The streets of Marana-Oro Valley-Catalina, the cloud ones

In case you didn’t notice, there was a prolonged street of clouds emanating from possibly as far away as Kit Peak, or maybe just the Tucson Mountains. Lasted for a few hours.Happens only on days with relatively shallow clouds (cloud-topped boundary layer) with a little wind, meaning that the thermals from the surface heating ended up being capped by an inversion or other stable layer, and those thermals form clouds in some places.  In this case,  a long line of intermittent clouds formed from an initial air bump caused by those mountains far to the SW of us. CTBL is more often invoked as a term by cloud folks when the sky is much cloudier in low clouds than these shots from two days ago, such as when the sky is covered in Stratocumulus clouds.

These kinds of streets occur over the same places whenever a day like this comes along.  Think of it, especially here in Arizona, as a row of shady air under which you might like to live compared to those areas on either side of this cloud “street.”  In Seattle, where the “cloud topped boundary layer” is almost a daily occurrence, you want to avoid being under the cloud street, where it can block the sun, and instead find the clearer slots!

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12:14 PM.
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1:07 PM.
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3:19 PM. Barely hanging on now, but still present. Dissipated within about a half hour of this shot.

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TUS rawin for June 18th, 5 PM AST (launched about an hour and a half before that).
TUS rawin for June 18th, 5 PM AST (launched about an hour and a half before that).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For those sharpies that day and logged in their cloud diaries that ice formed in those shallow Cumulus clouds, they will be a little chagrined by this TUS sounding.  This sounding suggests the clouds around the balloon were topping at -8 C, too warm for ice formation in shallow Cu.   Let us begin to explain this puzzle by presenting evidence of ice formation in those clouds on the 18th:

4:13 PM.  Virga hangs down from Cu mediocris, maybe only a km thick  toward Charoleau Gap.
4:13 PM. Virga hangs down from Cu mediocris, maybe only a km thick toward Charouleau Gap.  Lots of ice visible.

 

4:13 PM.  Overhead view of likely ice vellum between Cumulus clouds.  The clouds themselves may well have a few ice crystals, but too few to provide any visual indication except in the clearings between them.
4:13 PM. Overhead view of likely ice vellum between Cumulus clouds. The clouds themselves may well have a few ice crystals, but too few to provide any visual indication except in the clearings between them.  Ice crystals evaporate more slowly than liquid drops when encountering sub-saturated air between clouds, and so can be visible a bit longer if present between clouds in marginal situations like this.  Confidence level that this little patch is ice and not just haze is about 70-80 percent.

Simple answer to our connundrum; due to lifting of the air as it approached and went over the Catalina Mountains, the tops of the clouds reached those temperature below -10 C where is begins to form.  We would guess even closer to -15 C in that cloud in the distance beyond Charouleau Gap due to the amount of ice.  Ice increases with decreasing cloud top temperature, but the temperature at which ice onsets can change on a daily basis; higher onset of ice temperatures on days in which the clouds have larger drops in their tops (a phenomenon originally reported by Ludlam in 1952, then re-discovered by Rangno and Hobbs (1988) who did not, at that time, know of the Ludlam finding, and thus, did not cite it.  Pretty embarrassing, really.  Was cited later in an update, however.

The weather way ahead

Seasonal rains beginning to show up in southern Arizona now on models beginning around the 4th of July as a big anti-cyclone parks itself over the Four Corners area in the latest model run from 11 PM AST last night.  Very excellent run.

The End