SAR winds and Lake-Effect cloud bands

December 17th, 2021 |
SAR winds from satellite RCM2 at 23:48:58 UTC on 16 December 2021 (Click to enlarge)

Synthetic Aperture Radar (SAR) winds in select small domains are routinely available (with good latency, i.e., within 2 or 2-1/2 hours) at this website. Coverage over the Great Lakes typically occurs within an hour of 0000 UTC and 1200 UTC on each day. The image above (direct link) shows derived SAR winds (from the RCM2 satellite) over Lake Superior at 2348 UTC on 16 December 2021. Winds over Lake Superior are around 40 knots; weaker wind speeds are indicated in the lee of Isle Royale, the various Apostle Islands, Upper Michigan’s Keewenaw Peninsula, near Marquette Bay, and right offshore Minnesota. Fingers of stronger winds extend east-southeastward from just off the Minnesota shoreline. This small horizontal variability in the wind speeds is too small to be detected by other microwave detectors (such as ASCAT on Metop-B and Metop-C).

RCM2 SAR Winds at 2348 UTC, 16 December (left) along with annotated GOES-16 ABI Band 7 (3.9 µm) imagery at 2346 UTC, 16 December 2021 (right); arrows suggest the same structures in both figures (Click to enlarge)

The toggle above shows the winds with a time-matched image of GOES-16 Band 7 (3.9 µm) data (created using Geo2Grid), stretched to enhance low-level clouds. The second toggle includes best-guess location-matched arrows of features in the SAR winds and the GOES-16 ABI brightness temperature. That figure is reproduced below with an AWIPS display of GOES-16 3.9 µm imagery at 2346 UTC (again with a stretched colortable to emphasize low-level temperature contrast).

RCM2 SAR Winds at 2348 UTC, 16 December (left) along with annotated GOES-16 ABI Band 7 (3.9 µm) imagery at 2346 UTC, 16 December 2021 (right); arrows point to similar structures in both images (Click to enlarge)

GOES-16 satellite imagery over Lake Superior will have a parallax shift because the location is far from satellite nadir. Parallax shift is related to cloud-top heights, and derived Cloud-Top Heights (a Level 2 product) over the region show cloud tops between 3000 and 5000 feet near Minnesota, rising to about 7000 feet over Lake Superior in between Minnesota and the Keewenaw Peninsula. This parallax shift means that features will be displayed to the north and a bit to the west of their true location, displaced away from the GOES-16 sub-satellite point at 0oN, 75.2o W.

GOES-16 Derived Cloud Height, 2346 UTC on 16 December 2021 (Click to enlarge)

These images suggest that lake-effect clouds are regions of enhanced wind speeds. The inferred convective roll vortices present in the satellite imagery are also regions of enhanced convergence and upward moisture transport.

ASCAT winds versus SAR winds

November 22nd, 2021 |
ASCAT winds from Metop-B, 0616 and 1826 on 22 November 2021 (Click to enlarge)

From an email came this question: RADARSAT vs ASCAT winds, what are the differences between the two methods?

This comparison is not easy to make directly, as the orbits of Metop-B and Metop-C, the two satellites that carry the ASCAT instrument (now that Metop-A, which satellite also carried ASCAT, has been decomissioned), don’t sample the ocean at the same time/location as RADARSAT. The toggle above shows ASCAT winds from Metop-B (Metop-B orbits on 22 November 2021 are here, from this website) at 0631 and 1826 UTC on 22 November (from this source) in the region around Haida Gwaii (once known as the Queen Charlotte Islands). An obvious frontal passage occurred between those two times; this is also shown in the animation of surface charts (every 3 hours from 0600 through 1500 UTC shown here).

Imagery below shows SAR winds from RCM3 and RCM1 (RCM = RADARSAT Constellation Mission) at 02:23 (top) and around 15:03 (bottom) UTC on 22 November. The 15:03:52 image that follows the two images at bottom is here.

RCM3 RADARSAT SAR winds at 02:23 UTC on 22 November 2021 (Click to enlarge)
RCM1 RADARSAT SAR winds, 15:02:50 – 15:03:13 on 22 November 2021 (Click to enlarge)

How does scatterometery measure winds? If wind speeds over the ocean (or a lake) are very light, the water surface will be smooth. Microwave energy from a side-looking radar (ASCAT and SAR are both active radars; that is, they emit a ping and listen for a response) will reflect off it, and not scatter back to the instrument. As winds increase, small ripples develop and backscatter increases. Backscattered energy is greatest if the radar look and the wind direction are aligned; also, the backscatter is greater if the wind is blowing towards (vs. away from) the satellite. This is a source of ambiguity in direction. The backscatter distribution sensed has a name: Normalized Radar Cross-Section (NRCS); many different wind speed/direction combinations can produce the same NRCS. How can you mitigate these ambiguities?

For ASCAT instruments, the ambiguity is reduced through multiple measurements of the same surface — this gives NRCS values with different aspect and incident angles. Multiple measurements are achieved via the multiple antennas that are part of the ASCAT instrument (similarly, rotating beams on instruments such as AMSR-2 give multiple observations). Multiple observations allow for an accurate estimate of wind direction given the observations.

SAR processing mitigates the ambiguities by using numerical model output that suggests the correct wind direction. A challenge is that numerical model data has a far coarser resolution than SAR data. (Model data might also include errors!) As a result, artifacts can be introduced, and a good example is shown in the 02:23 UTC image above at 54.6 N, 131.8 W. In that region, where the windspeeds have an hourglass shape, the model wind direction is unlikely to be consistent with the observations. Keep that in mind when observing SAR winds.

One other aspect of the ASCAT v. SAR wind comparison bears notice: ASCAT winds have a typical upper bound, at around 45-50 knots. At stronger wind speeds, the backscatter to the ASCAT instrument is affected by foam on the sea surface that typically accompanies such strong winds. Special SAR wind processing (as discussed here) allows for observations of much stronger winds, as shown for 2020’s Hurricane Laura, where Seninel-1 SAR observations peaked at 150 knots! These computations use cross-polarization observations from SAR. Both SAR and ASCAT use co-polarization observations. Future ASCAT missions will support cross-polarization observations.

Some of the information above came from this link (specifically, here). If there are errors in the description, they’re this blogger’s fault however.

Wind and waves over Lake Superior

November 12th, 2021 |
RCM1 RADARSAT winds over Lake Superior, 11:59:40 UTC on 11 November 2021 (Click to enlarge)

A vigorous extra-tropical cyclone moving towards the western Great Lakes on 11 November 2021 generated significant winds and waves over Lake Superior. RADARSAT winds, above, from RCM1, (available at this website) show a large area of southeasterly winds between 30 and 40 knots at 1159 UTC on 11 November. Regions of lighter winds are present to the lee of both Isle Royale and Michicipoten Island. The animation below steps through the low-level water vapor and clean window infrared channels (Band 10 at 7.34 µm and Band 13 at 10.3 µm, respectively) and the airmass RGB. All three ABI products suggest a negatively-tilted trough moving into the western Great Lakes. Click here to see an animation of 3-hour surface plots from 0900 UTC 11 November through 0600 UTC on 12 November (also shown at the bottom of this post).

GOES-16 ABI Band 10 (7.34 µm), 13 (10.3 µm) and Airmass RGB, 1201 UTC on 11 Nov 2021 (Click to enlarge)

How do the SAR winds above compare to observations? Open-lake observations are scarce during Gales, and the mid-lake buoys (45028, 45006 and 45001, for example) have been retrieved for the season. However, Stannard Rock Lighthouse observations show sustained east-southeast winds near 40 knots at around 1200 UTC, as shown in the capture below, from this website (accessed from here). (Note that Stannard Rock wind observations are 35 m above the Lake surface)

Stannard Rock winds observations, 1100-1210 UTC on 11 November 2021. The format of the data is, from left to right: Year, Month, Day, Hour, Minute, Wind Direction, Wind Speed (m/s), Wind Direction, Peak Gust (m/s), Time of Gust.

At 1636 UTC, JASON-2 overflew eastern Lake Superior, and Altimetry data (from this website) from that satellite was used to compute wave heights, shown below. Peak Significant Wave Heights (defined as the average of the highest 1/3rd of all waves) under the satellite were in the 11-12 foot range. (Click here for a quick brief on JASON Wave Heights). Lake Superior Wave Heights at 1600 UTC (from this website (linked off here) maintained by GLERL) show rough agreement with the JASON observations.

JASON-2 Significant Wave Heights, 1624 UTC on 11 November 2021 (Click to enlarge)

GOES-16 Data at the time of the JASON overpass, below, shows the continued development/amplification of the extratropical cyclone. Note in particular the feature over western Iowa/eastern Nebraska that has dropped down the western side of this developing storm since 1201 UTC imagery, above.

GOES-16 ABI Band 10 (7.34 µm), 13 (10.3 µm) and Airmass RGB, 1626 UTC on 11 Nov 2021 (Click to enlarge)

At 2356 UTC on 11 November, RCM2 overflew western Lake Superior, and data at that time shows southwesterly winds between 25 and 30 knots over western Lake Superior. GOES-16 data at the same time, below (and surface data, bottom) show a mature storm to the north of Lake Superior. Data from the Devils Island C-MAN station (link) (see below) show sustained winds at 24 knots with gusts to 34 knots. (Note that the instruments for this reading are 25 m above ground)

RCM2 RADARSAT winds over Lake Superior, 23:56 UTC on 11 November 2021 (Click to enlarge)
GOES-16 ABI Band 10 (7.34 µm), 13 (10.3 µm) and Airmass RGB, 2356 UTC on 11 Nov 2021 (Click to enlarge)
Observed winds from C-MAN site DISW3 (Devils Island, Wisconsin), 1400 UTC 11 November – 1300 UTC 12 November 2021

Airmass RGB imagery from 1201, 1626 and 2356 UTC, below, the times of the wind and wave observations above, show the piecewise development of this system. Surface analyses for this system, from 0900 UTC on 11 November through 0600 UTC 12 November, are shown at bottom.

GOES-16 Airmass RGB at 1201 UTC, 1626 UTC and 2356 UTC on 11 November 2021 (Click to enlarge)
Surface Analysis, 0900 UTC 11 November to 0600 UTC 12 November 2021

Severe weather over northeast Illinois

August 9th, 2021 |
GOES-16 “Red”Visible (0.64 µm) Imagery, 1900 UTC 9 August – 0040 UTC 10 August 2021

Severe thunderstorms developed over northeast Illinois late in the afternoon on 9 August, and a series of tornadoes resulted. Storm Reports (from the Storm Prediction Center) are shown below. The mp4 animation above (click here for a large animated gif) shows 1-minute GOES-16 Mesoscale Sector 2 visible imagery (0.64 µm) from 1900 UTC on 9 August through 0040 UTC on 10 August. The active convection is apparent.

SPC Storm Reports from 9 August 2021 (Click to view site at SPC)

Part of the region hit by severe weather is in a persistent drought, as shown below (an image from this website). Rains that accompanied the severe weather provided some drought relief. (Click here for hourly CMORPH2 precipitation estimates from RealEarth)

Drought Intensity over the Midwestern United States, 3 August 2021 (Click to enlarge). Portions of southeast Wisconsin and northeast Illinois are under Severe Drought (Orange Enhancement)

RCM3 (RADARSAT Constellation Mission 3) Synthetic Aperture Radar (SAR) winds from a 2340 UTC overpass on 9 August (from this site) show a wind feature over Lake Michigan associated with the convection. These wind estimates might be affected by ice within the glaciated clouds

RCM3 SAR esimates of wind speed, 2340 UTC on 9 August 2021 (Click to enlarge)