TRMM captured an image of the wave at 04:07 UTC (12:07 am EDT) September 1, 2009 before the system became a named tropical storm. The image showed the horizontal pattern of rain intensity within the storm. Rain rates in the center swath are based on the TRMM Precipitation Radar (PR), and those in the outer swath on the TRMM Microwave Imager (TMI).The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). TRMM revealed that there areas of intense rain associated with strong thunderstorm activity at the center of the system. These strong showers and thunderstorms are releasing large amounts of heat, which bodes well for future intensification. However, due to the lack of banding or curvature in the rain features, TRMM also showed that the system is still immature and does not have a well-defined cyclonic circulation. Later in the day, the system was upgraded to a minimal tropical storm with sustained winds reported at 45 knots (~50 mph) by the National Hurricane Center (NHC). Despite the thunderstorm activity and warm waters, southwesterly wind shear kept Erika from intensifying and the system was still a minimal tropical storm at the time of the next TRMM overpass on the night of September 2. TRMM took that second image of Erika at 03:51 UTC on September 3 (11:51 p.m. EDT September 2) 2009. Once again TRMM showed that Erika did not have a well-defined cyclonic circulation, and had no eye or eyewall and very little evidence of curvature in the rain bands. At that time, Erika’s sustained winds were reported at just 35 knots (~40 mph) by NHC. Erika is expected to continue to track toward Puerto Rico and the Dominican Republic and weaken due to the continuing wind shear. |
Despite being downgraded to a remnant low, she’s still packing quite a lot of rainfall, and that’s what Puerto Rico and the Virgin Islands are getting today, September 4. The National Hurricane Center’s final advisory on Erika last night at 11 p.m. EDT noted that her ill-defined center was located 115 miles south-southeast of San Juan, Puerto Rico. At 8 a.m. today, the remnants of Tropical Depression Erika are located just south of Puerto Rico and are moving slowly westward at 5 to 10 mph. While shower and thunderstorm activity has increased over the past few hours, upper-level winds are not currently favorable for re-development of this system. Forecasters can’t count Erika out yet, so they’re giving her a “30 percent chance of regenerating” into a tropical cyclone over the weekend. Although the upper-level winds aren’t in her favor, the warm waters of the Caribbean are there, but it takes both to power a tropical cyclone. The Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite captured Erika’s remnant clouds on September 4 at 2:23 a.m. just to the east of Puerto Rico. The remnants don’t even resemble the shape of a tropical storm, and appear amorphous on satellite imagery. Infrared imagery measures temperatures and not only can it see cold, high cloud tops in tropical cyclones, but warm ocean waters. The ocean waters in the infrared image are clearly warm enough to support a tropical cyclone (80 degrees Fahrenheit), but the upper level winds have torn Erika apart. Looking back at her history, Erika formed from a strong tropical wave that moved off of the coast of West Africa and into the Atlantic on August 26. As the wave propagated westward through the central Atlantic, shower and thunderstorm activity associated with the wave increased. The Tropical Rainfall Measuring Mission satellite (known as TRMM) was placed into service in November of 1997. From its low-earth orbit, TRMM has been providing valuable images and information on tropical cyclones around the tropics using a combination of passive microwave and active radar sensors, including the first precipitation radar in space. TRMM is a joint mission between NASA and the Japanese space agency JAXA. |
