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Satellite Imager-Based Overshooting Convective Cloud Top Detection


Especially strong convective storm updrafts generated by buoyant energy can penetrate through the stable tropopause region and into the lower stratosphere. These penetrative updrafts have been described in the literature as "hot towers" and/or "overshooting tops" (OT). OTs have been recognized as a significant source of lower stratospheric water vapor which has important implications for the Earth's radiative balance and climate. (Dessler 2002; Corti et al. 2008) Thunderstorms with OTs are also often associated with strong horizontal and vertical wind shear and lightning through charge separation and accumulation in the storm updraft region (Ziegler and MacGorman 1994; Wiens et al. 2005), both of which represent serious turbulence and safety hazards for in-flight and ground aviation operations. Thunderstorms with OTs frequently produce hazardous weather at the Earth's surface such as heavy rainfall, damaging winds, large hail, and tornadoes. (see Dworak et al. 2012 and references therein).

An objective OT detection product has been developed that focuses on the attributes of OTs as depicted in weather satellite imagery (Bedka et al. 2010). Clusters of pixels significantly colder than the surrounding anvil cloud with a diameter consistent with commonly observed OTs (< 15 km) are identified through this approach. The algorithm was developed in support of the GOES-R Advanced Baseline Imager Algorithm Working Group, but this method can be applied to any polar- or geostationary-orbiting imager data. Algorithm performance is optimal when it is applied to high temporal and spatial resolution imagery. Figure 1 shows an example of MODIS OT detection algorithm output for a storm observed by the NASA CloudSat Cloud Profiling Radar. Cloudsat OT Detection Example
Figure 1: (top) A CloudSat Cloud Profiling Radar overpass through a convective storm with an overshooting top. (lower-left) OT detection algorithm output (red pixels) overlaid atop the corresponding MODIS 250 m visible image. (lower-middle) The MODIS 250 m visible image showing that each of the OT detections correspond the characteristic lumpy texture associated with an OT. (lower-right) MODIS 1 km 10.7 micron IR window image for the corresponding scene.

The algorithm is very efficient and can process a 4 km spatial resolution (at nadir) GOES image over CONUS in under 1 minute, allowing for processing in real-time or with large volumes of archived global geostationary and polar-orbiting satellite data. The efficiency and relatively high accuracy of the objective OT detection method allows for the creation of long-term OT databases that highlight when and where storms with tropopause penetrations most often occur. GOES-8, -12, and -13 data from 1995-2012 were used to produce an OT database covering a domain across Central America, the Eastern U.S. (east of 110° W), and offshore waters (see Figure 2). The patterns shown in the OT database are consistent with a long-term database of cloud-to-ground lightning strikes from the Vaisala National Lightning Detection Network (see Figure 3). The frequency and regional distribution of OTs varies markedly between day and night, which is related to diurnal differences in convective regimes across the U.S. (i.e. daytime orographically-induced or airmass thunderstorms vs. nighttime mesoscale convective systems) and the well-known nocturnal bias in convective activity over ocean (Alcala and Dessler, 2002; Liu and Zipser, 2005). The variability of tropopause penetrating deep convective storms across a day can also be seen in an animation of GOES OT detections (Figure 4).

1995-2012 Diurnal variability Figure 2: (top) The number of OT detections within a 0.25 degree grid box using half-hourly GOES-East imagery from 1995-2012. (bottom) The fraction of OT detections that occur during daytime (9 AM to 9 PM Local Time).
nldn climo
Figure 3: The number of cloud-to-ground lightning flashes per square kilometer and per year within 10 km grid boxes observed by the Vaisala National Lightning Detection Network.
Animation
Figure 4: An animation of the number of OT detections within a 0.25 degree grid box per two hour period using GOES-East imagery from 1995-2012.
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NASA Fact

On January 31, 1958, Explorer 1 became the first artificial satellite launched into space by the United States. Onboard was a cosmic ray detector designed to measure the radiation environment in Earth orbit.

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