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Satellite-based Nowcasting and Aviation Program

Enhanced-V Anvil Thermal Couplet Product Description

Jason Brunner¹, Kristopher Bedka², and Wayne Feltz¹

¹Cooperative Institute for Meteorological Satellite Studies (CIMSS), University of Wisconsin – Madison

²Science Systems and Applications Inc. at NASA Langley Research Center (SSAI/NASA LARC)

This research focuses on the development of an objective satellite-based enhanced-V anvil thermal couplet (ATC) signature detection product. The enhanced-V ATC product will be evaluated at the 2010 SPC Spring Experiment via the GOES-R Proving Ground.  Thunderstorms with enhanced-V ATCs frequently produce hazardous weather at the earth’s surface such as heavy rainfall, damaging winds, large hail, and tornadoes (Brunner et al. 2007).  The enhanced-V ATC develops when a deep convective storm updraft core of sufficient strength rises above the storms’ general equilibrium level near the tropopause region and penetrates into the lower stratosphere.  This results in an overshooting top (OT), which can be observed in ∼11 μm longwave IR window imagery as an isolated cluster of very cold pixels relative to the surrounding cirrus anvil cloud.  The OT acts as an obstruction to the strong upper-level wind flow and causes the flow to divert around it (Fujita 1978; Wang 2007).  As the flow is diverted around the OT region, it is hypothesized that the flow erodes the updraft summit and carries cloud debris downwind (McCann 1983).  The carrying of cloud debris downwind is reflected in the cold brightness temperatures (BTs) that compose the “arms” of the enhanced-V feature.  A region of anomalously warm BTs is often found between these arms downstream of the OT.  Figure 1 shows that the enhanced-V signature appears differently in almost every case, making direct pattern recognition of this feature nearly impossible.

ABI resolution enhanced-v events
Figure 1:  MODIS and AVHRR ∼11 μm IR window imagery degraded to the 2 km ABI spatial resolution for 8 enhanced-V events. All images cover the same horizontal distance and use same color enhancement, illustrating the significant variations in the V-signature across events.

Several hypotheses have been proposed to explain the existence of this warm region downstream of the OT (Figure 2).  One hypothesis is that the warm region is a result of subsidence of negatively buoyant overshooting cloud air downstream of an ascending cloud top (Heymsfield and Blackmer 1988; Adler and Mack 1986; Heymsfield et al. 1983; Negri 1982; Schlesinger 1984).  Another hypothesis is related to the existence of a stratospheric cirrus plume that extends downstream from the OT and radiates at warmer temperature than the cold anvil cloud underneath (Setvak et al. 2007).  This warm region and the OT cold region are dynamically linked and produce an ATC.  Thunderstorms with an enhanced-V and strong ATC signature in IR satellite imagery have been shown to be especially severe (Brunner et al. 2007).  McCann (1983) shows that the enhanced-V signature can appear 30 minutes before the onset of severe weather on the ground, providing a forecaster with crucial warning lead time.

Enhanced-V signature
Figure 2:  An example of an enhanced-V signature and associated severe weather reports on 9 July 2009.

The algorithm being evaluated at the 2010 SPC Spring Experiment focuses on the detection of the ATC signature, as an ATC is present in nearly all enhanced-V events and storms with high magnitude ATCs often produce severe weather.  Bedka et al. (2010) and Bedka (2010) describe the objective GOES-R ABI OT detection method and showed detection results using MODIS, GOES-12, and MSG SEVIRI imagery.  This method uses a combination of 10.7 μm infrared window (IRW) channel brightness temperatures (BTs), a numerical weather prediction (NWP) model tropopause temperature forecast, and OT size and BT criteria defined through analysis of 450 thunderstorm events within 1-km Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR) imagery.  This method is called “IRW-texture” because it utilizes BT spatial gradients (i.e. texture) to identify clusters of pixels that are significantly colder than the surrounding anvil cloud and have a size consistent with commonly observed OTs.  The enhanced-V ATC detection algorithm requires detection of the OT, as the OT is responsible for obstructing the upper-level flow which leads to formation of the enhanced-V and ATC signature.  A set of spatial IR BT and BT gradient checks are performed throughout the anvil cloud surrounding the OT to identify a significant downstream warm region that composes the ATC.

Enhanced-V: 9 July 2007
Enhanced-V: 9 July 2007
Figure 3:  Enhanced-V ATC detections with color-enhanced proxy GOES-R ABI IR window channel imagery at 1845 UTC on 7 April 2006. Enhanced-V locations are outlined in black dashed lines. Blue symbols are OT detects and green symbols are associated downstream warm regions, which together form the ATC.

Figure 3 shows an example of enhanced-V ATC detections with proxy ABI data for 7 April 2006 at 1845 UTC. For this particular case, four of the five existing enhanced-V signatures are detected. Table 1 shows probability of detection (POD) and false alarm ratio (FAR) statistics when either proxy GOES-R ABI or current GOES-12 imagery is used as input to the enhanced-V ATC detection algorithm.  125 manually identified enhanced-V events from 2003-2008 were used in this comparison.  The substantial decrease in POD for GOES-12 is due to the coarser spatial resolution of the 4 km GOES-12 data compared to the proxy ABI data, which was derived from 1 km MODIS and AVHRR imagery. Since the enhanced-V ATC algorithm is dependent on accurate detection of both OTs and ATCs, the fact that the OT and ATCs are less prominent in GOES-12 imagery contributes to the significant reduction in POD.  However, the FAR is improved for GOES-12 compared to proxy ABI.  In addition, Table 1 shows the percentage of detected true enhanced-V ATCs that had severe weather within 30 minutes of image time and within 60 km of OT location for proxy ABI (77.4%) and GOES-12 (100.0%), respectively.  It is important to note that all severe storms do not produce an enhanced-V signature in IR imagery, so a lack of an enhanced-V ATC detection near a severe report should not be construed as a “missed detection”.

Input Imagery to Enhanced-V ATC Detection False Alarm Ratio Probability of Detection % of Accurately Detected Enhanced-V ATCs with Severe Weather within +/- 30 minutes and 60 km of the OT location
Proxy ABI from MODIS/AVHRR 21.5% 67.2% 77.4%
GOES-12 5.9% 12.8% 100%
Table 1: Validation statistics for the Enhanced-V ATC detection. GOES-12 image times used were within ±10 minutes of the proxy ABI image time.

When an enhanced-V ATC is detected in GOES-12 imagery, the above results indicate that there is a very good chance that the detection is correct and that severe weather is associated with it.  Since there were very few GOES-12 true enhanced-V ATCs detected (only 16 of 125), the severe weather stats from proxy ABI (around 80%) are likely more representative of the probability of severe weather near detected enhanced-V ATCs.  These results suggest that enhanced-V ATC detection output could be used as an additional parameter to increase forecaster confidence that a given storm is producing severe weather.  For the 2010 SPC Spring Program, the enhanced-V ATC detection product will operate on both operational and rapid-scan GOES East imagery over the eastern two-thirds of the continental U.S. 


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