Orographically-trapped waves near Haida Gwaii

April 22nd, 2019 |

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with topography [click to play animation | MP4]

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with topography [click to play animation | MP4]

GOES-17 (GOES-West) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) revealed orographically-trapped waves propagating westward against the ambient flow over the Haida Strait (between Haida Gwaii and British Columbia) in the wake of a cold frontal passage (surface analyses) on 22 April 2019. The waves initially formed downwind of the 2000-3000 foot terrain of Haida Gwaii, and moved eastward — but were then reflected back to the west by the higher 6000-8000 foot terrain farther inland over British Columbia.

Note that the wave signatures became more attenuated — especially over the southern portion of the Strait — as middle-tropospheric moisture began to overspread the area. This moisture at higher altitudes absorbed radiation being emitted from below, and re-radiated energy at the colder temperatures found within that layer of mid-level moisture.

A plot of GOES-17 Water Vapor weighting functions calculated using 00 UTC rawinsonde data from Annette Island, Alaska (below) showed significant contributions for Band 10 (7.3 µm, violet) and Band 9 (6.9 µm, blue) radiation coming from within the 700-850 hPa range, so it’s likely that many of the waves resided within that layer. Higher-altitude contributions from the 500-600 hPa layer were due to the aforementioned high-level moisture that later moved over the region.

GOES-17 Water Vapor weighting functions calculated from 12 UTC rawinsonde data at Annette Island, Alaska [click to enlarge]

GOES-17 Water Vapor weighting functions calculated from 00 UTC rawinsonde data at Annette Island, Alaska [click to enlarge]

In a toggle between Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images at 2137 UTC (below), cloud-top infrared brightness temperature of cloud features in the Haida Strait were generally in the -5 to -10ºC range, corresponding to altitudes of 4400-6400 feet (1.4-2.0 km, 850-780 hPa) on the 00 UTC Annette sounding. On 2140 UTC GOES-17 Water Vapor imagery, the waves were still apparent in the 7.3 µm image but were becoming less distinct in the 6.9 µm and 6.2 µm images due to the arrival of mid-tropospheric moisture.

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images at 2137 UTC [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images at 2137 UTC [click to enlarge]

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