.

The Saharan dust layer is composed of sand and other mineral particles that are swept up in air currents and whisked westward over the Atlantic Ocean. The extreme daytime heating of the Sahara creates instability in the lowest layer of the atmosphere, warming and drying the air near the surface and cooling and moistening the air near the top of the dust layer near 5 kilometers (16,500 feet). Once it exits the African coast, the dust-laden air moves over air that is cooler, and moister, and it's the temperature inversion of warm air over cold that prevents deep cloud development. This suppression of deep cloud formation along with the dry air within the dust layer is reasons why this Saharan air layer is sometimes thought to suppress tropical cyclone development. On the other hand, the southern boundary of this hot desert air essentially acts like a front whose attendant wind patterns are a major source of the African waves that are precursors to storm formation.

HS3 addresses the controversial role of the Saharan Air Layer, or SAL, in tropical storm formation and intensification by taking measurements from three instruments on board the Global Hawk. These instruments include a cloud physics lidar which uses a laser to measure vertical profiles of dust; a dropsonde system that releases small instrumented packages from the aircraft that fall to the surface while measuring profiles of temperature, humidity, and winds; and an infrared sounder that measures temperature and humidity in clear-sky regions.

...The dust data collected by the Global Hawk is important for scientific studies on the SAL. Other data was useful operationally to the National Hurricane Center (NHC), the entity that issues forecasts for tropical cyclones. The forecasters at the NHC used data from dropsondes released from the Global Hawk in the discussion of Nadine at 11 a.m. EDT on Sept. 20, "The current intensity is kept at 45 knots (51.7 mph/83.3 kmh)…is in good agreement with dropsonde data from the NASA global hawk aircraft and AMSU [satellite instrument] estimates."...

The Global Hawk's flight pattern, from NASA

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