The following is an evaluation of a low cloud and fog (IFR/MVFR ceilings and MVFR visibility) case in South Texas using the 0900 UTC 2 January 2014 Nighttime (NT) Microphysics and MODIS-VIIRS 11-3.9 micron 1-km products. (The 0900 UTC 2 January 2014 MODIS-VIIRS 11 micron 1-km product and corresponding METAR observations were also utilized in the analysis.)
During the early morning hours of 2 January 2014, a drier air mass moved southward across South Texas with mid and upper level drying advancing faster than drying at the surface (note relative humidity/equivalent potential temperature cross-section in Figure 1, and surface equivalent potential temperature in Figure 2.) Thus the moisture profile ahead of the surface boundary became consistent with the development of fog or a mixture of low clouds and fog.
At 0900 UTC, NT Microphysics imagery was available (Figure 3.) At this time, the fog was located primarily over Deep South Texas and over the Coastal Waters. This evaluation is focused on the region within the circle depicted in Figure 3.
The MODIS-VIIRS 11-3.9 1-km micron imagery (Figure 4) revealed mainly yellow color, which suggests low clouds or fog. Note that the corresponding METAR observations LRD (Laredo International Airport, Laredo Texas) and HBV (Jim Hogg County Airport, Hebbronville Texas) depict low clouds (1100 feet and 1600 feet ceilings, respectively), and no fog. The southern observation APY (Zapata County Airport, Zapata Texas) depicts a combination of four (4) statute miles in fog (mist) and 800 feet AGL ceiling, yet is surrounded by limited blue color in the spectral difference imagery suggestive of higher clouds. In fact, SPoRT MODIS-VIIRS 11 micron 1-km imagery (Figure 5) depicts high clouds that correspond to the blue color in the spectral difference imagery. Thus, the 11-3.9 micron imagery does not identify the low cloud/fog mixture at APY.
However, the SPoRT NT Microphysics product (Figure 3) does provide the information necessary to distinguish between the APY and the LRD/HBV regions, yet additional analysis is needed. Note that the northern observations are located in predominately dull white-colored regions, yet the cloud/fog location is surrounded by dark blue-colored regions. A technique was utilized to determine the actual RGB values before interpretation of the imagery. A local application (D2D Image Maker) was utilized to generate PNG-formatted files of the NT Microphysics Imagery. These files were then imported into Adobe Photoshop computer software in order to determine the specific Red, Green, and Blue contributions at each point in the imagery.
The following are the approximate RGB values representative of the region surrounding each of the three (3) METAR sites, and the corresponding visibility restriction and cloud portion of the METAR observation at 0900 UTC 2 January 2014. (RGB values were determined in the following manner: For each color, determine the value at four points along the ~0.5 mile radius surrounding the location of the METAR, then compute the arithmetic average.)
KLRD: R123, G148, B144 10SM OVC011
KHBV: R132, G132, B156 10SM OVC016
KAPY: R100, G61, B130 4SM BR OVC008
Note that the blue and red contributions were significant, suggestive of a lower level/near surface phenomenon with significant optical depth (low clouds and/or fog). Note that although the resultant color of the NT Microphysics imagery suggests that the region surrounding APY has a higher concentration of blue, the actual blue contribution for APY is less relative to the other sites. (It’s important to note that the blue is not bright, which gives a clue that the blue contribution, though visible, is not necessarily high.)
When examining the RGB value differences between the locations, there is a significant difference in green contribution between the APY region and that of the other two regions; the green contribution of APY is significantly lower. A lower concentration of green for the combination MVFR visibility and MVFR/IFR ceiling (fog and cloud mixture) region of APY is consistent with a greater likelihood for fog.
This case illustrates the utility of using NT Microphysics imagery to separate the combination MVFR visibility and MVFR/IFR ceiling regions from the MVFR/IFR ceiling-only regions, if knowledge of the actual RGB values is available.