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 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

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.

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SSEC/AMRC co-investigator Matt Lazzara was down in Antarctica in January (don’t worry, it was summer down there at the time) heping to install a new Automated Weather Station (AWS) on the Ross Ice Shelf. He sent a nice Terra MODIS visible channel image that they received at McMurdo Station (above), as well as a photo of the actual weather station after installation (below).

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A cold frontal boundary moved southeastward across Wisconsin early in the day on 12 January 2007 — an extensive deck of post-frontal stratus cloud covered much of the southern half of the state during the afternoon hours, as seen on the MODIS true color image (above). As cold air advection in the wake of the frontal passage began to cool surface temperatures below freezing, patches of drizzle began to precipitate from the stratus deck; AWIPS imagery of the MODIS Cloud Phase Product (below) indicated that this stratus cloud deck covering southern Wisconsin was composed primarily of supercooled water droplets (blue enhancement = water phase clouds). This scenario of supercooled liquid precipitation falling onto surfaces that have cooled below freezing created icing conditions that prompted the issuance of freezing rain advisories for many of the counties across southern Wisconsin (farther to the south, ice accruals of 1-2 inches were reported across parts of OK, KS, MO, and IL during the 12-14 January period). Regional rawinsonde profiles at 12:00 UTC (6am local time) showed that the arctic air mass was fairly shallow (especially evident on the Minneapolis sounding), but temperatures were below freezing throughout the atmospheric column on both the Minneapolis MN (KMPX) and Green Bay WI (KGRB) soundings; however, the sounding at Davenport IA (KDVN) did reveal two layers of above-freezing air located between the surface and the 700 hPa pressure level.

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During periods when the GOES-12 imager is placed into Rapid Scan Operations (RSO) — as was the case on 05 January 2007 to monitor severe convection over the southeastern US — the coverage of the southern 2/3 of the South American continent is limited to only 1 image every 3 hours (when GOES-12 does a full disk scan). The color-enhanced IR window (IRW) image comparison above shows the difference in South American coverage between GOES-12 (in RSO) and GOES-10 (in routine operations) at 20:25/20:28 UTC, and Java animations of GOES-12 and GOES-10 imagery from that day further demonstrate the value of having the GOES-10 satellite positioned at 60º W longitude to support the Earth Observation Partnership of the Americas (EOPA) project. During the GOES-12 RSO period, which began at 18:30 UTC on that day, the entire South American continent was sampled 3-4 times per hour with GOES-10, allowing southern hemisphere meterologists to monitor the widespread convection and other phenomena that were occurring over that continent. The latest GOES-10 sounder and imager products are available on the CIMSS GOES Realtime Derived Products site.

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