The complex structure and evolution of the Gulf Stream

April 28th, 2009 |
MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product

An AWIPS image of the MODIS Sea Surface Temperature (SST) product (above) showed the complex structure of the Gulf Stream off the east coast of the US on 28 April 2009. The SST values were as high as upper 70s F (darker red colors) in one of the warm eddies along the southern edge of the Gulf Stream, while water temperatures were as cold as the upper 30s to low 40s F (darker blue colors) closer to the mainland. Note the appearance of a number of warm and cold “meanders” and “eddies” along both sides of the Gulf Stream axis — these warm and cold water eddy features can reach to depths of almost 4000 m, and the water temperature within these eddies can have an influence on the productivity of fishing areas.

A comparison of the MODIS SST product with sea surface temperature data from the RTG_SST and the RTG_SST High-Resolution analyses (below) showed that both the RTG_SST and the RTG_SST_HR  had a difficult time properly depicting some of the more subtle meanders and eddies along either side of the Gulf Stream axis. At some locations, differences between the MODIS SST value and the RTG_SST/RTG_SST_HR analyzed SST values were as large as 4-6 degrees F.

MODIS SST + RTGSST and HR-RTGSST model temperature fields

MODIS SST + "RTG_SST" and "RTG_SST High-Resolution" model temperature fields

A comparison of the 1-km resolution MODIS SST product with the corresponding 4-km resolution GOES-12 10.7 µm IR image (below) demonstrated the obvious advantage of better spatial resolution for detecting the smaller-scale meanders and eddies. Some of the darker green features seen on the GOES-12 IR image to the east of the Gulf Stream  were clouds (which were “blacked out” by the cloud mask algorithm on the MODIS SST image).

1-km reslution MODIS SST product + 4-km resolution GOES-12 10.7 µm IR image

1-km resolution MODIS SST product + 4-km resolution GOES-12 10.7 µm IR image

However, the clear advantage of GOES is the higher temporal resolution of the data — with images available more frequently, one can actually see the evolution and motion of the Gulf Stream itself, as well as some of the meanders and eddies along the periphery of the Gulf Stream during the 28-29 April period (below). The maximum speed of the Gulf Stream at the ocean surface is usually about 2.5 meters per second (5.6 miles per hour).

GOES-12 10.7 µm IR images

GOES-12 10.7 µm IR images

Lingering snow cover in the Upper Peninsula of Michigan

April 28th, 2009 |
MODIS visible, 11.0 µm IR, and Land Surface Temperature images

MODIS visible, 11.0 µm IR, and Land Surface Temperature images

A late-season winter storm dumped as much as 29.0 inches of snow across parts of the Upper Peninsula of Michigan during the 19-21 April 2009 period — in fact, the 20.5 inches that fell at Marquette was their 3rd largest late season 2-day snowfall on record. AWIPS images of the MODIS visible channel, 11.0 µm IR window channel, and the Land Surface Temperature (LST) product (above) showed that a few areas of snow cover still remained on 28 April 2009. MODIS IR brightness temperatures were as cold as +2º to +5º C (darker blue colors) over the patches of snow cover, which still appeared as varying shades of white on the visible image. While there were some MODIS Land Surface Temperature values as cold as the middle 40s F (darker green colors) over the patches of snow cover, the coldest areas showed up as black “NO DATA” pixels in the LST product, due to the product algorithm mistakenly identifying the sharp temperature gradients as cloud features.

Unfortunately, there were no National Weather Service Cooperative Observer locations in the region that reported any snow depth on the morning of 28 April, so the true depth of the remaining snow cover was not known — however, according to an email reply from meteorologist  John Dee (who lives on the Keweenaw Peninsula):

The snow that remains is from the season and is quite variable in depth, with shaded areas in the higher terrain still having a foot or a bit more, but unshaded areas being bare and those that catch some sun and some shade having anywhere in between zero and a foot. I’d say probably 2-6″ still remaining if you took the bare with the other areas with varying depth and averaged things out.

AWIPS examples of a 250-meter resolution MODIS true color image and a 1-kilometer resolution MODIS Sea Surface Temperature (SST) product (below) showed two items of interest: (1) there was a good signal of the runoff of snow-melt water as it flowed northward from the Ontonagon River basin into Lake Superior (note the reddish hue of the water immediately offshore, due to the iron-rich sediment), and (2) the water temperatures in Lake Superior were still quite cold, with MODIS SST values generally in the 35º to 38º F range (darker blue colors).

MODIS true color image + MODIS Sea Surface Temperature

MODIS true color image + MODIS Sea Surface Temperature