Exploring the effects of GOES-17 parallax over Alaska
GOES-17 (GOES-West) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) displayed the formation of an orographic rotor cloud downwind (north-northeast) of the Kigluaik and Bendeleben Mountains in the Seward Peninsula of Alaska on 27 June 2020. Even though the highest terrain in those mountain ranges was only 3700-4700 feet (1.1-1.4 km), the coldest cloud-top infrared brightness temperatures within the rotor cloud feature were around -50 to -51ºC.A plot of rawinsonde data from Nome (below) showed the strong southwesterly winds that existed within most the troposphere on that day. The tropopause temperatures were around -51ºC at altitudes of 9.4-9.6 km — indicating that these high-altitude rotor clouds were forced by vertically-propagating waves initiated by interaction of the anomalously-strong southerly/southwesterly lower-tropospheric flow with the west-to-east oriented mountain ranges.
Comparisons of topography and Visible/Infrared images from Suomi NPP and GOES-17 around 1320 UTC and 2140 UTC are shown below. Since there is generally very little parallax offset associated with imagery from polar-orbiting satellites (such as Suomi NPP), the rotor cloud appeared closer to the topography that helped to force development of that cloud feature. Plots of GOES-17 parallax correction vectors and displacements (in km) for a 30,00-feet (9.1 km) cloud feature at select points over the Alaska region (from this site) are shown below. For such a cloud feature over the Seward Peninsula, the parallax offset would be about 40 km (25 miles) — which closely corresponded to the offset seen between the GOES-17 and Suomi NPP images shown above.