Moisture above, dry air below can mean "Virga"
Posted March 9, 2015
There are times when part of the atmosphere sets up with enough moisture and lift to produce precipitation, but another part of the atmosphere limits the ability of that precipitation to reach the ground. We started the week with such a situation this Monday, and it can be worth noting because sometimes it can make for radar displays that look as though it should be raining in places where it really isn't.
The images here help to illustrate that principle. First, a regional radar image showing echoes over a large part of western and west-central NC and parts of southern VA, and a view of DualDoppler 5000 with some light echoes that appear to "circle" the radar from southeast of Roanoke Rapids to southern VA to the Triad area and down to the south and east of Charlotte. However, the map of surface reports, while showing a lot of cloudy skies across the state, indicates that the only stations reporting precipitation (red dots to the left of the station plots) at this time were Andrews and Asheville in the southwest corner of the state, while other sites showed clouds with ceilings around 8-12,000 feet off the ground, but no rainfall.
A look at a forecast profile (the fourth image) of temperature, dew point, wind and vertical motion shows why this would be the case. On this image, the red line is temperature from the ground up to about 30,000 feet, the green line is the dew point (which depends on how moist the air is), and the solid white line shows how fast air is rising (when that line is to the left of the vertical white dashed line) or sinking. When temperature and dew point are very close to one another, air is saturated, and given a little upward motion tends to condense into clouds and to potentially produce precipitation. Where those two lines are far apart, the air is quite dry.
On this profile, then, we have the moist air and upward motion indicated from about 10,000 feet upward, while the air appears much drier below that. This is a good setup for producing virga, and you may find that if you watch the clouds in situations like this you can sometimes see streaks of precipitation extending below the cloud bases, or sometimes it simply causes the base of the clouds to have a kind of fuzzy, indistinct appearance.
Back to that DualDoppler 5000 image, the "circle" at which the echoes appear represents the point at which the radar beam has climbed to about 6-7,000 feet above the ground, indicating that light snow and rain falling from some of the clouds is probably making it about 3-4,000 feet downward before evaporating in the drier air, and the radar beam passes beneath any precipitation that is closer to Raleigh (the echoes immediately around and just southeast of the radar are mainly ground returns and other non-meteorological echoes such as birds, insects and the like).
While this pattern was likely to hold for most of the event on Monday, with perhaps a sprinkle or two making it to the surface in spots, in other cases a period of virga of this sort can be followed by a gradual lowering of the cloud base and moistening of the lower atmosphere, allowing more and more precipitation to reach the ground.