Utility of the 1-km Enhanced MODIS SST Composite Product

The following two scenarios occur with significant frequency over the Northwest Gulf of Mexico (NW GOM) during the Fall, Winter, and Early Spring months that historically present a challenge to forecasters at WFO Corpus Christi (CRP.) These scenarios illustrate the utility of the NASA SPoRT 1-km Enhanced MODIS SST Composite Product to WFO CRP in the context of two (2) forecast events.

15 November 2011 Wind Case

With regard to the first case, an upper level disturbance approached Texas from the West 14 November 2011 (not shown.) In response, increasing onshore flow occurred aloft and at the surface over the NW GOM. Although forecasters have experienced this condition a myriad of times, it’s critical that the forecasters determine whether sustained surface wind at each of the six (6) Marine Zones within 60 nautical miles (NM) offshore (two zones within 20 NM of the coast, and the other two zones between 20 and 60 NM offshore), and over the bays, between Baffin Bay and Port O’Connor, Texas, are in the 35 knots (kt) or greater, 20-34 kt, 15-20kt, or 15kt or less category. This is because we would issue a “Gale Warning” marine hazard, “Small Craft Advisory” (SCA) marine hazard, “Small Craft Exercise Caution” wording in the Coastal Waters Forecast (CWF), or no marine hazard or cautionary wording, respectively (there exist sea height criterion over the Coastal Waters for SCA, yet will not discuss.) However, during many occasions it’s difficult to determine whether the sustained surface wind will remain in a particular category at a given marine zone.

In many cases, the surface wind will be determined in part by the wind speed aloft and the extent of vertical mixing. In many cases similar to this case, strong onshore flow of 25-35kt at 925mb occurs over the NW GOM. Thus, the thermodynamic profile within around 1-km of the surface will determine whether the faster wind aloft vertically mixes toward the surface, resulting in an increase in surface wind. The 1-km MODIS SST Composite provides a high resolution SST field over the NW GOM which allows us to infer the atmospheric stability above the surface. Note from the SST image valid at 15 November 2011 0400 UTC, that significantly cooler SST values occurred within the Nearshore Coastal Waters (0-20NM) than over the Offshore Coastal Waters (20-60 NM.) Thus, faster wind aloft (not shown) would be more likely to vertically mix toward the surface over the Offshore Waters. This is manifested in surface wind in the 15-20kt range at the two (2) buoys within Offshore Waters, while around 10kt wind occurred near the Coast. We issued an SCA for the Offshore Waters at 349 PM CST 14 November 2011 (the SST pattern depicted in the 15 November 2011 0400 UTC image did not change significantly from the corresponding image 24 hours earlier.) This SCA verified; based on subsequent conditions, this SCA was re-issued at 1007 PM CST on the 14th, and again at 344 AM CST on the 15th, for the same Offshore region. Without the SST product, we may have been tempted to include the Nearshore Waters in the SCA due to strong wind aloft.

29 February – 2 March  2012 Dense Sea Fog Case

For several days, persistent onshore flow occurred over the NW GOM. As the warmer moist onshore flow moved over the cooler water surface, advection fog (sea fog) developed and moved onshore. In particular, during the nighttime hours, sea fog would develop over the Coastal Waters and move onshore. During the daytime hours, modest increases in temperature over the Waters would allow drier air to vertically mix (the depth of moist air was shallow; not shown) and dissipate much of the fog. This cycle continued for several days, until a cold front moved offshore during the early morning hours of 3 March 2012. In the interim, we issued a myriad of Dense Fog Advisories (criteria over the water: visibility of 1 NM or less for at least 2 hours) for the Bays and Coastal Waters. Sometimes the Advisories were restricted to only the Bays and Nearshore Waters.

Sea fog typically develops when the surface dew points exceed the corresponding SST values. The 1-km Enhanced MODIS SST Composite product proved critical in determining whether sea fog would develop and where. Note the next figure depicts the SST product for 2 March 2012 0400 UTC. The three (3) observations offshore are from AWOS-III automated weather sensors placed on offshore platforms and measuring at elevations ranging from 25 to 35 meters. Note that two (2) of the platforms, and an observation at the Coast, reported visibilities of 1/4 nautical mile. The dew points from those 2 platforms and from the coastal site are greater than the corresponding SST values from the MODIS SST product. The platform without visibility restrictions has a dew point which is near the SST value. This image illustrates the extreme dependence of sea fog development on the dew point and SST values. Although we also utilized human, METAR, and other observations of conditions along the coast, and visible satellite imagery, to locate the fog, the use of the SST product allowed for the determination of the SST values that were critical in accurately forecasting sea fog.

In summary, the use of the 10-km Enhanced MODIS SST Composite allows us (WFO CRP) to increase forecast accuracy/skill with regard to conditions/phenomena that correlate with SST values, such as surface wind speed and sea fog. Further, I feel that this SST product has improved our ability to more accurately forecast during these conditions at higher horizonal resolutions (forecast more confidently within each of the 6 marine zones.)

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