Train of standing waves south of Hawai’i
* GOES-17 images shown here are preliminary and non-operational *
GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) revealed an interesting train of standing waves about 100-150 miles south of the Big Island of Hawai’i on 25 November 2018. With the presence of moisture aloft, the 3 water vapor weighting functions — calculated using the 00 UTC Hilo sounding — were shifted to high enough altitudes to eliminate the sensing of radiation from features in the lower troposphere. There were no pilot reports of turbulence in the vicinity of these standing waves — but they were located outside of the primary commercial air traffic corridors to/from the islands.
GOES-17 “Clean” Infrared Window (10.3 µm) and Near-Infrared “Cirrus” (1.37 µm) images (below) showed that these wave clouds were radiometrically transparent to longwave thermal energy being emitted from/near the surface — note that marine boundary layer stratocumulus clouds could be seen drifting westward within the easterly trade wind flow. As a result, the satellite-sensed 10.3 µm infrared brightness temperatures of the standing wave clouds were significantly warmer than that of the air at higher altitudes where they existed. These standing wave cloud features were, however, very apparent in 1.37 µm Cirrus imagery, along with what appeared to be other thin filaments of cirrus cascading southward overhead. The southward motion of the features seen on Cirrus imagery suggests that they existed at pressure levels of 370 hPa (26,900 feet / 8.2 km) or higher — altitudes where northerly winds were found on the Hilo sounding.A comparison of all 16 ABI spectral bands is shown below. Note that in the longwave infrared bands along the bottom 4 panels, the brightness temperatures are progressively colder (darker shades of green) on the 11.2 µm, 12.3 µm and 13.3 µm images — each of these bands are increasingly affected by water vapor absorption aloft, therefore more effectively sensing the thin layer of higher-altitude standing wave clouds. AWIPS cursor sampling showed the differences in detected brightness temperature at 3 different points along the feature: here, here and here. The increasing sensitivity to radiation emitted from higher altitudes can also be seen in a comparison of weighting functions for ABI bands 13, 14, 15 and 16. GOES-15 (GOES-West) Water Vapor (6.5 µm), Infrared Window (10.7 µm) and Infrared CO2 (13.3 µm) images (below) showed that the lower spatial resolution of the legacy GOES Imager infrared bands (4 km at satellite sub-point) was not able to resolve the individual waves as well as the 2-km GOES-17 ABI images . Also, as was seen with the GOES-17 imagery, the 13.3 µm CO2 brightness temperatures of the standing wave clouds were significantly colder (shades of blue) compared to those of the conventional 10.7 µm Infrared Window. The corresponding GOES-15 Visible imagery (0.63 µm) is also available: animated GIF | MP4. In comparisons of VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) images from Suomi NPP and NOAA-20 visualized using RealEarth (below), note the highly-transparent nature of the standing wave clouds on the RGB images; only the earliest 2256 UTC VIIRS 11.45 µm image displayed brightness temperatures of -20ºC and colder (cyan to blue enhancement). Terra (at 2043 UTC) and Aqua (at 2347 UTC) MODIS True Color RGB images along with retrievals of Cloud Phase, Cloud Top Temperature, Cloud Top Height and Cloud Top Pressure from the WorldView site (below) indicated that the standing wave feature was composed of ice crystal clouds exhibiting temperature values of -53ºC and colder (dark purple enhancement) located at heights of 12 km or higher (and at pressure levels at or above 250 hPa). These temperature/height/pressure values roughly corresponded to the upper portion of a layer of increasing relative humidity between 200-274 hPa on the Hilo sounding. However, an experimental CLAVR-x version of GOES-17 Cloud Type, Cloud Top Temperature and Cloud Top Height products (below; courtesy of Steve Wanzong, CIMSS) indicated Cirrus clouds having temperature values in the 210-200 K (-63 to -73ºC) range at heights within the 13-16 km range. These colder/higher values raise the question of whether the wave clouds might have formed and been ducted within the shallow temperature inversion near 15 km on the Hilo sounding. GOES-17 False Color RGB images (below) vividly portrayed the transparent nature of the high-altitude standing wave cloud feature, which allowed westward-moving stratocumulus clouds within the marine boundary layer to plainly be seen. The RGB components are 1.38 µm / 0.64 µm / 1.61 µm. A coherent explanation of this feature and what caused it to form remains elusive, earning it a distinguished place in the what the heck is this? blog category. Perhaps one clue existed in the winds aloft, as depicted by the NAM at 200 hPa, 250 hPa and 300 hPa (below), which showed northerly/northeasterly flow that was decelerating as it entered a trough axis (the region within the red box). Could this flow deceleration have induced a “reverse flow” which then caused enough weak lift to form the thin standing wave clouds within the aforementioned semi-moist 200-274 hPa layer seen on the Hilo sounding? No other obvious forcing mechanisms were in the immediate area — a slowly-approaching surface cold front was too far north of Hawai’i to have played a role.