![\"Slide03\"](\"https:\/\/rammb2.cira.colostate.edu\/proving-ground-blog\/wp-content\/uploads\/sites\/6\/2017\/01\/Slide03.jpg\")
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All this moisture produced widespread rain across far southern California, as depicted in the radar image shown below.<\/p>\n IR imagery at 1130 UTC indicates the clouds producing this rain did not extend to very high levels. \u00a0Sampling of the IR temperature near SAN yields a cloud top temperature of +1C or 274K. \u00a0Using the 12z SAN sounding this corresponds to a cloud top of about 580 mb.<\/p>\n First let’s look at a water vapor image near the time of the above imagery (1145 UTC) from GOES-15, which has a single channel at 6.19 microns (\u03bc). \u00a0The weighting function (discussed further below) for the imager channel is fairly broad, extending generally from 300 to around 400 mb. \u00a0Note that over San Diego and areas to the south this water vapor channel shows a large area of warm brightness temperatures, which forecasters often interpret as indicative of a dry atmosphere. \u00a0The color table being used is the “RAMSDIS water vapor” table available on AWIPS2.<\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n We can use water vapor imagery derived from the GOES-15 Sounder to get an idea what the 2 new water vapor channels that are on GOES-16 (-R) would show. \u00a0While the central wavelengths of the Sounder-based water vapor imagery compare to what is on GOES-16, the resolution is of course far worse (~8 km vs 2 km), and of course not as good as what is currently on GOES (4 km). \u00a0We can see this resolution difference (between GOES and the Sounder) by comparing the above image to the equivalent mid-level channel from the Sounder shown below. \u00a0Besides the resolution the imagery otherwise are fairly similar.<\/p>\n Perhaps the most intriguing of the new water vapor channels is the lower level channel, which from the Sounder would sense near 600 mb or lower. \u00a0The image for 1201 UTC from this channel at 7.4 \u03bc is shown below.<\/p>\n Even though this channel senses lower into the atmosphere, we still see warm brightness temperatures from the SAN area southward. \u00a0These warm temperatures and by inference perhaps dry conditions are found even above an area where rain is occurring AND the environment has a record TPW level. \u00a0How is this possible? \u00a0To answer this question we need to look in more detail at what levels the various channels are actually sensing. Remember, the weighting functions depend on the temperature and moisture distribution in the environment, and the satellite viewing angle, and this means they will be sensing different levels across a domain. \u00a0Below is what the weighting functions look like for the SAN sounding that was shown earlier.<\/p>\n In the above figure the channel number is given for each band; Sounder Channel 10 = 7.4\u00a0\u03bc, 11 = 7.0 \u03bc\u00a0and 12 = 6.5 \u03bc, and the GOES imager water vapor channel is #3. \u00a0The brightness temperatures that are listed in the above figure would be equivalent to what one would find using the sampling tool on AWIPS2 for the various channels when sampling over SAN.<\/p>\n The weighting functions above differ from what would be calculated for the “standard atmosphere”, shown in the figure below.<\/p>\n The actual weighting functions are generally narrower and more complex than those from the standard atmosphere. \u00a0 One important caveat to note about the weighting functions calculated for the 1200 UTC sounding is that while the viewing angle has been considered, the profiles are calculated using a forward model for clear<\/strong> sky conditions. \u00a0In our case we know that there are overcast conditions with a cloud top temperature derived from the IR imagery of 274K or ~580 mb over the SAN area. \u00a0The cloudy conditions would quickly saturate a given channel if it were to sense into the cloud layer, so there will be no contribution much below the top of the cloud layer. \u00a0In our case we note that if it were clear two of the Sounder channels would penetrate below the 580 mb level, so for these two channels the presence of the cloud layer should shift the actual weighting function upwards (colder), moreso for the 7.4 \u03bc channel compared to the 7.0 \u03bc channel. \u00a0Indeed, when we sampled these two channels over the SAN area the brightness temperatures found were 261 K for 7.4 \u03bc and 259 K for 7.0\u00a0\u03bc. \u00a0For the lower 7.4\u00a0\u03bc channel this is 3 K colder than the brightness temperature listed for this channel as calculated for clear sky conditions, so a shift to higher in the atmosphere, as expected. \u00a0For the 7.0 \u03bc channel the difference is only one degree, as would be expected smaller since that weighting function under clear sky conditions did not penetrate below the level where the cloud top occurred (580 mb). \u00a0With this in mind, for this case there was just enough moisture in the environment above the deep moist layer to saturate even the lowest water vapor channel before it sensed down to the cloud top, so\u00a0it looked relatively “dry” even though we had a record TPW present in the atmosphere.<\/p>\n This case shows that while the new water vapor channels will be powerful tools for forecasters, we must be careful to note that they do not necessarily tell us what the moisture profile looks like through the entire atmosphere. \u00a0In other words, we cannot determine from the water vapor imagery how much PW is present in the atmosphere. \u00a0There is, however, satellite imagery that does allow one to detect the actual TPW present. \u00a0Most forecasters are familiar with this imagery on AWIPS2 known as the NESDIS Blended TPW product, which is shown for this case below.<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a>The 7.0 \u03bc water vapor image shown above would generally sense somewhat lower in the atmosphere compared to the GOES imager water vapor image shown earlier. \u00a0The next image is the upper level water vapor channel from the Sounder-based imagery, shown below for the same time (1201 UTC), which would sense vertically a little higher than the current GOES imager channel. \u00a0The area of warmer brightness temperatures is not as extensive as in the mid-level imagery, indicating contribution from moisture at higher levels (the average weighting function for the 6.5 \u03bc imagery is centered close to 300 mb for the standard atmosphere).<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n