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.

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