Tuesday, October 25, 2011

What A Difference A Day Makes

Take a look at this radar image from yesterday (top) at around 10 a.m., as compared to 10 a.m. today at the same time (bottom). At this time yesterday we were under a large precipitation shield hundreds of kilometers across associated with a “mesoscale convective system”. This precipitation shield is nicely shown from the S-Pol radar using the “centimeter wavelength” radar band that senses the presence of precipitation-sized particles including rain drops, snow, etc. Today, not much is going on.

The radar sends out radiating waves that have a trough and a crest and a trough about every 10 centimeters or so, which produces a return echo to the radar if a precipitation-sized particle is encountered. Birds and insects can also detected, and it has been noticed that sea birds tend to ride the leading edge of gust fronts generated by precipitation systems. The S-Pol allows beams of two different polarizations to be sent out, that allows both better sensing of precipitation, as well as determination of the types of particles encountered (e.g. hail vs. graupel vs. insects). S-Pol also includes a millimeter wavelength radar, which can sense smaller sized particles like drizzle and clouds droplets. I might talk about this instrument in a future post.

In addition to the S-Pol radar, we also have a precipitation radar at Addu Radar from Texas A&M called SMART-R, which is mounted on a truck and hence is more mobile. The wavelength for SMART-R is about 5 centimeters, which allows the radar to detect smaller particles (and also get blocked by smaller particles, so that you can’t see as well behind the clouds/precipitation detected). The SMART-R doesn’t do dual polarization.

Visual snapshots verify the vastly different cloud conditions between yesterday and today. The first image is from yesterday taken by the camera (facing east) on the S-Pol radar site. The second was taken by me this morning. The tropical field program veterans I’ve been talking to on this trip cite the tendency for there to be a 2-day cycle of precipitation in the tropics alternating between dry and wet days. It is hypothesized by these folks that a 2-day time period is necessary for the lower atmosphere (called the boundary layer) to recover from the previous precipitation event. This would imply that tomorrow will be wetter.

2:30 p.m addendum: Convection starting to develop like popcorn near us:

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