Severe convection « CIMSS Satellite Blog

Tropical Storm Katia forms over the far eastern Atlantic

August 30th, 2011

GOES-13 Visible (0.63 µm) images (click image to play animation)

An area of disturbed weather that emerged off the coast of Africa over the weekend has acquired sufficient organized convection to be classified as a Tropical Storm, Katia, the 11th named storm of this active Atlantic Hurricane Season. Analyses from the CIMSS Tropical Weather Website show the storm just south of a region of dry air from the Sahara. Shear analyses at the site show that Katia is projected to move into a region of decreasing shear in the next 24 hours. In addition, sea surface temperatures are warm. The forecast from the National Hurricane Center suggests slow strengthening over the next 3 days.

Overshooting Tops diagnosed with MSG data (click image to play animation)

Overshooting tops diagnosed using MSG data (at this site) (above) show a decrease in OT generation over the center of the system today, coincident with warming of the cloud tops. Variability in the number of OTs is common, as shown here.

Posted in General interpretation, GOES-13, Severe convection, Tropical cyclones | No Comments »

Hurricane Irene over the Atlantic Ocean

August 26th, 2011

GOES-13 Visible (0.63 µm) images (click image to play rocking animation)

GOES-13 Visible (0.63 µm) images (click image to play animation)

Category two Hurricane Irene (shown in the rocking animation above) is over the South Atlantic Bight after moving through the Bahamas on August 25th. The projected path is over coastal North Carolina and then towards New England. What factors will control future changes in strength?

Morphed Microwave imagery (click image to play animation)

Morphed microwave imagery, above (and available from the CIMSS Tropical Cyclones site) shows a slow erosion in the inner eyewall during the first 12 hours of August 26, which suggests a possibly temporary decrease in organization to the storm. (Morphed microwave imagery from the 24th and 25th of August show how the eyewall structure changed as the hurricane moved through the Bahamas).

GOES Water Vapor (6.5 µm) image

Water vapor imagery (above) shows dry air west of Irene and ahead of Irene. Recall, however, that GOES Water Vapor signals are showing the temperature at the top of the water vapor. Over the hurricane, where water vapor is associated with deep convective clouds, cold temperatures (white and green in the enhancement) are indicated. The yellow enhancement surrounding Irene indicates warm water vapor that is therefore low in the atmosphere, but it does not quantify how much moisture is present. That is, there could be abundant moisture very low in the atmosphere. Total Precipitable Water (TPW) Percent of Normal maps, however, blended from GPS and GOES Sounder data over land, and from Microwave imagery over water, show relatively dry air over the southeastern part of the United States. MIMIC TPW also shows dry air west of Florida over the Gulf of Mexico. If that dry air can be entrained into the circulation of Irene, significant strengthening would not be expected. (Note also that the MIMIC TPW shows a very rich moisture source just off the coast of the northeast United States. As Irene moves northward, that moisture-rich air should fuel heavy rains).

GOES-13 Visible (0.63 µm), IR (10.7 µm) and auto-detected Overshooting Tops (click image to play animation)

Thunderstorms in the outer rainbands are still producing overshooting tops, a sign of the intensity of the storm, as shown above. Overshooting Tops and Thermal Couplets can also be viewed here. Detections from 1215 UTC on August 26th show overshoots (and thermal couplets) near the eye.

Aqua MODIS False Color RGB Image

The Aqua satellite overflew Irene before sunrise on August 26th. The above false color image, using water vapor, infrared and short-wave infrared data from the MODIS instrument onboard Aqua as red, green and blue input, shows that dry air had suppressed convection north of the center of the storm at that time. Since then, as shown in the visible imagery at the top of this blog entry, convection has eliminated this dryer region. (Aqua MODIS image courtesy of Dave Santek at SSEC).

MODIS 11.0 µm and POES AVHRR 10.8 µm IR images

AWIPS images of 1-km resolution MODIS 11.0 µm and POES AVHRR 10.8 µm IR data (above) displayed cloud top IR brightness temperatures as cold as -85ºC (darker violet color enhancement) at 09:45 UTC within some of the spiral bands of Irene.

NOAA-19 AVHRR false-color Red/Green/Blue (RGB) image

A NOAA-19 false-color Red/Green/Blue (RGB) image (above; courtesy of Dave Santek, SSEC) showed a well-defined spiral band structure to the storm.

Posted in AVHRR, General interpretation, GOES-13, MODIS, POES, Red/Green/Blue (RGB) images, Severe convection, Tropical cyclones | No Comments »

Overshooting Tops in Hurricane Irene

August 24th, 2011

GOES-13 10.7 µm IR channel images (click image to play animation)

Automated detection of overshooting tops allows identification of regions of intense convective updrafts that penetrate into the stratosphere. Overshooting tops correlate well with regions of intense rainfall and with severe weather. In tropical systems, overshooting tops are most frequent during intensification. The loop above shows overshooting tops (identified as the yellow thunderstorm symbol) superimposed on GOES-13 10.7 µm IR channel imagery as Hurricane Irene moved over the southern Bahamas. This is during a time when the hurricane was slowly strengthening.

The automatic detection of overshooting tops is done using a GOES-R Proving Ground algorithm, developed in preparation for GOES-R that is scheduled to be launched in late 2015 or early 2016. Overshooting tops that are detected over the Continental United States using data from GOES-East (GOES-13) are available at this website. As Irene moves northward over the Atlantic south of Cape Hatteras and east of Georgia and South Carolina on August 25th through the 27th, overshooting tops, if they are present, will be indicated; their presence suggests a vigorous hurricane. This website shows overshooting tops (automatically detected using Meteosat data) over the tropical Atlantic west of Africa; there is a correlation between tropical wave intensification and OT frequency.

GOES-13 IR/WV Difference image

GOES-13 Overshooting Tops

Overshooting tops can also be detected by subtracting the GOES-13 10.7 µm and 6.5 µm imagery (that is, IR – Water Vapor brightness temperatures). Overshooting tops inject water vapor into the stratosphere where it is more readily detected and therefore produces a larger signal on the difference product. For example, this image, also above, left, (taken from the CIMSS Tropical Cyclones site) shows the WV/IR difference at 1945 UTC on 24 August. Compare it to this image (also above, right) that shows the auto-detected tops at 1932 UTC (None were detected at 1945 UTC). (Click for Visible imagery for 1932 UTC and 1945 UTC) Note that the difference field shows a very large region where overshooting tops are indicated to the west of the hurricane eye — this is more likely a cirrus shield than a region of many overshoots which are brief in nature — and a region to the northeast of the hurricane eye is in a region where an overshoot was detected at 1932 UTC. By 1945 UTC, the initial overshoot may be collapsing, but the high cirrus shield will still be present, and the difference field detects it.

Overshooting tops continue to be detected in Irene’s eyewall at 2115 UTC, 2125 UTC and 2132 UTC. The burst of overshoot production seems to end by 2140 UTC, however.

Some of the convective bursts that were producing the overshooting tops can be seen in an animation of GOES-13 0.63 µm visible channel images (below; click image to play animation; also available as a QuickTime movie). The GOES-13 satellite continued to be in Rapid Scan Operations mode, providing images as frequently as every 5-10 minutes. It is interesting to note the “trochoidal motion” (or wobble) of the eye of Hurricane Irene as it moved northwestward across the Bahamas during the day.

GOES-13 0.63 µm visible channel images (click image to play animation)

A sequence of 1-km resolutiion POES AVHRR 12.0 µm IR images (below) revealed a number of areas which exhibited cloud top IR brightness temperatures of -80ºC or colder (purple color enhancement) — the coldest seen was -87ºC at 12:30 UTC.