A Tropical Winter Wonderland

It’s not every day the official National Weather Service forecast from Honolulu, HI calls for freezing rain and snow in parts of Hawaii, but that’s what happened 19 February 2012 for the Big Island. Mauna Loa and Mauna Kea both received snow over the last 24 hours, and VIIRS shows both volcanoes are covered with the white stuff.

True color image of the island of Hawaii from VIIRS, taken at 23:16 UTC, 19 Febraury 2012

True color image of the island of Hawaii from VIIRS, taken at 23:16 UTC, 19 Febraury 2012

A false color image of the island of Hawaii taken from VIIRS at 23:16 UTC, 19 February 2012

False color (or "pseudo-true color") image of the island of Hawaii from VIIRS at 23:16 UTC, 19 February 2012

The top image is an RGB composite of moderate resolution channels M3 (blue), M4 (green) and M5 (red). As these channels observe radiances in the blue (0.488 μm), green (0.555 μm) and red (0.865 μm) portions of the visible spectrum, respectively, the image represents what the human eye sees, and is thus a “true color” representation.

The bottom image is an RGB composite of high resolution channels I-1 (blue), I-2 (green) and I-3 (red). These channels observe radiances centered on 0.64, 0.86 and 1.61 μm, respectively. As a result, differences in the optical properties of liquid droplets and ice particles at these wavelengths allow liquid and ice clouds (and snow) to be distinguished more easily. Liquid clouds appear white (or a dirty, brownish-white), while ice clouds appear blue. The snow on top of Mauna Loa and Mauna Kea stands out as a deeper blue. Vegetation still shows up as green, and barren ground as brown, producing an image that may be called “pseudo-true color”.

Both peaks, which are more than 13,600 ft above sea level, receive snow during most winters. The Mauna Loa observatory, at 11,100 ft above sea level, averages 3.7 inches of snowfall each year. While there are many observatories on Mauna Kea with weather stations, none of them seem to collect routine snowfall information. You can find the latest webcam imagery of Mauna Kea by clicking here to find out how much snow there is in the tropical paradise now. Aloha!

Tropical Cyclone Giovanna

Back in January, Madagascar was brushed by tropical cyclone Funso, which caused periods of heavy rain, but was a bigger deal for neighboring Mozambique. This time around, Madagascar took a direct hit from tropical cyclone Giovanna, which reached “Super Cyclone” status as category 4 storm just prior to making landfall.

VIIRS got a great look at Giovanna while it was a category 4 storm.

Visible image of Super Cyclone Giovanna

A visible image of Super Cyclone Giovanna from VIIRS channel I-1 taken at 09:47 UTC, 13 February 2012 (Thanks to Dan Lindsey)

IR-window image of Super Cyclone Giovanna

IR-window image of Super Cyclone Giovanna taken by VIIRS channel I-5 at 09:47 UTC, 13 February 2012 (Thanks to Dan Lindsey)

Fortunately, Giovanna weakened rapidly upon making landfall. Moving east to west across the center of the island and over the mountains of central Madagascar, Giovanna could not maintain its Super Cyclone status. At the time of this post, Giovanna was located over the west coast of Madagascar and down to sustained winds of 35 knots, making it a weak tropical storm. Early reports suggest between 2 and 5 people were killed, and some towns on the east coast had 60% or more of their buildings damaged or destroyed.

All things considered, the situation could have been much worse, considering residents were only given a half a day’s warning, and the primary method for disseminating weather warnings is through the use of town criers.

A squall line over Texas as seen by VIIRS

VIIRS RGB "true color" composite

A severe squall line formed over eastern Texas on 25 January 2012. There were 19 tornado reports and 48 reports of wind damage, including “a house destroyed by a possible downburst”, according to the Storm Prediction Center. The high resolution imager on VIIRS captured this squall line as it was rapidly intensifying. Shown below are images collected from channel I-5, the high-resolution infrared window channel (11.45 μm). (Click on images for full resolution.)

VIIRS Channel I05

A squall line over eastern Texas observed by VIIRS channel I05 (11.45 um) at 19:24 UTC on 25 January 2012.

This squall line had several overshooting tops over the Gulf of Mexico that reached a temperature of -77 C. A zoomed-in view of these tops are shown below.

VIIRS Channel I05

A squall line over eastern Texas observed by VIIRS channel I05 (11.45 um) at 19:24 UTC on 25 January 2012.

The dark blue pixels near the center of the image indicate an overshooting top approximately 5 km in diameter where temperatures were less than -77 C. Several pixels in a storm top at the bottom center of the image and in a storm top at the top center of the image (near Galveston, TX) also reached that temperature.

A sounding was taken at 18:00 UTC at the Lake Charles, LA, National Weather Service (NWS) office, which observed a minimum temperature of -74 C at 17.9 km above sea level, indicating that these are some tall thunderstorms. Image courtesy the University of Wyoming.

Radiosonde sounding

NWS sounding taken at 18:00 UTC from the Lake Charles, LA office.

The VIIRS imagery was collected right as the squall line was intensifying. Shown below is the radar loop from the Houston/Galveston radar between 18:00 UTC and 21:00 UTC. Note, at the beginning of the loop, the southern end of system consists of two rather disorganized lines of cells. These lines of cells merge at around 19:25 UTC (the time of the Suomi NPP overpass), and a much stronger and more organized squall line develops.

Radar loop

Radar loop from the Houston/Galveston NWS WSR-88D radar beginning at 18:00 UTC, 25 January 2012.

At roughly 375-m resolution at nadir, the I-5 channel on VIIRS is providing some of the highest resolution infrared imagery available to the atmospheric science community. We are just beginning to see the capabilities of this powerful instrument.