Sunday, December 12, 2010

Virga dissipates in a time lapse on 10 December 2010

My threshold for being excited at weather is really going downhill this winter.  Now I'm excited to see interesting virga.  Given the state of this year virga seems to be a lot more frequent and sometimes there are some really interesting phenomena to be seen.  Today was no exception and so I ran the camera for another event at 12 frames/minute.  I started at noon facing out of my office window to capture a turbulent appearing virga covering the sky.  I'll spend some time here trying to explain what's happening in the sky.

Photograph at 16mm focal length facing south from the NWC at 1807 UTC.

About 40 minutes later and the sky completely changed appearance with a much drier layer of altocumulus.  This picture shows how much the scene had changed.

Same view but at 1840 UTC.

This time lapse  shows how the sky changed.  Set the video resolution to at least 720p.

What are we seeing?
 The layer of virga appears relatively turbulent with regions of descending precipitation next to compensating ascending air.  This is not all the surprising since the evaporating/sublimating precipitation is probably creating small downdrafts.   The 3.5 and 4.5 deg scans from KTLX facing south appears to show evidence of this turbulence too.  The beams scanned the best turbulence around 6.5 - 9.5 kft ARL.

The 1807 UTC KTLX 4.5 deg elevation scan of velocity and reflectivity.  The two yellow lines in the left side represent the approximate horizontal field of view of the time lapse. My camera was about where the yellow lines converge.

Did the dry altocumulus surge in as if there was a cold front aloft?
What was interesting to me about the time lapse was the impression that the dry altocumulus surged in from the west.  I'm not sure that there was any change in wind speed aloft that would give this impression.  What's more likely is that the precipitating layer was moving to the east and it was followed by the drier altocumulus layer.  So just to check I pulled up this profiler image of radial velocity and horizontal winds from Purcell which is about 10 miles south of my position.  Here is that image below.
Purcell profiler radial velocity (shaded) and horizontal winds ending at 19 UTC.  The double blue line along the bottom represents the duration of the time lapse.

The time height plot doesn't show any change in winds aloft.  It's westerly from the start to finish of the time lapse which is visually represented by the double blue line at the bottom of the time height plot.  The plot also helps me explain why the virga appeared to be left behind.  While the top of the virga layer was around 14 kft according to the radar, the precipitation evaporated from 5.4 to 6.8 kft ARL.  The winds in this layer were southerly and fairly light.  So the source of the virga pushed on the east leaving the evaporating/sublimating remnants behind.

Was the virga frozen?
The big question is whether or not the virga was frozen?  The imagery certainly seems to suggest falling snow, almost like a turbulent form of mammatus.  But according to the morning and evening sounding taken from Norman, the freezing level was nearly 9 kft, or almost 3 kft above the base of the precipitation according to the radar.  I should've seen the frozen virga turn into fall streaks or something like that.  But then I noticed that the sounding was dry below the freezing layer and the resulting wetbulb zero height was more like 4.8 kft MSL in the 12 UTC observation (first image below).  That value rose to 5.8 kft MSL in the evening sounding.  Subtract roughly 1.1 kft for the radar height and I can easily see snow flakes surviving well below the freezing level.

How representative were the morning and evening soundings to the time of the time lapse?
There is an interesting parallel between the morning sounding and the first picture and then the evening sounding and the second picture (end of the time lapse).  The morning sounding shows a much deeper saturated layer.  If that saturated layer extended down to somewhere around 8-9 kft AGL along the lines of the top of the turbulent cellular echo layer detected by radar, then the sounding would agree much better with what I believe is happening at 18 UTC.  The bottom image taken at the end of the time lapse is much more similar to the evening sounding some 5 hours later.  Both the sounding and the imagery suggests a shallow, more elevated moist layer that manifested itself visually as a thin altocumulus layer.  In fact, the clouds deck at sounding time was very similar to the second image time of 1840 UTC.

What is likely causing the turbulent appearance in the virga?
There's something I said about detecting the top of the turbulent layer by radar being the top of the saturated layer.  Why choose that layer to assume that's the top of the saturated layer?  If I may conjecture, I believe that the base of the precipitating layer is subject to strong lapse rates because of the evaporational/sublimational cooling relative to the completely precipitation free layer below.  Because of this, there is significant overturning seen in the time lapse shows and inferred by radar.  This mixing layer is not likely to be saturated.  The morning sounding also shows that the layer of saturation features small lapse rates relative to the unsaturated layer underneath.  Perhaps by 18 UTC, the sounding would show the saturated layer at a lower level featuring poor lapse rates and then underneath lies the unsaturated, steep lapse rate layer.

Sounding at Norman 12 Z, 10 Dec 2010

Sounding at Norman  00 Z, 11 December 2010

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