The GOES-16 ABI Night Microphysics RGB monitored convective outflow boundaries from major thunderstorms in Argentina in early March 2019.
By Jochen Kerkmann (EUMETSAT)
Argentina is one of the global hotspots for intense thunderstorms (see this convection case from 2012. Geography plays a very important part in thunderstorm formation in Argentina. More precisely, the Andes mountains help to lift warm, moist air coming from the north (from the Amazon Basin), which collides with cooler, drier air, from the south, promoting the formation of powerful storms.
Apart from moisture and instability, Argentina is also a place of high wind shear, with the subtropical or polar jet streaks crossing the country, which is the third mandatory ingredient for the formation of severe thunderstorms (see GOES-16 IR10.3 image with absolute topography at 500 hPa of this case(Figure 1).
Argentinian thunderstorms, which can reach very cold cloud top temperatures (-80 to -90 deg C, 18 km height), have a long life cycle, lasting for many hours in the afternoon and into the night. They move like squall lines along the eastern side of the Andes in Central Argentina, causing floods and destruction of the vineyards in the region's booming wine industry.
Figure 2: GOES-16 infrared animation, 4 March 00:00–10:00 UTC. Source: CIRA
The animation of the IR10.3 band on the ABI instrument of GOES-16 at 15-minute intervals (Figure 2), shows the northward progression of the convective system, along the Andes mountains. The coldest clouds (overshooting tops, OTs) are denoted in black. Note the abrupt appearance and disappearance of black dots (OTs), which is due to the inadequate time sampling of the system. 15-minute imagery is not good enough to smoothly follow the development of OTs, which often have a lifecycle of just a few minutes (3-10 minutes according to the author's experience of American storms).
Unfortunately, rapid scan imagery is not available for this case. While the IR animation reveals the high-level features of the storms (OTs, plumes, gravity waves etc), the low-level features of the storms, like low-level moisture and low-level clouds (e.g. outflow boundaries), are best seen in the Night Microphysics RGB product. For the GOES-16 ABI instrument this RGB uses the difference IR12.3-IR10.3 on the red beam, IR10.3-IR3.9 on the green beam and the IR10.3 on the blue beam.
Figure 3 shows the Night Microphysics RGB at 10:00 UTC, a time when the eastern part of the image is already in daylight (see the Day-Night terminator line). Low level clouds appear with a green colour (in the night part of the image), high level clouds in dark red (thick clouds) or dark blue (thin clouds). On the north-western side of the huge convective system, several outflow boundaries can be seen, best seen in the animation (Figure 4), which move north-westward in the direction of the moist inflow air.
Figure 4: GOES-16 Night Microphysics RGB animation 00:00–10:00 UTC. Source: CIRA
Addendum: To monitor both, high-level and low-level features of thunderstorms (at night) in one image, a combination of enhanced IR10.3, and Night Microphysics RGB is very useful. The enhanced IR image is shown on the Night Microphysics background image for clouds colder than -40 deg C. An example of this is given in this case study from August 2009, see bottom right AVHRR image.