July 30th, 2007
Our recent fascination with the MODIS Sea Surface Temperature (SST) product continues, with AWIPS imagery of the MODIS SST that revealed an intricate pattern of cold water eddies across the southern portion of Hudson Bay in Canada on 30 July 2007 (above; upper left panel). The 2 MODIS image sets are about 90 minutes apart, and the image animation indicates that these cold water eddies were moving rather rapidly westward during that short time interval — this water feature motion was in the opposite direction of the boundary layer winds, which were light westerly to northwesterly around the northern periphery of a surface anticyclone that was centered over northern Ontario.
There were patches of low-level cloudiness over the water in the eastern and western portions of the satellite scene — these clouds showed up as darker (warmer) features on the MODIS 3.7Âµm IR images (above; lower right panel) due to the solar radiation reflected off the tops of these water droplet clouds. If you look closely, you can also see several small white “specks” in the water near the middle of the MODIS visible images (above; lower left panel) which were also moving westward — these were small ice floes that were floating in the still-cold waters of Hudson Bay (the coldest SST values seen in that area on this day were around 37Âº F or +3Âº C, dark blue enhancement). The ice floes did not exhibit a darker signal on the MODIS 3.7Âµm IR image, since the component of solar radiation reflected off ice surfaces is minimal.
These small ice floes were more clearly depicted in false-color composite images using the 250-meter resolution visible channels 1 and 2 from the Terra abd Aqua MODIS instruments (above). Hudson Bay retained significant ice cover well into the month of June 2007 (MODIS true color image | MODIS false color image: ice features have a red enhancement), and a good deal of ice was still present as recently as early July 2007 (MODIS true color image | MODIS false color image: ice features have a red enhancement).
A comparison of GOES-13 and GOES-12 visible channel images (Java animation, below) better showed the motion of these ice floes during the 6-hour period from 14:02-20:15 UTC. Note the improved image navigation and registration (INR) evident with the GOES-13 satellite: the coastline and island features remain fairly steady from image to image, in contrast with the GOES-12 images which exhibit a notable amount of “wobble” in the animation. Using McIDAS, we tracked the speed of 2 different ice floe features on the GOES-13 imagery: one of the fastest-moving ice floes was seen to have a speed of about 1.8 kilometers per hour (1 knot), while most of the other ice floes seemed to be moving more slowly at a speed of around 0.5 kilometers per hour (0.3 knots). Using the AWIPS “Distance Speed” tool, the speed of displacement of the ice floes and cold water eddies between the 2 MODIS images was found to be about 3 kilometers per hour (2 knots).
July 29th, 2007
The low-lying areas of the Wisconsin River and other tributaries that drain into the upper Mississippi River are favored areas for nocturnal valley fog formation, as shown by the MODIS “fog/stratus product” and topography image comparison (above).
AWIPS images of the MODIS and GOES-12 fog/stratus product from 29 July 2007 (below) demonstrated important differences in the detection of these narrow fingers of river valley fog (yellow to orange enhancement) that were forming during the nighttime hours over parts of southwestern Wisconsin, southeastern Minnesota, and northeastern Iowa. The 1-km resolution MODIS fog/stratus product at 07:50 UTC (below, left) was able to give a more precise indication of the areas where fog was beginning to form, while the corresponding 4-km resolution GOES-12 fog/stratus product at 08:01 UTC (below, right) could only provide a vague signal that fog was starting to develop over portions of the region.
The fog continued to increase in depth and areal coverage later that night, with surface visibility eventually dropping to 0.15 miles at Wisconsin sites Lone Rock KLNR and Boscobel KOVS (0.01 inch of precipitation was also recorded at each location as mist developed) — and as the fog thickened, the GOES-12 fog/stratus product did begin to exhibit a better signal of the fog structure (especially over the lower Wisconsin River valley: Java animation).
A comparison of the AWIPS GOES-12 fog/stratus product and GOES-12 visible channel image at 13:15 UTC (below) shows that much of the river valley fog was still present after sunrise; however, note that after sunrise the fog features on the GOES-12 fog/stratus product began to change in appearance from yellow or orange enhanced features (11-3.9Âµm brightness temperature difference values around -7Âº to -8Âº C) to darker gray enhanced features (11-3.9Âµm brightness temperature difference values of around -4Âº to -5Âº C) — this is due to the fact that the 3.9Âµm channel is very sensitive to solar radiation reflected off the tops of water droplet cloud features (such as fog), which increases the brightness temperature value sensed by the 3.9Âµm detectors (and therefore decreases the 11-3.9Âµm brightness temperature difference that constitutes the fog/stratus product). Because of the solar reflection issue, the fog/stratus product is not valid during daytime hours.
July 25th, 2007
An mesoscale phenomenon that sometimes emerges out of Mesoscale Convective Systems (MCS) is the Mesoscale Convective Vortex (MCV). Intense latent heating within the rain core of an MCS can help spin up a vortex that will occasionally live on even as the MCS that spawned it withers away. The spin-up can be visualized as a potential vorticity response to latent heating in mid levels that increases static stability and therefore increases the potential vorticity, inducing spin. The spin development can also be viewed in terms of changes in height via the Quasi-geostrophic height tendency equation: latent heat above causes height falls below and the development of cyclonic spin.
Atmospheres that support the development of MCVs have things in common. Abundant moisture and low stability are important. It’s also common to have low values of vertical wind shear; that is, the wind profile is fairly uniform. The degree of uniformity together with the amount of moisture and instability help determine if the MCV will be sustained. The key to persistence is ongoing warming through latent heat release at mid levels.
On 24 July, a large MCS over the northern Plains spawned an MCV that moved eastward and southward into Minnesota. The loop is above. Focus on the large cloud mass over the Dakotas that moves towards central Minnesota. The cyclonic spin of the MCV is subtle but present.
This MCV traversed a region of plentiful moisture, as shown by surface dewpoint plots at 15z and at 21z. Dewpoint plots at 850 hPa also show plenty of moisture over the upper midwest, and plots of 700-400 hPa wind speed shearshow a region of small shear over the upper midwest. You can also check out Chanhassen’s upper air sounding at 12z on the 24th and 00z on the 25th.
What do the conventional data show? Abundant moisture in a region of small vertical shear. That is precisely the kind of atmosphere that supports MCVs.
Added: A visible loop that more clearly shows the cyclonic spin of the MCV is available here. (Caution: this is a 12 megabyte animated gif)
July 23rd, 2007
An AWIPS image of the MODIS Sea Surface Temperature (SST) product (above) revealed a pocket of warmer SST values (72-75Âº F or 22-24Âº C, orange enhancement) over the mid-lake waters of southern Lake Michigan during the afternoon (around 19:41 UTC, or 2:41 PM local time) on 23 July 2007.
An animation of daily MODIS SST images (below) indicates that this particular warm water feature was not evident on afternoon MODIS SST imagery over southern Lake Michigan on 20 July or 21 July, but the water temperatures did begin to increase over that general region on 22 July.
A time series plot (below) of the air temperature and water temperature from the southern Lake Michigan Buoy 45007 (data courtesy of the National Data Buoy Center) shows that water temperatures at that location (along the northern fringe of the warm SST feature) warmed at a rate of nearly 1Âº F per hour during the afternoon hours on 23 July (with a total increase of 9Âº F or 5Âº C in the 12-hour period from 12:00-00:00 UTC). It is interesting to note that the Buoy 45007 water temperature (blue; measured at 0.6 meter below the site elevation) warmed more quickly than the Buoy 45007 air temperature (red; measured at 4 meters above the site elevation) during the daytime hours (12 GMT to 00 GMT) on both on 22 July and 23 July. Relatively light winds (4 knots or less) and low wave heights during the morning and afternoon allowed for such a rapid warming of the lake’s “skin temperature” (a similar diurnal change in SST values over the Gulf of Mexico and the western Atlantic Ocean — as much as 3Âº K (5Âº F) in regions of light winds — was previously reported using GOES-8/GOES-9 IR satellite data [Wu, Menzel, and Wade, 1999]). The maximum MODIS SST values of 77Âº F (25Âº C) seen on 23 July were the warmest observed over southern Lake Michigan during the 8-day period from 17-24 July.