Sensing the surface in GOES-16 Water Vapor imagery

January 10th, 2022 |

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images, with and without map overlays [click to play animated GIF | MP4]

GOES-16 (GOES-East) Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images (above) showed that portions of the coastline of Lake Superior, Lake Michigan and Lake Huron were apparent during the day on 10 January 2022. When a very cold/dry arctic air mass is present over a particular area, the water vapor “weighting functions” are shifted to lower altitudes — which in this case allowed the strong thermal contrast between (1) the cold, snow-covered land surface across Minnesota, Wisconsin, Michigan and Ontario and (2) the warmer ice-free nearshore waters of the Great Lakes to be sensed by GOES Water Vapor spectral bands. The coldest surface air temperatures that morning across the region included -34ºF at Badoura MN, -27ºF at Grantsburg WI and -15ºF at Ironwood MI.

Plots of GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor weighting functions (below) — calculated using 12 UTC rawinsonde data from Green Bay, Wisconsin (KGRB) — showed peak radiation contributions for the 7.3 µm and 6.9 µm spectral bands were at the 853 hPa and 730 hPa pressure levels, respectively, with some contribution coming directly from the surface. This example underscores the fact that “water vapor” spectral bands are essentially infrared bands — and with very little water vapor within the atmospheric column to absorb then re-radiate any upwelling energy (at its colder ambient temperature aloft), the signature of this land vs. water thermal contrast was able to reach the satellite sensors with minimal attenuation. 

Plots of GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor weighting functions at Green Bay WI [click to enlarge]

According to the Green Bay WI rawinsonde Total Precipitable Water (TPW) climatology (source), the TPW value of 0.04 inch at 12 UTC on 10 January was very close to the record low value (0.03 inch) for that date/time (below).

Green Bay WI rawinsonde Total Precipitable Water climatology [click to enlarge]

Other examples of GOES water vapor imagery sensing the surface in a cold/dry air mass: Feb 2020 | Jan 2019 | Dec 2019.

SAR data for winds and ice over the Great Lakes

January 10th, 2022 |
Radarsat Constellation Mission (RCM) observations over Lake Superior, 11:59 UTC on 10 January 2022 (Click to enlarge)

Data from this site shows SAR observations over the Great Lakes daily around 0000 and 1200 UTC. The image above shows SAR data over Lake Superior just before 1200 UTC on 10 January. The background flow used in processing shows strong northwesterly winds. Note the relative calm in the lee of the Keewenah peninsula, and an interesting boundary in the winds near Michipicoten Island. As noted in this blog post from December, the strongest winds are likely associated with enhanced Lake-Effect bands, as enhanced vertical mixing in those bands will allow stronger upper level winds to mix down to the lake surface.

Does ABI imagery show enhancements in the regions where the SAR data indicates enhanced mixing with convective bands? Consider the 3.9 µm image below (from this NOAA/STAR website) from 1201 UTC. Cold cloud tops northeast of Marquette MI do correlate well with the strong winds in that regions.

GOES-16 Band 7 (3.9 µm) at 1201 UTC on 10 January 2022 (click to enlarge)

Scatterometry can also be used to measure winds on the lake surface. The imagery below (from this website) shows vectors from the Chinese HY-2B scatterometer at 1330 UTC. Spatial resolution for this imagery is much coarser, and observations closer to shore do not occur. Northwest winds of at least 30 knots are indicated however.

HY-2B scatterometry at 1310 UTC, 10 January 2022 (Click to enlarge)

A careful observer of the SAR winds above might notice very strong winds in/around Little Bay de Noc, the northeastern part of Green Bay. Care must be taken to differentiate between ice and winds in regions where ice is present, as SAR data can be also used to identify regions of ice. The toggle below of NOAA-20 True and False color imagery over the western Great Lakes (from the VIIRS today website) does show cyan regions — typical of ice — over northeastern Green Bay. (Click here for highest resolution False Color imagery from NOAA-20 on 9 January)

NOAA-20 True and False color over the Great Lakes, 9 January 2022 (Click to enlarge)

What kind of wave heights are these strong northwesterly winds generating over Lake Superior? Altimetric data from SMAP, below, (source) shows 6-8 foot waves over western Lake Superior. The longer fetch for the region northeast of Marquette probably means much higher waves there.

Wave Heights (feet) at ~0315 UTC on 10 January 2022 (click to enlarge)