MODIS Fog Study – Fall 2009

Fig 1. MODIS fog over Morristown/Knoxville CWA for September 12 ~0700Z.

The inland NWS partners (BMX, HUN, OHX, MRX) participated from September 1-22 in evaluating the MODIS spectral difference (i.e. “fog”) product for use in aviation and public forecasts of visibility restrictions. Impact of the product was evident in the first half of the study as the product was able to better define the more coarse GOES imagery as well as help identify mesoscale areas of fog in river valleys and low lying topography (see Fig 1). Interestingly, it also did well to identify fog in elevated valleys of the Cumberland and Appalachian Plateaus (see Fig 2), sometimes prior to identification by GOES imagery (see Fig 3). Feedback from users was a mixed. Even with the positive items previously mentioned, users thought that the product over identified areas of fog and would like to see both increased frequency of the product and better timing (i.e. closer to sunrise when fog is most likely). SPoRT is working on a modification of the product to better separate areas of low clouds from fog as well as a hybrid GOES/MODIS product that would provide greater continuity to the product while still incorporating high resolution MODIS data.

Fig 2. The MODIS "fog" is shaded dark (yellow from Fig 1) and placed over the topography for easier viewing of the correlation with low lying areas and river valleys. Note the fog in elevated valleys of the various plateaus.

Fig 3. GOES Low Cloud Base at ~0700Z on September 12. Green areas indicate high cloud bases (i.e. ceilings for aviation) over the plateaus of TN, NC, and VA at this time, but the high resolution MODIS shows good correlation of fog with topography and suggests that fog is occurring. GOES LCB products would later indicate low bases (red) in the morning. MODIS provided some additional lead time and perspective in this case.

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