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

Fire on the Beach

April 24th, 2009

Fires in coastal northeastern South Carolina, (news links here and here, for example) near Myrtle Beach, have destroyed 70+ houses and forced residents to evacuate. A true-color MODIS image that shows the distinct smoke plume is available here.

The fires were visible from satellite in both the visible channels, as shown above, and in the near-infrared channels. MODIS imagery in the 3.7-micron channel shows hot spots where the peat and brush fires are active. The character of the radiation emitted by the fire is a function of the temperature, as described by Wien’s Law, with higher emitting temperatures leading to shorter wavelength emissions (as described graphically by this applet. Note in the applet how the wavelength of the peak emitted radiation decreases as the temperature increases; a fire burning with a temperature of 700-800 F will have peak emissions near 3.9 microns).

The near-infrared channel (3.9 microns) on the GOES imager is more sensitive to fire detection than the far-infrared channel (10.7 microns) in part because of the great increase in near-infrared emission that occurs as fires develop and mature. In the loop of GOES Imager information above (Visible, 10.7 micron and 3.9 micron, respectively), note the very dark (warm) pixels in the 3.9 micron image in the region of the fire. The warmest pixels have brightness temperatures of 318.5 K at 3.9 micron vs. 300 K at 10.7 microns. In comparison, both sensors have brightness temperatures of 290 K in the waters off the coast. The difference at hot temperatures arises from the enhanced 3.9 micron emissions due to the fires.

Fires are routinely monitored at CIMSS using GOES Imager data, principally visible data and the 3.9 and 10.7 micron channels. See this link for more information. The processed data for 1545 UTC on 23 April do indicated fires (red pixels) near Myrtle Beach (the color key is available here).

Using satellite data to detect differences in soil moisture

April 24th, 2009
GOES-12 visible images

GOES-12 visible images

GOES-12 visible images (above) showed that much of Iowa and northern Illinois were cloud-free during the morning and early afternoon hours on 24 April 2009. There appeared to be some subtle differences in the soil types over parts of those regions, with some areas exhibiting a slightly darker appearance on the visible imagery.

However, GOES-12 3.9 µm shortwave IR images (below) indicated the presence of a broad swath of notably cooler ground, oriented SW-NE across northern Illinois — this was due to moist soils from significant rainfall during the overnight hours (radar-estimated Storm Total Precipitation). Surface air temperatures appeared to be responding a bit more slowly to the daytime solar heating over the swath of cooler wet ground that was seen on the IR imagery.

GOES-12 3.9 µm shortwave IR images

GOES-12 3.9 µm shortwave IR images

AWIPS images  of the MODIS visible channel, 3.7 µm shortwave IR channel, and Land Surface temperature (LST) product (below) also showed a higher-resolution view of the swath of cooler ground  — the LST values over the swath of wet ground were generally in the upper 70s F (orange colors), compared to the upper 80s F (red colors) over the adjacent dry ground areas. Also note the very high LST values of 100-110º F (darker red colors) over parts of western Iowa — these high LST values corresponded to freshly plowed fields where newly-planted crops had not yet begun to come up.

MODIS visible, 3.7 µm shortwave IR, Land Surface Temperature images

MODIS visible, 3.7 µm shortwave IR, Land Surface Temperature images

Over Iowa, note how the MODIS Normalized Difference Vegetation Index (NDVI) values were lower (0.2 to 0.3) in areas where the Land Surface Temperature (LST) values was the highest (below). The MODIS NDVI values were similar over much of northern Illinois, but due to the wet condition of the soil the LST values were much lower in that region.

MODIS LST product + NDVI product

MODIS LST product + NDVI product