This blog entry will examine the synthetic low cloud / fog product produced by the 4-km NSSL WRF-ARW model. For detailed information on this product, see:
Let’s examine this product on the Washington and Oregon coastline from October 10, 2012:
Recall that low cloud or fog is depicted as blue in this color table (positive temperature difference) while the black / dark grey regions are mid- to high level clouds forecast by the model. This is from the 0000 UTC 10 October model run. At the start of the loop (0900 UTC) we see extensive regions of low cloud / fog along the Oregon and Washington coastline. As we move forward in the loop, we observe an expansion of low cloud / fog in a few areas. One of the more obvious places this is occurring is the Strait of Juan de Fuca just northwest of Seattle, WA. We can see an eastward of expansion of the low cloud / fog towards the coastline between Seattle and Bellingham, WA. Soon thereafter we see the expansion of the low cloud / fog signature in the vicinity of Seattle. We also observe a northeastward expansion of the low cloud / fog signature southwest of Seattle, in the Chehalis gap moving towards Tacoma. Less subtle is an eastward expansion of the low cloud / fog signature along the Columbia River that separates Washington and Oregon. By the end of the loop (1900 UTC) we begin to see dissipation of the low cloud / fog regions as daytime heating becomes more significant.
As a comparison with observations, we will utilize the CIRA GOES low cloud / fog product with corresponding visibility (bottom number, miles) and ceiling (top number, hundreds of feet AGL) observations:
In this color table, low cloud / fog corresponds to grey / dull white. Note the terminator crosses the scene at sunrise and the product is briefly not as usable at this time. Compare this loop with what we just discussed in the synthetic imagery model forecast to assess how well the model performed. Use a high loop speed and click the rock button, does this help identify anything you missed at the normal loop speed? Recall that this product (also the synthetic low cloud / fog product) does NOT discriminate between low cloud / fog, therefore we overlay the ceiling and visibility observations to help us make this discrimination.
At about the same time, over in the southeast we can see large areas of low cloud / fog in Texas / Louisiana and also Florida / Georgia forecast by the model:
Note the locations as well as the timing of low cloud / fog.
As before, we will use the CIRA low cloud / fog product along with ceiling and visibility observations for our comparison with what actually occurred:
Over Texas, we see a favorable comparison with the model forecast in that the model forecast a region of low cloud / fog advecting from southeast / southern Texas towards the northwest. Further north, a rapid expansion of low cloud / fog occurred that eventually merged with the other low cloud / fog field to the south so that most of Texas became covered in low cloud (and fog, in a few locations where visibility was quite low). Meanwhile, further east across Georgia and Florida, note the low visibilities which would correspond to fog over a fairly large area. By late in the loop, daytime heating helps to dissipate the fog in the 1500-1600 UTC time range, very close to the model forecast.
Note the mid- and high level clouds in the synthetic low cloud / fog product over southern Florida. This may make identification of the low cloud / fog underneath this cloud field difficult to identify. One way around this would be to look at the model output directly, this is a loop of the cloud liquid water at the lowest vertical level from the NSSL WRF-ARW forecast for the same time period:
The blue regions indicate where cloud liquid water is present at the lowest vertical level in the model, therefore this would likely be fog. This product has 2 advantages over the synthetic low cloud / fog imagery:
1) Mid- and high level cloud obscuration is no longer a problem.
2) You may discriminate between low cloud and fog.
The one disadvantage of the product is the comparison with GOES satellite imagery. This is more readily accomplished between the synthetic imagery and the GOES imagery.
This model output of fog from the model is referred to as the NSSL WRF-ARW experimental fog product and the real-time data can be found here:
The real-time synthetic and GOES imagery described above can be found on the GOES-R Proving Ground Real-time Products page.