Strong gap flow into the Gulf of Tehuantepec

November 26th, 2021 |
GOES-16 True Color imagery, 1330 – 1520 UTC on 26 November 2021

GOES-16 True-Color imagery from the CSPP Geosphere site (link showing the data above) on 26 November, above, show features associated with strong flow through Chivela Pass in southern Mexico, gap winds often called Tehuano winds or Tehuantepecers. Strong descent associated with these events can often limit the presence of clouds that can be used as tracers. However, scatterometry (from this website) will show surface winds, and an MetopB overpass shortly after the end of the animation above, below, shows a core of strong winds over the ocean.

ASCAT Winds from Metop-B, 1532 UTC on 26 November 2021 (Click to enlarge)

The GOES-16 CONUS domain extends southward to the northern part of the Gulf of Tehuantepec (about 14.6 N Latitude). Visible imagery from 1516 UTC, below, is overlain with the Derived Motion Wind vectors (in the surface – 900 mb layer) at the same time. Strong northerly winds north of Chivela Pass are apparent, but the lack of clouds to track in the Gulf prevented the inference of winds there from the GOES-16 data.

GOES-16 Visible Imagery (Band 2, 0.64 µm) and Derived Motion Winds, surface-900 mb, 1516 UTC 26 November 2021 (Click to enlarge)

The strong winds are also associated with a local increase in Aerosol Optical Depth (AOD), as shown below.

GOES-16 Aerosol Optical Depth (AOD) at 1520 UTC on 26 November 2021 (click to enlarge)

Strong winds will cause significant mixing in the upper part of the ocean, which will result in cooling. Imagery from this website (shown below) shows cooling in the Gulf from previous events. Here is an animation from that website, courtesy Tim Schmit, NOAA/NESDIS/STAR

SST analysis valid at 24 November 2021 (Click to enlarge)

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GOES-17 True Color RGB images [click to play animated GIF | MP4]

In GOES-17 True Color images created using Geo2Grid (above), enhanced forward scattering during the morning hours helped to highlight the offshore transport of airborne dust.

Other blog posts discussing Tehuano wind events can be found here.

Blowing dust in Argentina

November 24th, 2021 |

GOES-16 “Red” Visible (0.64 µm), Dust RGB and Split Cloud Top Phase (11.2 µm – 8.4 µm) BTD images [click to play animated GIF | MP4]

30-second Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm), Dust RGB and Split Cloud Top Phase (11.2 µm – 8.4 µm) brightness temperature difference (BTD) images (above) revealed a plume of blowing dust propagating northward across the San Juan Province of western Argentina late in the day on 24 November 2021. The dust was being channeled through a gap in higher terrain along the foothills of the Andes (below).

GOES-16 “Red” Visible (0.64 µm) and topography images [click to enlarge]

A larger-scale view of hourly GOES-16 Visible images with plots of surface reports (below) suggested that this dust occurred in the vicinity of a strong cold front that was moving northward across Argentina.

GOES-16 “Red” Visible (0.64 µm) images, with METAR surface reports plotted in cyan [click to enlarge]

Mendoza — located south-southwest of where the dust plume first became apparent in GOES-16 imagery — reported a thunderstorm with dust at 2000 UTC (along with a southeasterly wind gust to 32 knots), followed by a reduction of surface visibility to 0.5 miles at 2215 UTC as the air temperature sharply dropped with the cold frontal passage (below). About 260 miles (420 km) east of Mendoza at Rio Cuarto, a similar sharp temperature drop was seen as the cold front passed.

Time series of surface data at Mendoza, Argentina [click to enlarge]

Showers over the south Pacific

November 24th, 2021 |
GOES-17 ABI Band 13 (“Clean Window”) Infrared (10.3 µm) imagery, 0000 – 1440 UTC on 24 November 2021 (Click to enlarge)

GOES-17 Infrared imagery, above, centered on American Samoa, shows several low-level cloud lines from which showers are developing (and then rapidly dying, suggestive of strong shear, as noted in this 0600 UTC shear analysis taken from this website). There are also convective elements developing over/near some of the islands. What is the moisture/stability distribution around these showers?

GOES-17 Total Precipitable Water fields, below, show American Samoa at the edge of a moist (TPW exceeding 2″) band (associated with the South Pacific Convergence Zone), dryer air (TPW is around 1.2″) to the northeast and relatively dry air to the south (TPW is around 1.3-1.4″ in patches). There is an increasing amount of noise in this Level 2 product starting around 1200 UTC, manifest as horizontal lines, that arise because of the poor functioning of the GOES-17 Loop Heat Pipe. As the ABI instrument’s focal plane’s temperature increases, bands that are used in the computation of Total Precipitable Water (including Band 15), become noisy. (Note that Band 13 on this day is not obviously affected by the increase in the focal plane temperature).

Hourly estimates of Total Precipitable Water, a cloud-free Level 2 GOES-17 product, displayed on top of GOES-17 clean window (Band 13, 10.3 µm) infrared imagery, 0000 to 1400 UTC on 24 November (Click to enlarge)

GOES-17 ABI data can also be used to estimate atmospheric stability, as shown below. Lifted Index fields (also showing Loop Heat Pipe-related striping at the end of the animation) show strongest instability in the region where showers are most common to the north of American Samoa — in the moist band. The strongest instability is over the southwestern part of this domain (the diagnosed Lifted Index there is near -5). Level 2 products from GOES-17 can give hints as to where convection will form out over the open ocean where conventional observations are sparse, even when Loop Heat Pipe issues with GOES-17 start to become obvious.

GOES-17 Lifted Index, a cloud-free Level 2 Product, plotted on top of GOES-17 Clean Window Imagery (Band 13, 10.3 µm), hourly from 0000 – 1400 UTC on 24 November 2021 (Click to enlarge)

Special note for the Lifted Index animation above: The bounds of the Lifted Index values have been changed from the AWIPS default — -10 to 20 — to -5 to 7; this was done to better differentiate between small variations in stability.

NUCAPS use over Alaska

November 24th, 2021 |
NOAA-20 NUCAPS Sounding Availability Points, 0953, 1132 and 1310 UTC on 24 November 2021; the 1132 UTC imagery also shows 500-mb plots from 1200 UTC Radiosondes. (Click to enlarge)

A useful strength of NUCAPS profiles over Alaska is that at that northern Latitude, sequential overpasses will overlap so that one location will be sampled sequential times. In the example above, note for example that portions of Canada’s Northwest Territories are sampled three times. Much of southern Alaska is sampled twice. Thus, NUCAPS profiles there allow a user to ascertain routinely how the atmosphere is changing over 90 minutes, usually at times surrounding radiosonde observations at 1200 UTC (and 0000 UTC).

The animation below shows NUCAPS soundings and the upper air sounding at CYVQ (Norman Wells) in the Northwest Territories, at the edge of the Sounding Availability Plots imagery above. There is warming between the two NUCAPS profiles, and gross aspects of the NUCAPS and radiosonde profiles agree (for example: Tropopause Height, low-level inversion). Always remember that a NUCAPS profile is representative of a volume of air; radiosondes sample individual points as they ascend.

NUCAPS Profiles near Normal Wells, NWT, at 1000 and 1121 UTC, and the 1200 UTC CYVQ Upper Air sounding (click to enlarge)

The plots below compare the 1200 UTC PAFC (Anchorage AK) soundings with nearby NUCAPS soundings. General agreement here is better: Tropopause heights are similar, a low-level inversion is present, as is general drying with height. Note how the sequence of PAFC sounding and the two NUCAPS soundings show a slow lower-tropospheric warming trend.

The 1200 UTC PAFC (Anchorage AK) soundings, and two NUCAPS Profiles near Anchorage, at 1132 and 1301 UTC (click to enlarge)

Use the daily overlap of NUCAPS soundings to give yourself a twice-daily estimate of how the troposphere is changing over Alaska. Gridded NUCAPS fields (not shown) will also overlap and can also be used in this way. NUCAPS are an observational product that is largely independent of model data.