The effect of snow cover on minimum and maximum temperatures

January 16th, 2007 |

AWIPS MODIS visible channel image

An AWIPS image of the MODIS visible channel (above) shows fresh snow cover over much of the Upper Midwest region on the afternoon of 16 January 2007, following a winter storm that moved through the area 1-2 days earlier. This satellite scene is cloud-free for the most part, so the variation in visible “brightness” is due to differences in snow cover and/or density of forest vegetation (areas with a dense tree population will still aprear darker, even if there is a deep snow cover on the ground). Cooperative observer reports of new snow depth that morning (not shown; only first-order climate station data are plotted) were greatest across eastern Nebraska (5 inches), northern Iowa and southern Minnesota (8 inches), and southern Wisconsin (6 inches), while lake-effect snow had contributed to even greater snow depths farther to the north (17 inches in northern Wisconsin, and 13 inches in the Upper Peninsula of Michigan).
AWIPS MODIS IR image with minimum temps and snow depth

The swath of fresh snow cover had an important effect on the nocturnal radiational cooling of the surface, and therefore on the minimum temperatures that were reported that morning. An AWIPS image of the MODIS 11.0 µm InfraRed (IR) channel at 08:02 UTC (above) reveals significant variability in the regional “surface temperatures” that were sensed by the satellite — colder IR brightness temperatures (-30 to -40 C, -22 to -40 F) are shown by the darker blue colors on this enhancement. The coldest morning minimum temperatures that were reported by cooperative observers on 16 January included -37 C/-34 F at Embarrass, Minnesota, -32 C/-25 F at Minong, Wisconsin, -31 C/-23 F at Spencer, Iowa, and -30 C/-22 F at Wayne, Nebraska. Note how the larger lakes in northern Minnesota exhibited slightly warmer IR brightness temperatures (near -20 C/-4 F, cyan enhancement) compared to the surrounding land areas; while these lakes were frozen and snow-covered, the satellite was able to sense warmer radiation coming from the lake water below the ice surface.
AWIPS MODIS IR image with maximum temps and snow depth

An AWIPS image of the MODIS IR image at 19:09 UTC (above) shows a similar variability in the regional afternoon IR brightness temperatures. This IR image was near the time of the daily maximum temperatures — areas having little or no snow cover exhibited warmer IR brightness temperatures (0 to +5 C/32 to 41 F, red enhancement), while those locations having deeper snow cover had colder IR brightness temperatures (near -15 to -20 C/+5 to -4 F, green to cyan enhancement). The actual reported maximum air temperatures showed a similar pattern to the deepest snow cover and the colder IR temperatures; the daytime high was only -17 C/+1 F at Mankato, Minnesota, while temperatures reached a balmy +6 C/+43 F at Dickinson, North Dakota (where there was no snow cover). The warmer, unfrozen waters of Lake Superior and Lake Michigan also stand out with their warmer red enhancement (some colder lake-effect snow bands were still seen over parts of the Great Lakes). Use this handy Java image fader to interactively fade between the MODIS visible and IR images shown above (1280 x 1024 screen resolution required). Speaking of warm, unfrozen waters…due in no small part to the 13th warmest December on record, the larger lakes in the vicinity of Madison, Wisconsin were still not frozen (MODIS true color image).

The “water vapor” channel: sometimes, it’s more like an IR channel

January 16th, 2007 |
AWIPS GOES water vapor channel image

AWIPS GOES water vapor channel image

The most common interpretation of the GOES 6.5/6.7 µm “water vapor channel” imagery is that it provides a depiction of the “moisture content” of the middle to upper troposphere. While this is generally a valid interpretation, it is also important to keep in mind that first and foremost, the water vapor channel is essentially an InfraRed (IR) channel that senses the mean temperature of a layer of mid-tropospheric water vapor. The AWIPS GOES water vapor channel image on the morning of 16 January 2007 (above) provides a good example of the “temperature sensing characteristics” of this particular satellite channel. Bismarck, North Dakota (KBIS) was located within a continental arctic air mass that covered much of the central US (the KBIS surface air temperature at 12:00 UTC was -16º F / -27º C); on the other side of the cold front, Key West, Florida (KEYW) was located within a maritime tropical air mass (the KEYW surface air temperature at 12:00 UTC was +72º F / +22º C). Using the general rule of water vapor channel interpretation, “brighter” shades on the image (or with the color enhancement used here, the blue to white to green colors) are associated with a more moist middle troposphere, while “darker” shades (or the yellow to red colors on this enhancement) are associated with a drier middle troposphere (QuickTime animation of grayscale GOES-12 water vapor imagery). So was Bismarck (KBIS) more moist aloft than Key West (KEYW) at that time?

AWIPS GOES Sounder precipitable water product

AWIPS GOES Sounder Total Precipitable Water product

If we examine the GOES Sounder total precipitable water (PW) derived product (above) we see that in terms of moisture contained within the total atmospheric column, the air mass over KBIS is actually significantly drier (PW = 0.31 inches or 8 mm) than the air mass over KEYW (PW = 1.18 inches or 30 mm). However, the majority of the moisture that contributes to total column precipitable water is often contained within the lower troposphere, at altitudes below which is normally sensed by the GOES water vapor channel…so it’s not surprising that the 2 images above give us different answers regarding “moisture” at KBIS and KEYW.

AWIPS model cross section of temperature and specific humidity

AWIPS model cross section of temperature and specific humidity

Now let’s take a look at a vertical cross section of model-derived temperature and specific humidity, along a line from KBIS to KEYW (above). A plot of the GOES-12 water vapor channel weighting functions (derived using the rawinsonde data from KBIS and KEYW) indicates the altitude and vertical thickness of the “moist layer” that is being sensed by the satellite at each location — the height and thickness of this “layer” changes according to variations in air mass temperature/moisture distribution, as well as the actual satellite viewing angle (or “zenith angle”). It is interesting to note that despite very different air mass characteristics and satellite viewing angles (KBIS = cold and dry, zenith angle = 57.5º; KEYW = warm and moist, zenith angle = 29.5º), the water vapor channel weighting functions look very similar for those 2 locations — the altitude of the peak contribution is generally within the 400-500 hPa pressure range. The cross section above indicates that the specific humidity within that particular 400-500 hPa layer was generally between 0.2 and 0.5 g kg-1 at both KBIS and KEYW — however, note that the temperatures within that range of pressures were significantly colder at KBIS (-25º to -36º C) than at KEYW (-6º to -20º C). So assuming that the satellite was sensing similar values of mid-tropospheric moisture — specific humidity values between 0.2 and 0.5 g kg-1 — it is the temperature differences that contribute to the different water vapor “brightness temperatures” (and the resulting image brightness values or color shades) seen on the GOES water vapor image at those 2 locations.