Applications of an Objective Overshooting Top Detection Algorithm

A new method for objective satellite-based overshooting cloud top (OT) detection has been recently introduced. This “IRW-texture” method method uses a combination of 1) 11 um infrared window (IRW) channel brightness temperatures (BTs), 2) a numerical weather prediction (NWP) model tropopause temperature forecast, and 3) OT size and BT criteria defined through analysis of 450 thunderstorm events within 1-km IRW imagery to identify clusters of pixels significantly colder than the surrounding anvil cloud with a diameter consistent with commonly observed OTs. This algorithm has been shown to operate well with any high spatial resolution (≤ 4 km) geostationary or polar-orbiting ~11 micron infrared window channel image. This algorithm can be used to investigate the relationships between OT activity and: 1) the tropical cyclone genesis process, 2) aircraft observations and avoidance of turbulence, 3) the El Niño Southern Oscillation (ENSO). This presentation will highlight recent analyses and case studies of OT activity in tropical cyclones, aviation turbulence encounters, and throughout the ENSO.

Under the “vortical” hot tower theory of tropical cyclogenesis, updrafts within pre-genesis tropical cyclones can increase its vertical component of vorticity through tilting and stretching. Using data from 2009, potential areas of tropical cyclogenesis (invests) were tracked, averaging the number of overshooting tops experienced within 200 km of the invest's center. A comparison between the vertical component of vorticity when the invest reached 40W, and the number of overshooting tops experienced per day yielded a correlation of 0.628. In addition to tropical cyclone development, tropical cyclone intensification can also be explored. As seen with both Hurricanes Katrina and Dennis from 2005, a spike in the number of OTs within a boundary of wind speeds from the Automated Tropical Cyclone Forecast (ATCF) data is observed prior to rapid intensification of both storms.

OTs are strongly related to aviation turbulence incidents. Previous work by these authors has determined that, within 25 km of an OT, there is a 25% frequency of light or greater turbulence and a 5% frequency of moderate or greater turbulence. A case study of Pinnacle flight 2871, which made an emergency landing in Louisville, Kentucky on August 4, 2009 after a turbulence encounter, showed that the aircraft flew within 25 km of an overshooting top. In addition, field experiments, like The PRE-Depression Investigation of Cloud-systems in the Tropics (PREDICT), can use OT location to avoid areas of turbulence.

Finally, overshooting tops have a significant climatological impact, as water vapor, an important greenhouse gas, is injected into the stratosphere by intense storm updrafts. The difference between the number and location of overshooting tops with the El Niño Southern Oscillation (ENSO) is explored by comparing OT activity occurring during an El Niño phase and a La Niña phase. As expected, more overshooting tops occurring during El Niño when compared with La Niña over the equatorial tropical Pacific Ocean as warmer ocean temperatures increase the regional atmospheric instability. Regions of greater overshooting top occurrences during La Niña include the subtropical Pacific as well as the Eastern Pacific. A student's T-test was thus applied to investigate if the regions of greater OTs observations were statistically significant to ENSO.