Long-lived low cloud edge over the Eastern Pacific Ocean

August 25th, 2010 |

Visible and infrared GOES-11 imagery over the eastern Pacific Ocean have indicated a persistent southwestward-moving cloud edge during the past several days. The visible image, above, from 1830 UTC on 25 August, shows a distinct cloud edge arcing from northwest to southeast. What is the history of this feature? The visible imagery loop below, showing 1800 UTC images over the course of 4 days, show that the feature likely has its roots in dry air exiting the North American continent. A loop of 11-micron imagery from GOES-11 every two hours (here) shows the steady progression of the feature, and the persistent sharpness of the edge. Brightness temperatures of the clouds are steady near 287-288 K; brightness temperatures in the clear region are 290-291 K.

Hurricane Danielle

August 24th, 2010 |
GOES-15 Super Rapid Scan Operations (SRSO) 0.63 µm visible images

GOES-15 Super Rapid Scan Operations (SRSO) 0.63 µm visible images

As part of its NOAA post Launch Science Test, the GOES-15 satellite was placed into Super Rapid Scan Operations (SRSO) mode on 24 August 2010, providing images at 1-minute intervals to monitor Hurricane Danielle. McIDAS images of GOES-15 0.63 µm visible channel data (above) did show some interesting convective development associated with Danielle, but no eye was apparent.

While Danielle had intensified into a Category 2 hurricane during the preceding overnight hours, a collapse in both intensity and in satellite appearance began during the daylight hours. AWIPS images of the CIMSS MIMIC Total Precipitable Water (TPW) product (below) did show that some dry air became entrained into the southern and eastern quadrants of Danielle, but fairly far from the center of the tropical cyclone.

MIMIC Total Precipitable Water (TPW) product

MIMIC Total Precipitable Water (TPW) product

In addition to the entrainment of dry air, there was also a change from light easterly wind shear on 23 August to light northwesterly wind shear on 24 August, as seen on images from the CIMSS Tropical Cyclones site (below). It is unclear to what degree either of these factors played in the decrease in intensity  of Hurricane Danielle (which was actually downgraded to a Tropical Storm later in the day)..

GOES-15 IR image with Deep Layer Shear overlay + 85 GHz microwave image + MIMIC TPW

GOES-15 IR image with Deep Layer Shear overlay + 85 GHz microwave image + MIMIC TPW

There was an 8-hour gap in SRSO imagery due to a GOES-15 satellite maneuver, but once the 1-minute interval imagery resumed, it almost appeared as though Danielle was trying to form a more organized convective eyewall later during the day (below).

GOES-15 Super Rapid Scan Operations (SRSO) 0.63 µm visible images

GOES-15 Super Rapid Scan Operations (SRSO) 0.63 µm visible images

Tropical Storm Danielle

August 23rd, 2010 |
GOES-13 10.7 µm IR images

GOES-13 10.7 µm IR images

McIDAS images of GOES-13 10.7 µm IR data (above) revealed a classic convective burst signature as Tropical Storm Danielle began to intensity over the middle Atlantic Ocean on 23 August 2010. The coldest cloud top IR brightness temperature during this time period was -90º C at 07:15 UTC.

UPDATE: Danielle continued to intensify during the day, becoming the second hurricane of the 2010 Atlantic Basin Season. Images from the CIMSS Tropical Cyclones site (below) showed that Danielle continued to exhibit a large, cold Central Dense Overcast (CDO) on IR imagery, with a number of overshooting tops being indicated on the IR/Water Vapor difference product. However, a well-defined closed eyewall structure could be seen on SSMI/S-16 microwave imagery.

GOES-13 IR image + GOES-13 IR/Water Vapor difference product + SSMI/S-16 microwave image

GOES-13 IR image + GOES-13 IR/Water Vapor difference product + SSMI/S-16 microwave image

The CIMSS Deep Layer (200-850 hPa) Wind Shear product (below) showed that Danielle was within (and moving into) a very low shear environment, which was a favorable factor for further intensification. The nearly circular CDO was characteristic of tropical cyclones that are located within such a low shear environment.

GOES-13 IR image + Deep Layer Wind Shear product

GOES-13 IR image + Deep Layer Wind Shear product

Unusually dry water vapor image signature over the central US

August 21st, 2010 |
GOES-15 full disk 6.5 µm water vapor images (at 30-minute intervals)

GOES-15 full disk 6.5 µm water vapor images (at 30-minute intervals)

The normal operational GOES scan schedule provides one full disk image every 3 hours — but as part of the GOES-15 NOAA Science Test, the GOES-15 satellite was placed into a mode which allowed one full disk image to be scanned every 30 minutes. McIDAS images of GOES-15 6.5 µm “water vapor channel” data (above) showed a series of these 30-minute interval full disk images during a 24-hour period during 21 August – 22 August 2010. It should be pointed out that the ABI instrument on the GOES-R satellite will provide a full disk image every 5 minutes!

The image artifacts seen moving from west to east across the northern portion of the images (from 05:15 UTC to 06:45 UTC) were due to stray light contamination. This stray light problem affects the image quality to varying degrees during the Autumn and Spring season “eclipse periods”, since the newer GOES satellites (GOES-13 through GOES-15) have larger batteries that allow them to continue to operate through the eclipse periods when the satellite is in the Earth’s shadow.

Note that some of warmest water vapor brightness temperatures (yellow to orange color enhancement) — which normally indicate areas of very dry air in the middle to upper troposphere — were found over the central US during this particular period. This region of warm/dry air showed up well on GOES-11, GOES-12, GOES-13, and GOES-15 water vapor images (below), although the signal was less obvious on the 6.7 µm GOES-11 imagery (which is a spectrally narrow water vapor channel at an 8 km spatial resolution, compared to the spectrally broad 4-km resolution channels on GOES-12 and later). Each of the water vapor images is displayed in the native projection of the particular satellite.

GOES-11, GOES-12, GOES-13, and GOES-15 water vapor images

GOES-11, GOES-12, GOES-13, and GOES-15 water vapor images

A closer view using AWIPS images of GOES-13 6.5 µm water vapor channel data (below) showed the evolution of this feature, which was becoming warmer/drier as a middle-tropospheric ridge of high pressure was building across the region. Water vapor brightness temperatures were as warm as -9.5º C (orange color enhancement), which is very unusual to see covering such a large area over the central US.

GOES-13 6.5 µm water vapor images (with rawinsonde locations)

GOES-13 6.5 µm water vapor images (with rawinsonde locations)

4-panel images displaying the 3 GOES Sounder water vapor channels (6.5 µm, 7.0 µm, and 7.4 µm) along with the standard GOES-13 imager 6.5 µm water vapor channel are shown below. The water vapor channel weighting function of each of these channels peaks at different altitudes, which is obvious by the difference in water vapor brightness temperatures on each of the images (warmer/drier areas are enhanced with yellow to red colors).

GOES Sounder and GOES Imager water vapor channel images

GOES Sounder and GOES Imager water vapor channel images

The GOES-13 sounder and GOES-13 imager water vapor weighting functions for Topeka, Kansas (below) indicated that the altitudes of the weighting function peaks did indeed descend from 12 UTC on 21 August to 00 UTC on 22 August as the middle troposphere became warmer/drier — but the altitudes of the various weighting function peaks were not significantly lower than those computed using the US Standard Atmosphere.

Topeka, Kansas water vapor weighting function plots (compared to US Standard Atmosphere)

Topeka, Kansas water vapor weighting function plots (compared to US Standard Atmosphere)

The rawinsonde date from Topeka, Kansas at 12 UTC on 21 August and at 00 UTC on 22 August are shown below. While the air aloft was certainly dry, it was also quite warm at those altitudes — and the warm temperature of this mid-tropospheric air was likely contributing to the unusually warm/dry appearance on the water vapor imagery. This case helps to highlight the fact that the water vapor channels are also IR channels, so they are sensitive to temperature as well — and some of the signal of the features seen on the imagery may be due to temperature as well as moisture aloft.

Topeka, Kansas rawinsonde plots (12 UTC 21 August, 00 UTC 22 August)

Topeka, Kansas rawinsonde plots (12 UTC 21 August, 00 UTC 22 August)