Monthly Archives: April 2020

Sea Fog near Corpus Christi

GOES-16 ABI Band 02 (0.64 mm “Red Visible”) visible imagery, 1201-2311 UTC on 20 April 2020

GOES_16 Visible imagery, above (along with surface observations of ceilings and visibilities), shows fog and low clouds over south Texas and offshore waters. The observations plotted can allow you to determine where IFR conditions are apparent — where visibilities are between 1 and 3 statute miles and ceilings are between 500 and 1000 feet. It’s difficult to determine the area of IFR conditions based solely on cloud cover however.

The animation below shows the probability of IFR conditions, a product that fuses satellite information with low-level saturation information from the Rapid Refresh model (Click here for an animation with no observations). The morning fog over east Texas burns off fairly quickly, and only dense sea fog is left after about 1700 UTC (10 AM CDT). In many offshore regions (and over east Texas before sunrise), the IFR Probability field has a flat character to it that is typical of a IFR Probability field determined mostly by model data. More pixelated data (and somewhat higher probabilities) occur where breaks in the cloud allow for satellite data to identify low clouds.

GOES-R IFR Probability, 1101 UTC to 2311 UTC on 20 April 2020

The night fog brightness temperature difference field, below, highlights a challenge in identifying low clouds using satellite data alone (in contrast to the IFR Probability above that uses satellite and model data, fusing the strengths of both). When cirrus is present, it can mask the satellite’s view of the low cloud beneath. In addition, the Night Fog Brightness Temperature difference product is not consistent through sunrise, as the emissivity differences that drive the signal at night time (small cloud droplets are not black body emitters of 3.9 µm radiation, but they are blackbody emitters of 10.3 µm radiation) become overwhelmed by reflectivity differences during the day when far more 3.9 µm solar radiation is reflected than 10.3 µm solar radiation. Thus, in the day, both low clouds and high clouds show up as black in this enhancement: they are both able reflectors of 3.9 µm radiation.

Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) fields from 1101 UTC to 2311 UTC on 20 April 2020

The Day Fog Brightness Temperature Difference field, below, shows how cirrus ice crystals are initially more reflective of solar radiation, but as the sun climbs higher in the sky, low clouds start to reflect just as much solar radiation.

Day Fog Brightness Temperature Difference (3.9 µm – 10.3 µm) fields from 1201 UTC to 2311 UTC on 20 April 2020
GOES-16 Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), 1101 UTC on 20 April 2020

The Night Fog Brightness Temperature Difference field, above, is a key component (the ‘green’ part) of the Advanced Night Time Microphysics RGB, shown below. Where the Brightness Temperature Difference field is unable to view low clouds, similarly the Night Time Microphysics RGB will be unable to highlight them.

GOES_16 Night Fog Microphysics, 1101 UTC 20 April 2020

Thanks to Penny Harness, WFO Corpus Christi, for alerting us to this event. (Link)

Detecting low ceilings over California

GOES-17 Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) and Night Time Microphysics RGB, 1256 UTC on 01 April 2020 (Click to enlarge)

Detecting stratus at night, and thereby inferring the presence of fog, usually involves the Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) field that identifies clouds made up of water droplets owing to the droplets’ different emissivity properties at 10.3 µm (droplets emit energy at that wavelength mostly as a blackbody) and at 3.9 µm (droplets do no emit energy at that wavelength as a blackbody). This difference field is a crucial component in the Night Time Microphysics Red-Green-Blue (RGB) Product as evinced in the toggle above. The regions shown to have low clouds (blue and cyan in the Brightness Temperature Difference field, pale yellow in the RGB) are not necessarily those regions with IFR conditions, i.e., where fog and low ceilings are present. The satellite can sense the top of the cloud, but it is a challenge to infer from the satellite data alone where the cloud base sits.

GOES-R IFR Probability fields combine satellite information with model estimates of low-level saturation. An accurate model simulation can allow the product to highlight regions of low ceilings (where fog is more likely) and screen out mid-level stratus. Consider the toggle below, and note how is emphasizes regions where observations show low ceilings and/or reduced visibilities (Blue Canyon airport northwest of Lake Tahoe and Paso Robles airport). Note also how the signal at Bakersfield, at the southern end of California’s Central Valley, is de-emphasized.

GOES-R IFR Probability fields provide a consistent signal for low ceilings and reduced visibility. The fields marry the strengths of satellite detection and model data.

GOES-17 Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), GOES-17 IFR Probabilities, and GOES-17 Night Time Microphysics RGB, 1256 UTC on 01 April 2020 (Click to enlarge)