Hawaiian Islands Lee-Wave Clouds
January 24, 2003
While checkin' out the daily weather on-line one day, I came across this interesting weather phenomenon. As I always do when I check out the current weather in the west, I started out on the NRL EPAC Satellite page, to see if there was anything interesting headed our way. As I was about to click on over to another weather website, a satellite image in the bottom corner caught my eye. I noticed some well formed Lee-Wave clouds. Now this event in and of itself isn't all that unusual, as they can be seen quite frequently in the winter over the Sierra Nevada's and Rocky Mountains. However, what was suprising was that they were occuring downwind from the the peaks on the northern Hawaiian Islands. The mid and upper level winds are usually not strong enough to support such cloud formations, not to mention, it usually takes the approach of a strong pressure system to create conditions favorable for lee-wave cloud formation. Well on this day, the conditions came together in the Hawaiian Islands, and produced numerous wave clouds on the lee side of the peaks. The clouds were particularly noticeable downwind from Oahu.
The Surface Conditions
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Notice the tight pressure gradient setting up over the northern Hawaiian Islands.
12 Z sounding out of Honolulu
Notice the presence of sustained 30-40 Knot winds at ridge top elevation. Also critical is the shallow layer of dry, stable air sandwiched between the moist air at 800mb. The stability of the atmosphere at the 800 mb pressure surface is the result of a small temperature inversion. A temperature inversion is an area where temperature increases with height in the atmosphere. Normally, temperature decreases with height in the troposphere. These conditions, combined with a few other factors invloving things such as Kelvin-Helmholtz Instibility, made for an awesome display of lee-wave clouds downwind from the Hawaiian Island chain.
Some Satellite Images: Starting with the Big Picture
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Here you can see the trough really diving south, almost to 20N! Also notice the convection forming along the leading edge of the front as it trails across Kauai.
1 km GOES 10 Visible Image
Here you can really see the lee waves lined up one after another behind Oahu, and little off of Maui.
Here's a loop that I saved from the day.
Notice that the waves are for the most part stationary. However, this does not indicate staginate air. In fact as was stated earlier, winds in those regions were around 40 knots. I have made some loops of the entire day's lee-waves, but I have not had success putting them online yet. The waves were present during the entire day, and the final images before sunset show convection firing along those lee-waves as the front hit.
Here is an Infrared Satellite Image.
This image clearly shows the approaching front and associated lee-waves. What is interesting to note is the convection beginning to fire along the leading edge of the front as it passed over Kauai. The convection would only become more enhanced as the front neared the lee-waves coming off of Oahu.
Here is a BREF image out of PHKI taken about 15 minutes after the image above.
Once again, notice the convection along the leading edge of the front. A severe thunderstorm warning was issued by the NWS Honolulu, mainly for damaging winds, as the line of convection passed over the island of Kauai.
This is what the radar looked like as the line later passed over Oahu.
Notice how much more intense the line became as it interacted with the lee-waves that lined up downwind from Oahu. As you can see, upwind from Oahu, the average reflectivity along the leading edge is only around 35-40 DBZ, while downwind from Oahu, where the lee-waves once were, reflectivity is on the order of 50-55 DBZ. There is definately some interaction occuring between the leading edge of convection and the lee-wave clouds. Very Interesting I think.
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This 1km Visible image shows the initial convective towers firing along the lee-waves about an hour and a half prior to the radar image pictured above.
Here are a few more interesting data/graphics I collected this day.
Severe T-storm Warning for Kauai & High Wind Watch for Oahu
I also saved text from this day, but I am unable to locate some of the files at this time. I do remember that winds on the islands were SW at around 25 to 35 knots all day, with higher gusts. In addition, frontal passage brought with it a slightly drier air mass and strong NW winds. The front eventually stalled out on its way to the Big Island, and Hawaiian weather returned to normal a couple days later.
SOME INTERESTING LINKS
Neat little graphic which depicts these clouds
Another Explanation of Mountain Waves