Hurricane Gustav

August 26th, 2008 |
GOES-12 IR + SSM/I microwave images (Animated GIF)

GOES-12 IR + SSM/I microwave images (Animated GIF)

Hurricane Gustav rapidly intensified into a hurricane on 26 August 2008 (less than 24 hours after the first advisory was issued). As noted by the National Hurricane Center’s Gustav discussion number 6:

HURRICANE GUSTAV DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL072008
1100 AM EDT TUE AUG 26 2008

AN AIR FORCE RECONNAISSANCE MISSION EARLY THIS MORNING FOUND PEAK FLIGHT-LEVEL WINDS OF 91 KT…AND AN SFMR SURFACE WIND OF 78 KT. THESE MEASUREMENTS SUPPORT AN INITIAL INTENSITY OF 80 KT.  A DROPSONDE RELEASED IN THE EYE OF GUSTAV DURING THE LAST AIRCRAFT PASS THROUGH THE CENTER JUST BEFORE 1200 UTC…FOUND A SURFACE PRESSURE OF 982 MB…BUT WITH 12 KT OF WIND AT THE SURFACE…SO THE MINIMUM PRESSURE HAS BEEN ESTIMATED SLIGHTLY LOWER.  THE AIRCRAFT AND EARLIER MICROWAVE SATELLITE IMAGERY INDICATE THE PRESENCE OF A VERY SMALL EYE…HOWEVER THE EYE HAS NOT YET BECOME APPARENT IN CONVENTIONAL SATELLITE IMAGERY.

A comparison of  imagery from the CIMSS Tropical Cyclones site (above) shows the shield of central dense overcast (cold cloud tops) on the GOES-12 IR image, with evidence of an eye structure showing up on the SSM/IS microwave image. 1-km resolution GOES-12 visible imagery (below) also did not display an eye with Hurricane Gustav during the morning hours.

GOES-12 visible images (Animated GIF)

GOES-12 visible images (Animated GIF)

Wildfire in southeastern Montana: a view from 4 different satellites

August 22nd, 2008 |
PS images of MODIS and GOES shortwave IR channels (Animated GIF)

AWIPS images of MODIS and GOES shortwave IR channels (Animated GIF)

Lightning was the cause of a wildfire in southeastern Montana (about 45 miles east-southeast of Miles City) during the afternoon hours on 22 August 2008, which burned a total of about 2600 acres. AWIPS images of the MODIS 3.7 µm and GOES 3.9 µm shortwave IR channels (above) showed a significantly warmer “fire hot spot” on the MODIS 18:20 UTC image (51.0º C, orange pixels) compared to the 18:25 GOES image (36.5º C, dark black pixels). The fire hot spot on GOES imagery did not approach the temperatures seen on the MODIS image until about 20:15 UTC.

This wildfire could be viewed using GOES-11 (GOES-West), GOES-12 (GOES-East), and GOES-13 (undergoing a period of operational testing as “GOES-Central”) — you can see that there was a slight amount of image “wobble”  on the GOES-11 and GOES-12 shortwave IR and visible images, while the corresponding GOES-13 image navigation was very steady (below). Changes to the GOES-13 spacecraft have resulted in improved image navigation compared to the previous GOES satellites.

Also note that the smoke plume on the visible images was much more apparent on the GOES-12 and GOES-13 images — this is due to the fact that the forward scattering angle was more favorable from the viewing perspectives of GOES-12 (positioned at 35º W longitude) and GOES-13 (positioned at 105º W longitude) during the late afternoon hours, which helped to highlight the smoke plume as it drifted southeastward.

GOES-11 + GOES-12 + GOES-13 shortwave IR and visible images (Animated GIF)

GOES-11 + GOES-12 + GOES-13 shortwave IR and visible images (Animated GIF)

A plot of the GOES-11, GOES-12, and GOES-13 3.9 µm shortwave IR brightness temperature values (below) showed a fair amount of variability between the maximum “fire hot spot” temperatures sensed by the different satellites. Although there was general agreement in the trend of increasing temperatures as the wildfire continued to burn, at times the measured IR temperatures differed by as much as 20º K (for example, at 19:45 UTC).  The different satellite viewing angle was a contributing factor to some of these differences — at times the fire was partially obscured by cloudiness that was moving through the region during that period.

GOES-11 / GOES-12 / GOES-13 3.9 µm IR brightness temperatures

GOES-11 / GOES-12 / GOES-13 3.9 µm IR brightness temperatures

Tropical Storm Fay: GOES-13 images during GOES-12 KOZ

August 22nd, 2008 |
GOES-12 and GOES-13 IR images (Animated GIF)

GOES-12 and GOES-13 IR images (Animated GIF)

On the current operational GOES-11 and GOES-12 satellites, data losses occur for several hours each day during the weeks centered around the Spring and Autumn equinox due to (1) Keep-Out Zones (KOZ), and (2) Eclipse. During KOZ, sunlight impinging upon the optical path of the instrument detectors requires the GOES Imager and Sounder to be turned off; during Eclipse, the satellite is in the Earth’s shadow, so the solar panels cannot generate power for the Imager and Sounder instrument packages. On the animation of GOES-12 and GOES-13 10.7 µm IR imagery (above) from 22 August 2008, you can see the period of no data from GOES-12 during such a KOZ — however, data continued to be available using the GOES-13 satellite, due to changes in the spacecraft design that mitigate the KOZ problem.

On AWIPS, data from the GOES-11 (GOES-West) satellite are remapped and used to replace the missing GOES-12 (GOES-East) data during such KOZ and Eclipse periods. During this particular KOZ period, a strong rain band associated with Tropical Storm Fay was moving inland across the Jacksonville, Florida region, creating wind gusts up to 60 mph with heavy rainfall, severe street flooding, and multiple power outages due to downed trees and power lines. On the AWIPS IR images from the KOZ period (below) you can see that the re-mapped GOES-11 data at 05:00 and 06:00 UTC appear somewhat “distorted” due to the large satellite viewing angle (GOES-11 is positioned at 135º W longitude over the Pacific Ocean), and the high-altitude cold cloud top features are shifted significantly eastward due to the associated satellite parallax error.

AWIPS images of GOES IR channel (Animated GIF)

AWIPS images of GOES IR channel (Animated GIF)

The GOES-13 satellite had recently been brought out of on-orbit storage for a period of operational testing. Real-time GOES-13 images are available here and here; GOES-13 sounder derived products are available under the “GOES-Central” heading.

Satellite products for interrogating low clouds

August 20th, 2008 |
MODIS + GOES-12 IR window images (Animated GIF)

AWIPS images of the MODIS + GOES-12 "IR window" channel (Animated GIF)

Areas of low cloudiness (such as stratus and/or fog) at night can be important aviation hazards, but they are sometimes difficult to identify simply by examining standard IR imagery. A comparison of 1-km resolution MODIS 11.0 µm and 4-km resolution GOES-12 10.7 µm “IR window” images (above) does not give the sense that there was widespread fog and stratus clouds in place over much of eastern Colorado during the pre-dawn hours on 20 August 2008. Even with a color enhancement that highlights the warmer range of IR brightness temperatures (below), the edges of the low cloud feature are difficult to pick out unambiguously.

MODIS + GOES-12 IR window images (Animated GIF)

AWIPS images of the MODIS + GOES-12 "IR window" channel (Animated GIF)

There are a variety of satellite products available in AWIPS that can be used to better identify and characterize areas of low cloudiness and fog. In this case, the 1-km resolution MODIS fog/stratus product (below) showed very good details about the edges and structure of the area of low clouds and fog, with the corresponding  4-km resolution MODIS Cloud Top Temperature and Cloud Phase products  indicating that the cloud tops over eastern Colorado were composed of water droplets (blue enhancement) with cloud top temperatures around +5º C (red enhancement). However, note that there was another separate area of supercooled cloud tops farther to the east (located over western Kansas), where the Cloud Top Temperatures were below freezing (yellow enhancement) while the Cloud Phase product still indicated water droplet clouds (blue enhancement) — regions such as this might pose a higher risk for aircraft icing.

MODIS fog/stratus  + cloud top temperature + cloud phase products (Animated GIF)

AWIPS images of the MODIS fog/stratus + cloud top temperature + cloud phase products (Animated GIF)

Using the GOES-12 satellite, one can examine the 4-km resolution fog/stratus product (below), and also utilize the 4-km resolution Low Cloud Base product along with the 10-km resolution sounder-derived Effective Cloud Amount and Cloud Top Height products to gain additional information about the cloud base height, cloud coverage, and cloud top height.

GOES-12 fog/stratus + low cloud base + effective cloud amount + cloud top height products (Animated GIF)

AWIPS images of the GOES-12 fog/stratus + low cloud base + effective cloud amount + cloud top height products (Animated GIF)