Mysterious Gravity Wave Over the Eastern Pacific Ocean
We received the following email from Ken Waters of the National Weather Service forecast office in Phoenix, Arizona:
I noticed something interesting in this morning’s visible imagery off the Baja California coast.
Here’s a link: https://docs.google.com/open?id=0B2ktDMIN5qWfODI2OTYzYjgtZjJkMS00MTU5LTk2ODctYzdhNzY5M2Y2MWIx
I’m looking at the apparent wave pattern that’s going “upstream” towards the northeast whereas the low level flow is mostly towards the south. Is that a gravity wave? If so, what causes it? The vis can only go so far back so I looked at the IR and couldn’t find anything obvious.
Great question Ken — it certainly appears to be some sort of internal gravity wave, but what caused it and why was it propagating against the ambient flow shall remain a bit of a mystery until we can dig into this case a bit further. One more event for the “What the heck is this?” blog category.
GOES-15 6.5 µm water vapor channel images + GOES-15 0.63 µm visible channel images
McIDAS images of GOES-15 6.5 µm water vapor channel data (prior to daylight) and then GOES-15 0.63 µm visible channel data after sunrise (above) on 14 November 2011 tell us one thing: this gravity wave was apparently fairly deep in the vertical, since it exibited a signal on both the water vapor channel imagery (which generally senses radiation from the middle troposphere: San Diego 12:00 UTC rawinsonde water vapor chanel weighting function profile) as well as on the lower-tropospheric cloud features seen on the visible channel imagery.
Note that there was a second packet of shorter-wavelength gravity waves that could be seen in the far southwestern portion of the GOES-15 visible image satellite scene toward the end of the animation. This second packet of gravity waves was very evident on a 500-meter resolution Aqua MODIS Red/Green/Blue (RGB) true color image at 21:21 UTC (below).
Aqua MODIS true color image
Gravity waves are usually ducted within a well-defined temperature inversion. A look at the 12:00 UTC rawinsonde profile from San Diego, California (below) did indicate the presence of a few inversions that might have been capable of ducting such a gravity wave — but the inversions existed at multiple levels.
San Deigo, California 12:00 UTC rawinsonde profile
An AWIPS image of 18:00 UTC MODIS 0.65 µm visible channel data with overlays of 1-hour interval MADIS satellite winds (below) did not reveal any atmospheric motion vectors with a southwesterly component – but these would likely have been rejected by the winds quality control algorithms, since such a motion would have differed too greatly from the model first guess wind fields at 850 hPa, 500 hPa, 300 hPa, and 250 hPa.
MODIS 0.65 µm visible channel image + MADIS 1-hour interval satellite winds
Regarding the effect of the gravity wave seen on the lower-tropospheric clouds bands, a MODIS 11.0 µm IR image detected cloud top IR brightness temperatures around +4ºC, which on a RUC model sounding at that location apparently corresponded to a cloud top height around 12,550 feet (below) — however, this value seemed to be a bit high judging from the appearance of the cloud band features on the GOES and MODIS visible and IR imagery.
MODIS 11.0 µm IR image + RUC model sounding
On the other hand, POES AVHRR Cloud Type and Cloud Top Height products indicated that these low-level cloud bands were water droplet clouds, with cloud top heights of around 1 km (below) — much more typical for marine boundary layer cloud features over this region.
POES AVHRR Cloud Type product
POES AVHRR Cloud Top Height product