Altostratus opacus virgae transitions to Altocumulus during day! Storms to bring rain!

What a day for cloud maven juniors and me, too, watching the Altostratus opacus (but sometimes “translucidus” cuz you could see where the sun was) become Altocumulus! It happens pretty often and is the result of lowering, and warming of the cloud tops, but I need to generate some excitement on an otherwise somewhat dull day.

What else is happened as tops warmed? Good-bye virgae (“virga”, in plain speak), except in a couple of locations that raised the question, “Was it hers (Mother Nature’s) or ours (aircraft effects)?”

2016122912Z_SKEWT_KTUS — The TUS balloon sounding through all that Altostratus opacus virgae. Launched at about 3:30 AM. The top temperature is so cold (-60° C, -76° F) we don’t even want to know here in Arizona that such temperatures are possible. So, you can imagine all the ice that might form in a moist layer of air. The bottom is even cold at nearly -15° C, there were the temperature jumps out to the right. This is a situation we call “overrunning”, where warmer air is going over a colder air mass. This cloud was about 22,000 feet thick, 7 km at this point.

The slight spread between the two lines illustrates the classic representation of what we measure when the balloon passes through an all ice crystal/snowflake cloud like this version of Altostratus was yesterday morning. The humidity element on the balloon measures the humidity relative to liquid water, not ice, so there will be some spread between the dew point temperature (line on the left) and the temperature (line on the right) when the balloon ascends through an ice cloud. Saturation with respect to ice is indicated here in that deep “overrunning” layer, something also likely to happen tomorrow to the writer’s “company” fubball team tomorrow.

And here's the TUS sounding launched yesterday afternoon when we only had Altocumulus opacus clouds, just as dark as Altostratus opacus, but much thinner. Tops around -10° for the most part, but there may have been some turrets to almost -20° C. The balloon almost certainly passed between clouds, did not go exactly through a Altocumulus cloudlet. — And here’s the TUS sounding launched yesterday afternoon when we only had Altocumulus opacus clouds, just as dark as Altostratus opacus, but much thinner. Tops around -10° for the most part, but there may have been some turrets to around -20° C. The balloon almost certainly passed between clouds, did not go exactly through a Altocumulus cloudlet. Don’t worry if you can’t make out the actual temperatures on the lines sloping up to the right, just take my word for everything I say. You can easily see how much it dried out in the middle and upper cloud regions between the morning sounding and this one.

Yesterday’s clouds and the transition

8:29 AM. Altostratus opacus virgae (has some downward pendants of ice and snowflakes coming out of it). The TUS radar had some sprinkles showing up here and there.

Now let’s look over here:

9:19 AM. Thin spot in the overcast. — 9:19 AM. “Thin spots in overcast”: we used to say that a lot in our human weather reports of ages gone by. Here the thin spot makes this Altostratus translucidus.. While there is an irregular look to this Altostratus due to virga hanging down, there is no indication of liquid water elements, ones that would show up as sharply-outlined darker elements. While this is hours later than that morning TUS sounding, it is likely that in spite of this thin spot, the Altostratus layer was still many kilometers (thousands of feet thick). Ice crystals and snowflakes are far less numerous than droplets in liquid clouds, and, therefore clouds composed of ice are more transparent given equal depths. Compare the visibility in a dense fog with being in a light snowfall.

9:38 AM. Example of a some sharply-outlined liquid clouds embedded in the Altostratus layer have formed. The growth of ice crystals and snowflakes is enhanced in liquid clouds because they represent regions where it is saturated with respect to WATER, and highly supersaturated with respect to ice (the relative humidity with respect to ice is well over 100%). Also, if the droplets in these clouds are large enough (larger than about 15 microns in diameter) they can be collected by the falling ice and snow, adding to their mass of those, causing them to fall faster.

10:29 AM. Had numerous, dramatic outbreaks of mammatus around this time, probably representing the fall back of turrets on top of the Altostratus as this time. We will say no more about mammatus since the author has tended toward the prurient to break up the tedium in past notations about mammatus. — 10:29 AM. Had numerous, dramatic outbreaks of mammatus around this time, probably representing the fall back of turrets on top of the Altostratus as this time. We will say no more about mammatus since the author has tended toward the prurient to break up the tedium in past notations about” mammatus.” This might be viewed as an upside down look at the cloud tops at this point, BEFORE they collapsed and dropped below the main bottom of this layer. At the top (rumpled area), regions of a liquid cloud layer are beginning to appear, a sure sign that tops are receding.

11:55 AM. Moving along,looking upwind across the Oro Valley. Still looks composed mostly of ice, but liquid clouds are on the far horizon. — 11:55 AM. Moving along,looking upwind across the Oro Valley. Still looks composed mostly of ice (Altostratus opacus virgae here), but liquid clouds are on the far horizon.

1:21 PM. Altocumulus opacus rules. The deep icy cloud is all gone by now. No virga. Notice, too, in spite of being less than a kilometer thick, this cloud looks as gray as the Altostratus that was many kilometers thick. The droplet concentrations in a liquid cloud such as this might be 200, 000 per liter, while the ice concentrations in that Altostratus cloud were likely in the 10s per liter. The smaller particles in Altocumulus clouds, average perhaps only 15-20 microns in diameter also are able to reflect far more sunlight back into space, and less sunlight reaches the bottom making it darker. In contrast, the (ice) particles in the Altostratus would be hundreds of microns to millimeters in diameter (i.e., precip-sized).

2:22 PM. Looking around at these cold Altocumulus clouds, generally not showing virga, you begin to wonder if those areas you do see have been the result of an aircraft passage, as here in that little spot of virga.

2:28 PM. Some breaks in the overcast allowed some nice scenes to fall upon our mountains. Here, the Charouleau Gap is highlighted.

The weather just ahead

Here’s the latest projected rain totals from the U of AZ Wildcat Weather Department Weather Calculator:

From the global ingest of data at 11 PM AST last evening. Indicates that green Catalina will be in the half inch to three quarters of an inch between now and New Year's Day afternoon. Comes in two segments, the first overnight tonight, and then another starts New Year's Eve. — From the global ingest of data at 11 PM AST last evening. Indicates that green Catalina will be in the half inch to three quarters of an inch between now and New Year’s Day afternoon. Comes in two segments, the first overnight tonight, and then another starts New Year’s Eve. Seems reasonable. Probably not quite reasonable is the red on the Cat Mountains, indicating 3-4 inches accumulation during this time, probably a bit overdone. Both storms are rather small in size, so the amount of rain depicted in these model runs has varied a lot. But, they seem to be settling on something decent. Seems the least we’ll end up with has to be more than a third of an inch, worst case scenario. See Bob and the NWS for a good look at these incoming events. We’re mostly about clouds here.

Undercutting flow from the tropical Pacific is on schedule. So, a good chance for major rains along the southern portions of the West Coast in a few days, with a pretty good chance they’ll leak into Arizony.

The End

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.

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