Overshooting Tops (OT’s) are domelike bulges atop an anvil cloud that indicate a particularity strong updraft within a convective system that has vertically penetrated the tropopause. Areas of convection possessing OT’s are often associated with hazardous weather at the surface, including lightning, heavy rainfall, and severe weather. An OT Detection (OTD) algorithm has been developed to highlight the presence of overshooting tops, and quantify their strength. This product is especially useful at night in the absence of higher resolution visible imagery and in areas where radar coverage is limited or absent.
The OTD algorithm is currently under evaluation at the Storm Prediction Center (SPC) as part of a GOES-R Proving Ground demonstration. Thus far, SPC forecasters have found this product to be especially useful at night, but also valuable during the daytime. They like that it makes the OT’s stand out, making it easy to observe trends in the feature over several hours. It provides them with additional situational awareness, confirming some of what they may already know in areas of good radar coverage. Finally, forecasters have found it to be useful to monitor trends in OTD’s as a way to monitor mature convective evolution. A persistent OT (or cluster of OT’s) indicates a long-lived strong updraft and storm, while the disappearance of said OT’s over the next few scans is a sign that the updraft is weakening, and the storm may be dissipating.
This final point was on display last night, when an SPC forecaster used the OT product in his decision-making process when issuing an SPC Mesoscale Discussion (http://www.spc.noaa.gov/products/md/md0401.html). He noted “The GOES-R OVERSHOOTING TOP PRODUCT INDICATED A DIMINISHING TREND OVER EAST CENTRAL MS SUCH THAT UPDRAFT INTENSITIES ARE LIKELY WEAKENING”. The animation below shows the IR imagery, storm reports, and OT magnitude with this particular event in NAWIPS.
Figure 1: April 24, 2014: 2332 – 0245 UTC. GOES-East IR imagery, Overshooting Top Magnitude, and SPC storm reports. For storm reports, a=hail, A=large hail (2″ dia. +), w=wind). Colorbar on left is IR brightness temperature, colorbar on right is overshooting top magnitude (difference in brightness temperature between overshooting top and surrounding anvil).
Notice an OT was first detected with the storm of interest at 0015 UTC in west central Mississippi. This feature was detected by the algorithm over the next several scans (0015-0200 UTC) until 0215, when the OT had collapsed and was no longer detected (as another storm with OTD’s approached from the northwest). The associated storm produced severe winds and large hail at the surface along its path before weakening during the hour after the last scan in which the OT was detected. Particularly large hail (4.25″!) was reported in central Mississippi at 0120 UTC, just after a measured increase was observed in the OT magnitude. This case shows the value in using this product to monitor trends in mature convective evolution and particularly long-lived storms, and how decreasing trends in OTD’s may be a precursor to the dissipation of the convection.
Forecasters will continue to evaluate this product and provide feedback on its operational utility in the SPC. They understand that the algorithm will be much improved and more valuable in the GOES-R era with the increased spatial and temporal resolution of the GOES-R Advanced Baseline Imagery (ABI).
– Bill Line, SPC/HWT Satellite Liaison