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NASA Sees Cyclone Evan Blown Apart by Wind Shear
Cyclone Evan is no more than a remnant low pressure area in the South Pacific Ocean now. NOAA's GOES-15 satellite captured an image of the remnants from its fixed orbit in space on Dec. 20 that showed strong wind shear had basically blown the storm apart.
The last official bulletin by the Joint Typhoon Warning Center was issued on Dec. 19 at 2100 UTC (4 p.m. EST/U.S. or 12:56 a.m. Fiji local time on Dec. 20). At that time, Evan's maximum sustained winds were still near 35 knots (40 mph/64.8 kph) and it had transitioned into an extra-tropical storm. It was located 400 nautical miles south of Nadi, Fiji, near 24.3 south latitude and 178.5 east longitude. Evan was moving to the south-southeast at 4 knots (4.6 mph/7.4 kph). Evan has since become a remnant low pressure system.
NOAA's GOES-15 satellite captured an infrared image of Evan's remnants on Dec. 20 at 1500 UTC (10 a.m. EST) showed the remnants of former Tropical Cyclone Evan blown apart by very strong wind shear, northeast of New Zealand. The low pressure center appears northwest of the plume of clouds associated Evan's remnants. The image was created by NASA's GOES Project, located at NASA's Goddard Space Flight Center in Greenbelt, Md.
Northwesterly wind shear continued to be very strong, blowing between 40 and 50 knots (46 and 57.5 mph/74 and 92.6 kph) and cooler ocean waters were weakening Evan quickly.
Evan has now gone into the history books as one of the strongest cyclones to hit Fiji and American Samoa in recent memory.
Text Credit: Rob Gutro
NASA's Goddard Space Flight Center
Update Two: 
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This time series of infrared images from the AIRS instrument aboard NASA's Aqua satellite show the changes in intense thunderstorms (purple) within Cyclone Evan as it weakened from Cyclone to Tropical Storm strength in 36 hours. The left image is from Dec. 18, center from Dec. 19 at 0159 UTC, and right from Dec. 19 at 1259 UTC, showing a smaller area of intense storms, and the cyclone appears more disorganized. Credit: NASA JPL, Ed OlsenTime Series of Infrared NASA Images Show Cyclone Evan's Decline
Cyclone Evan is now far south of Fiji and wind shear and cooler sea surface temperatures have been taking their toll on the storm and weakening it. Infrared data from NASA's Aqua satellite has shown a quick decline in the storm's structure over one day.
A time series of infrared images from the Atmospheric Infrared Sounder (AIRS) instrument that flies aboard NASA's Aqua satellite showed changes in intense thunderstorms within Cyclone Evan between Dec. 18 and Dec. 19. Over a time period of 36 hours, Evan weakened from Cyclone strength to Tropical Storm strength. In an AIRS image captured on Dec. 18 there were two large areas of strong thunderstorms with very cold cloud top temperatures colder than -63 Fahrenheit (-52 Celsius).
By Dec. 19 at 0159 UTC (Dec. 18 at 8:59 p.m. EST/U.S.) the area of strong thunderstorms had become smaller, and the storm appeared less organized. In the AIRS infrared image from Dec. 19 at 1259 UTC (7:59 a.m. EST), the area of strongest thunderstorms had been reduced further and cloud top temperatures throughout the storm were warming, indicating cloud heights were falling because of less evaporation. Evan had moved over sea surface temperatures below the 80 degree Fahrenheit (26.6 degree Celsius) threshold, so evaporation and thunderstorm development had waned.
Wind shear had increased as well, pushing the bulk of the thunderstorm activity about 65 nautical miles (74.8 miles/120.4 km) to the southeast, according to the Joint Typhoon Warning Center. Northwesterly wind shear was very strong, blowing between 40 and 50 knots (46 and 57.5 mph/74 and 92.6 kph). Animated multi-spectral satellite imagery also showed the low-level circulation center remains fully exposed.
On Dec. 19 at 0900 UTC (4 a.m. EST/U.S.), Evan was a tropical storm with maximum sustained winds near 45 knots (51.7 mph/83.3 kph). It was located about 335 nautical miles (385.5 miles/620.4 km) south of Nadi, Fiji, near 23.7 south latitude and 178.3 east longitude. Evan was moving to the south-southeast at 7 knots (8 mph/13 kph).
Forecasters at the Joint Typhoon Warning Center noted that because of the strong wind shear and cooler sea surface temperatures, Evan may dissipate sometime on Dec. 20.
Text Credit: Rob Gutro
NASA's Goddard Space Flight Center
Dec. 19, 2012
Update One: 
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TRMM captured data on Cyclone Evan's cloud heights and rainfall on Dec. 16 and spotted an extremely tall storm-cell in the north side of the eyewall. The updrafts in this tower extended high enough to lift precipitation-size ice 17 km above the ocean surface (red in the image). The insert shows the infrared cloud top temperatures. The clockwise arrow shows the direction of the winds circling the eyewall. At the north side of the eyewall, the explosive circular shape is the upper-level outflow from the extremely tall tower, Credit: NASA/Owen KelleyNASA Satellite Finds an Unusually Tall Storm-cell in Cyclone Evan
NASA's Tropical Rainfall Measuring Mission or TRMM satellite found an unusually tall towering thunderstorm in Cyclone Evan.
According to Owen Kelley of the TRMM satellite team at NASA's Goddard Space Flight Center in Greenbelt, Md, the most startling feature of the December 16 overflight of Tropical Cyclone Evan was the extremely tall storm-cell in the north side of the eyewall. At the time TRMM passed overhead and captured an image of the storm, Evan was about to rake across the northern coast of the islands of Fiji.
The updrafts in this tower extended high enough to lift precipitation-size ice 17 km (10.5 miles) above the ocean surface. Tall precipitation cells are generally taken to be anything at least 14.5 km (9 miles) high and are nicknamed "hot towers," but what was seen in Evan's eyewall was a different category of storm cell.
Storm-cells as tall as the one in the eyewall of Evan have been long known to occur occasionally over land, but before the TRMM satellite, there were not thought to occur over ocean far from land. While field campaigns have periodically studied one location or other over the ocean, what TRMM has taught us is that such sporadic observations are insufficient if you want catch rare events. After 15 years of continuous operation, TRMM satellite reveals the rare features and challenges our understanding of how the weather works. The ocean is an unlikely place to find extremely tall oceanic cells because the ocean surface stays roughly constant in temperature, unlike the land which quickly heats up over the course of a day, increasing low-level instability, and encouraging tall cells to form.
