The active phase continues to live up to our hopes. Today I focus on the weather this morning in Addu. Early this morning a strong, very coherent line of convective cells went through. Here it is on S-Pol reflectivity map for 0117 UTC (6:17 AM local time).
This line was moving roughly parallel to itself, that is, southwest to northeast, which was the way the wind was blowing at low levels.
When we went to work in the morning a little after 8, this line had broadened (or a new one had developed and merged with it) and when we got to the S-Pol site about 8:30 (0330 UTC), it was pouring very hard. The reflectivity map looked like this (S-Pol is at the center):
The wind was howling as well. To quantify this, here is a plot of doppler radar velocities measured along a cross-section pointing roughly east-northeast - that is, down the axis of the line. The x axis is range from the radar along that direction, the y axis is height, and the color gives the speed of the wind according to the color bar scale on the right, in meters per second. The radar measures only the component of the wind velocity towards or away from the radar. The strong red blob at the lower right of the plot tells you that the winds at the surface were greater than 20 m/s (~45 mph).
The strong decrease in wind speed as you move right from the low-level wind maximum around 20km range indicates very strong low-level convergence. Convergence is a technical term that means exactly what it sounds like - flow converging like water does when it goes down the drain. Just as in that case, where the water can't disappear so it has to be going somewhere in the third dimension (down the drain), in this case there is no drain so the flow has to be going up. This convergence is consistent with a very strong updraft.
The rain gauge at Gan (about 7 miles away from S-Pol) showed rain rates instantaneously above 100 millimeters per hour, which is pretty respectable. The accumulation for the event (not shown) was around 30 mm.
What was interesting about this is that the rain appears to have been mostly "stratiform" by which we mean, forming from thick continuous layer clouds that usually have modest updraft speeds, rather than intense convective cells with strong updrafts and less horizontal homogeneity. A classic indicator of the stratiform nature is the "bright band" apparent on this radar cross-section. This plot is similar to the one above of velocity, but it shows reflectivity instead, vs. range and height.
The bright band is the thin layer of high reflectivity around 4 km, which is just below the "melting level" - the level at which the atmospheric temperature is zero degrees C. At this point frozen hydrometeors (snow etc.) are melting as they fall. They reflect the radar beam very effectively for a short time until they melt more as they fall further into warmer air below. Commonly stratiform rain is lighter than convective, but clearly that needn't always be the case.
PS: For those of you who don't recognize the quote in the title of this post, try this. No marine mammals were harmed in the production of this blog.
No comments:
Post a Comment