Lake-effect Showers in August

August 24th, 2009 |


Unseasonably cool air over the Midwest combined with warm late-Summer lake surface temperatures to sustain a line of lake-effect showers that came ashore over southern Lake Michigan on Sunday Morning. A very sharp-edged narrow cloud feature extended north-northeastward from southern Lake Michigan towards northwestern lower Michigan. Lake surface temperatures temperatures in this region, as observed by MODIS, were in the 60s. At the same time, very cool temperatures prevailed in the lower troposphere (see the plotted sounding from Gaylord, MI, from 1200 UTC 23 August here showing temperatures near 5 C at 850 hPa). The resultant temperature difference of about 13 C over the lowest 1-2 kilometers of the atmosphere helped sustain showers as shown on the Chicago (LOT) radar here. Peak dbZ values in this loop were around 24. Expect the lake-effect cloudiness to become more common in the coming months as cooler and cooler airmasses move over the relatively warm Great Lakes.

Note also the widespread valley fog over the Wisconsin River in southwestern Wisconsin and the upper Mississippi River at the beginning of the satellite imagery loop. Daytime heating is suppressed over the river valleys because energy is lost to evaporating cloud droplets, and the environment closest to the River therefore is cooler than surroundings later in the loop. For that reason, cumulus development over the rivers is delayed, and the river valleys are clear later in the loop.

Added: This case was also mentioned in Tom Skilling’s WGN Weather Center blog.

Stereoscopic views of Hurricane Bill

August 20th, 2009 |


GOES-14, which was launched on 27 June and is now undergoing post-launch check-out, is over the Equator at 90 degrees W longitude, whereas GOES-12, the operational GOES-East, is over the Equator at 75 W Longitude. The two satellites give slightly different views of Hurricane Bill, and stereoscopy can be used to view the visible imagery, allowing three dimensions to be perceived. This is done by crossing your eyes to produce three images; when the middle image comes into focus, three dimensions can be perceived.

In this case, stereoscopy shows vertical structure to the clouds, and also the presence of cirrus over the Hurricane eye.

Hurricane Bill and SRSO Scanning

August 20th, 2009 |


Super Rapid Scan Operations are called on GOES-East or GOES-West when meteorologists want to investigate phenomena that occur over very short timescales. Typically, SRSO imagery is taken every minute. However, gaps exist because of responsibilities to other regions. For example, when the National Hurricane Center requested SRSO observations of Hurricane Bill, satellite imagery was still required to observe tornadic thunderstorms over the upper midwest, and to fulfill international treaty obligations to provide full disk imagery every three hours. GOES-R, scheduled for launch in 2015, will have enhanced observational capabilities, enabling SRSO and full-disk scanning simultaneously. Indeed, ABI on GOES-R will scan a full disk image every 15 minutes, a CONUS image every 5 minutes, and a 1000×1000 km area every 30 seconds, simultaneously.

In the case of Hurricane Bill, the SRSO helped define the small cloud vortices within the hurricane eye, as seen in the image above. These vortices have been observed in previous hurricanes as well — mostly notably in Hurricane Isable in 2003. The SRSO visible loop is here (Warning: 25 M animated gif) and the SRSO color enhanced infrared loop is here. IR Brightness temperatures within the eye are in the 290-300 K range, somewhat cooler than the sea surface temperature in this region. The small vortices within the eye are low clouds.

Hurricane Bill

August 17th, 2009 |
GOES-12 IR images

GOES-12 IR images

Bill became the first Atlantic hurricane of the 2009   season  on 17 August 2009. GOES-12 IR images from the CIMSS Tropical Cyclones site (above) displayed a canopy of very cold cloud tops around the center of the hurricane. The CIMSS satellite-derived wind shear product indicated that the storm environment was characterized by low shear, allowing intensification to progress.

A comparison of a 10:15 UTC GOES-12 IR image with a DMSP SSM/IS 85 GHz microwave image from a couple of hours earlier (below) showed a well-organized banding structure beneath the central dense overcast of cold cloud tops.

GOES-12 IR image + DMSP SSM/IS microwave image

GOES-12 IR image + SSM/IS microwave image