Friday, April 5, 2013
Thunderstorms in Seattle are very rare. Seattle doesn’t tend to get much in the way of ‘severe’ weather, where ‘severe’ is defined to be tornadoes/hail/rain/wind etc. from strong thunderstorms. When we do get thunderstorms though, it’s always fun to see, and we get occasional storms that drift westward over the Cascade crest in the summer. These storms don’t usually bring much in the way of precipitation, but they can make for a fantastic light show.
First, a recap of today – we saw some fairly heavy rain come through Western Washington this morning, but the Seattle area was largely shadowed due to the Olympics. Our prevailing winds associated with storms coming off the Pacific are from the southwest, so places northeast of the Olympics like Sequim often enjoy dry and sometimes even sunny weather while it is pouring 60 miles to the southwest. When the upper level flow is westerly, however, places directly to the east of the Olympics end up getting shadowed. That’s why Seattle has been drier this morning and afternoon afternoon than I originally thought it would be. Quite a bit of rain fell in the super early morning hours though… almost a half inch as of noon today.
The picture below shows the prominent shadowing over the Seattle metropolitan area and points north this morning. When the flow is perpendicular to the Cascades as it is in this radar picture, the mountains are the most efficient at forcing air to rise, and there is a lot of orographic precipitation on the west slopes. The east slopes are completely dry.
Another system is currently coming into the area right now, and you can see it on the latest radar image and the latest visible, infrared, and water vapor satellite images.
|01:00 pm PDT Fri 05 Apr 2013: UW West Coast 2 Visible Satellite|
|01:00 pm PDT Fri 05 Apr 2013: UW West Coast 2 Infrared Satellite|
|01:00 pm PDT Fri 05 Apr 2013: UW West Coast 2 Water Vapor Satellite|
Now, I usually don’t show all these satellites, but since I have time, I thought I’d give you a brief rundown of the differences between the three.
The first satellite image I posted is known as a visible satellite image. The idea behind this type of imagery is pretty simple; a satellite takes a picture of what the surface of the Earth looks like at any given time. Visible satellite images are great because they show exactly what the clouds look like and they can do this in pretty high resolution. I’ll often show visible satellite pictures when we have a showery pattern and/or convergence zone over our region because they are the most effective at illustrating the finer details of the clouds over our area. One caveat is that these these images are useless at night since there is no sunlight over the area. It would be really cool if we equipped a satellite with some night-vision goggles to show us what the clouds looked like then.
The good thing is that we have done that! The second type of imagery I posted is infrared satellite imagery. All objects emit radiation proportional to their temperature, and an infrared satellite measures the amount of radiation emitted from an object and converts it to temperature. Infrared imagery is extremely useful because it tells us the temperature of a cloud. In this color-enhanced picture, the coldest cloud tops are in color, and the warmer ones are in grey. One interesting thing to note is that the land also emits infrared radiation, and one can therefore use an infrared image to determine the temperature of the land. See how the land around 60 degree latitude line in Canada is much more gray than the Southwest? As evidenced by the visible satellite picture, there are no clouds in both regions, but since the ground temperatures in the Southwest are much warmer, it appears blacker than the northern Canada area.
A downside to infrared images is that they have trouble picking up clouds with warm tops. Visible satellite images clearly show fog, but infrared images often do not because temperatures at the tops of the fog are relatively warm compared to other clouds.
The third type of satellite image is a water vapor image. A water vapor image is a specific type of infrared image, but whereas the infrared image I described above primarily measures wavelengths in the 10-12 micrometer range, water vapor sensors on satellites measure wavelengths in the 6.5-6.9 micrometer range. Water vapor is transparent at wavelengths longer than this and on visible satellites, so that is why it doesn’t show up on the previous two satellite images. A water vapor image is used to detect the amount of water vapor in the atmosphere.
Water vapor imagery is my favorite type of imagery because it can tell us the general flow of the atmosphere, specifically the 300-600 mb level of the troposphere which is a key region for storm growth and formation. Water vapor is always present, so you can see areas where the air is dry and areas where it is humid. In the picture above, red colors represent areas with low water vapor and green areas represent areas with high water vapor. Water vapor imagery is also very useful for determining where the center of low pressure in a specific storm is, as a storm undergoing cyclogenesis carves out a “dry slot” near the area of lowest pressure. I remember a storm back on October 18, 2007 that was poorly forecast and the center of low pressure was impossible to detect on infrared or visible imagery. The water vapor image, however, showed a clear dry slot, and this was used by the National Weather Service to determine where the low actually was.
One other note: there are only two satellites that the U.S. uses for most of these images: the two geostationary satellites that are located over the equator at a height of 36,000 km (GOES-West and GOES-East). There are also polar orbiting satellites, but I’ll touch on those in a later blog. There are not separate satellites for each image; each satellite has the capability of taking visible, infrared, water vapor, and a host of other satellite images.
Alright, now a perfectly seamless transition back to thundershowers. I’m pretty slick.
We get our thundershowers when we have have unstable air over the region (i.e. a large decrease in temperature with height). Most of our thundershowers occur during spring because the upper atmosphere is very cold and the increasing solar insolation due to higher sun angles heats up the lower part of the atmosphere. Some of you may be familiar with lapse rates… as air rises, it cools at a rate of 9.8 degrees Celsius per km if it is unsaturated and 6.5 degrees Celsius per km if it is unsaturated. If the environmental lapse rate (the change in temperature in height in the environment) is higher than either of these lapse rates, an air parcel that rises will cool at a slower rate than the environment around it, and it will continue to rise because warmer air is less dense. This is what is meant by instability – the tendency for air parcels to rise.
We’ll see if we get any thunderstorms tomorrow. It’s kind of a crap-shoot, so don’t get mad at me if I’m wrong. In case you haven’t noticed, that happens a lot.