Atlantic Eddies along the Gulf Stream

May 21st, 2007 |

AWIPS MODIS sea surface temperature image

An AWIPS image of MODIS sea surface temperature (SST) on 21 May 2007 (above) shows an intricate pattern of Atlantic Ocean eddies along either side of the axis of the Gulf Stream. SST values in the 70-78º F / 21-26º C range (orange enhancement) marked the Gulf Stream axis as it meandered northeastward across the western Atlantic. Some of the eddies were offset a considerable distance from the Gulf Stream axis, most notably the pronounced cool eddy (SST values around 64º F / 18º C, dark green enhancement) located well to the southeast. The black areas over water on the image are regions where the MODIS cloud mask product indicated that clouds were present — no SST data is generated for such “cloudy” pixels. Although some cloud patches were indeed present, the strong thermal gradients associated with the Gulf Stream sometimes cause the MODIS cloud mask product to falsely indicate “cloud” over fairly large regions that in reality are not cloudy.

Note how the Real-Time Global SST model analysis (below) does a poor job resolving the details of these particular eddy features.


Days 35 and 36 of the Georgia/Florida fires

May 21st, 2007 |

MODIS false color RGB images

The largest wildfire in Georgia history started on 16 April, and continued to burn on 20 May and 21 May 2007. The Terra and Aqua false-color red/green/blue (RGB) images using MODIS channels 07, 02, and 01 (above) reveal the extensive burn scars resulting from the 472,000 acre Sweat Farm Road / Big Turnaround fire south of Waycross, Georgia (station identifier KAYS), as well as the 125,000 acre Bugaboo fire that straddled the Georgia/Florida border. The locations of the hottest active fires were indicated by the pink-colored pixels along the periphery of the burn scars.

Fire and ice in Quebec, Canada

May 18th, 2007 |

MODIS 4-panel image

The AWIPS 4-panel display of MODIS imagery (above) shows a portion of west-central Quebec, Canada (between Nemiscau Airport, CYHH, and Chibougamau, CYMT) on 18 May 2007. The 3.7µm Band 20 shortwave IR image (upper left panel) and the 2.1µm Band 7 snow/ice image (upper right panel) both indicate a cluster of hot pixels (dark black on the Band 20 image, bright white on the Band 7 image) associated with a wildfire that started to burn at that location several hours earlier. This fire was hot enough to saturate the 3.9µm IR detectors on GOES-12, causing the indicated brightness temperatures to “roll over” and be displayed as very cold (bright white) pixels. The corresponding MODIS visible Band 1 image (lower right panel) shows the smoke plume from this fire, which was drifting northeastward.

There are numerous lakes in that region, including the long, curved Lake Mistassini (the largest lake in Quebec) located north-northeast of CYMT. Since the lakes in that area had only recently thawed, their water temperatures were still quite cold as confirmed by the light gray brightness temperatures on the MODIS Band 20 and Band 31 IR images (upper left and lower left panels). However, a close examination of the northern portion of Lake Mistassini on the MODIS visible Band 1 image (lower right panel) tells us that part of the lake still had significant ice (indicated by the lighter gray shades over the northern half of the lake). A MODIS true color image of Hudson Bay (below) includes the partially-ice-covered Lake Mistassini (as well as the fire smoke plume) in the extreme southeastern corner of the image.

MODIS true color image

Occluding cyclone in the Gulf of Alaska

May 17th, 2007 |

GOES-11 water vapor image

GOES-11 6.7µm “water vapor channel” imagery (above; QuickTime animation) revealed a textbook signature of an occluding cyclone over the Gulf of Alaska on 17 May 2007. A dry slot begins to interact with a baroclinic leaf feature after 00:00 UTC, with a distinct cloud head or “comma head” forming after 06:00 UTC. After 12:00 UTC, the characteristic “dry swirl” pattern is evident on the water vapor imagery, indicating that the cyclone was entering the occluded phase of its life cycle.

A classic hook-shaped pattern was also seen on AWIPS imagery of the DMSP SSM/I rainfall rate product (below), although rainfall rates were fairly light (in the 1-2 mm per hour range). NOAA Ocean Prediction Center guidance was forecasting gale and storm force winds in the northwestern quadrant of the occluding cyclone.

DMSP SSM/I rainfall rate


GFS90 model cross section (PV)

A cross section oriented west to east through the cyclone at 18:00 UTC using AWIPS GFS90 model fields indicated that the dynamic tropopause (taken to be 1.5 Potential Vorticity Units, purple to blue enhancement, above) was fairly low across the region of the cyclone (below the 500 hPa pressure level), with a hint of stratospheric air (greater than 1.5 PVU) extending downward to near the surface. A corresponding cross section of Relative Humidity (below) showed the very dry air (RH values less than 20%, orange to red enhancement) associated with the elevated dry slot that was wrapping around the cyclone.

GFS90 model cross section (RH)