Mostly cloud-free skies in the wake of a cold frontal passage (MODIS true color image) allowed some interesting temperature signatures to be observed across the Upper Midwest and western Great Lakes states on 09 July 2008. An AWIPS image of the MODIS Land Surface Temperature (LST) product (above) revealed significantly warmer LST values (90-95º F, red colors) associated with the many cities across the region. The higher density of buildings, roadways, and paved surfaces in cities and urban areas contributes to the markedly higher surface (or “skin”) temperatures observed on satellite imagery — in fact, MODIS LST values were as hot as 101º F in the Chicago area. However, outside of the cities and urban areas, the MODIS LST values were generally in the 70s F, closer to the observed air temperatures (which are usually reported at suburban airports, away from the city centers).

The MODIS Sea Surface Temperature (SST) product (below) revealed a ribbon of significantly colder SST values (around 50º F, cyan colors) in Lake Michigan, along the nearshore waters off eastern Wisconsin. A cold front had moved eastward through the region on the previous day, and post-frontal winds gusting to 20-25 mph helped to aid the process of cold water upwelling immediately off the coast. Mid-lake MODIS SST values were about 10º F warmer (green colors), which was supported by Lake Michigan buoy data.

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Fog that formed in the Gulf of Maine on Tuesday July 8th developed when relatively moist air moved from the continent over the cooler ocean waters and was cooled from beneath by conduction to the dewpoint. In other words, Advection Fog.

An obvious question arises from the loop: Why does the fog persist over the ocean east of Nantucket and along the coast of Maine even as it clears out in the central part of the Gulf of Maine? The answer is shown in the color-shaded image of SST (available from the Applied Physics Lab at Johns Hopkins University) below. Warmer waters over the central Gulf of Maine will promote the faster erosion of ocean fog because of enhanced vertical mixing because of lower stability. Ocean fog that moves over land — as over southwestern Nova Scotia — also dissipates as heating over land promotes vertical mixing. Dryer air aloft mixed towards the surface reduces the boundary layer relative humidity and the fog evaporates. The dryer air just above the surface is readily apparent in Skew-T plots from Yarmouth, Nova Scotia and from Gray, Maine.

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Many large wildfires continued to burn in parts of California on 08 July 2008. A comparison of MODIS false color and true color images from the SSEC MODIS Today site (above) demonstrated the use of the false color imagery for determining the areal coverage of burn scars in the Los Padres National Forest south of Monterey, California; the burn scars show up as reddish-brown features, with active fire hot spots having a brighter light pink appearance on the false color imagery. In the true color imagery, it is difficult to discriminate between thick smoke from the fires and stratus clouds right along the coast, but the smoke exhibits a light blue tint on the false color imagery.

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Hurricane Bertha became the first hurricane of the season in the Atlantic Basin on 07 July 2008,  setting a new record for the furthest-east named storm formation in the tropics. GOES-12 IR images from the CIMSS Tropical Cyclones site (above) showed increasing coverage of cold cloud top temperatures and the formation of an eye; microwave imagery from the polar-orbiting SSM/I instrument (below) began to show better evidence of an eye structure a few hours before the geostationary satellite imagery.

GOES-12 visible imagery (below) showed a closer view of the forming eye of the hurricane.

A plot of the CIMSS Advanced Dvorak Technique intensity estimate (below) showed that Bertha began a period of more rapid intensification during the early morning hours of 07 July.

Bertha had been moving northwestward over increasingly warmer Sea Surface Temperatures (below), which may have played a role in the intensification of the tropical cyclone.

UPDATE: Hurricane Bertha rapidly intensified into a Category 3 storm during the afternoon hours on 07 July, with the CIMSS ADT intensity estimation technique suggesting peak wind speeds near 115 knots. During this period of rapid intensification, Bertha also displayed a nice eye on satellite imagery (QuickTime animations: GOES-12 visible | GOES-12 IR). It is interesting to note that the MIMIC Total Precipitable Water product (below) indicated that dry Saharan Air Layer (SAL) air had wrapped completely around Bertha during the 05-08 July period — the presence of such dry air in close proximity to a tropical cyclone would normally be thought of as a negative factor for rapid intensification!

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