July « 2008 « CIMSS Satellite Blog

Mesoscale Convective Vortex (MCV) over southeastern Wisconsin

July 11th, 2008

A mesoscale convective system (MCS) moved southward across southern Wisconsin during the morning of 11 July 2008, producing a well-defined bowing segment along the southern flank. As the MCS continued to decay into the late morning hours, the signature of a weak Mesoscale Convective Vortex or MCV (see Interesting Clouds on Satellite: “Mesoscale Convective Vortex” from the NWS Milwaukee) became apparent on AWIPS images of the GOES-12 visible and IR channels (above, upper 2 image panels; see also a GOES-12 visible animation from Dave Santek, SSEC). While the presentation of the MCV was fairly impressive on the GOES-12 satellite imagery, this feature was much less obvious on the Milwaukee/Sullivan radar base reflectivity and base velocity data (above, lower 2 image panels).

Northerly winds gusted to 37 mph at the Madison airport KMSN (41 mph on top of the SSEC building in downtown Madison) as the bowing segment convection moved through; however, it is interesting to note that Juneau KUNU later reported a southeasterly wind gust of 43 mph at 17 UTC (Noon local time) as the MCV passed over that area; apparently a boundary layer “wake low” was responsible for producing this brief period of gusty southeasterly winds (which also included gusts to 20 mph at West Bend KETB and 17 mph at Watertown KRYV).

A comparison of MODIS true color and false color images from the SSEC MODIS Today site (below) showed finer detail in the cloud structures of both the leading edge of the convective outflow (the aforementioned bowing segment) and the MCV. The MCV signature was comprised primarily of lower to middle level clouds, and became more obvious as the canopy of high-level cirrus clouds (cyan colors on the MODIS false color image) began to dissipate as the MCS decayed.

Posted in GOES-12, MODIS, Severe convection | No Comments »

MODIS detects warm cities and cold upwelling

July 10th, 2008

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.

Posted in Marine weather, MODIS | 1 Comment »

Fog in the Gulf of Maine

July 8th, 2008

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.