Kind of got distracted with chores after the big trip to NC and didn’t get to this until today… If you can remember as far back as April 26th, we had a “FROPA” (“frontal passage” in weatherspeak) that day. The U of A weather model indicated beforehand that the bases of the clouds last Thursday would lower to the tops of Samaniego Ridge.
Well they did, though it seemed in doubt for a time, and occurred a bit later than the model had predicted.
Also, a few drops came down here late in the morning; more precip was visible to the north of us and that was reflected in the NCAR precip estimate for Arizona the following morning, an estimate that suggested the heaviest rains were up to half an inch just 150 miles away.
Here are a few of last Thursday’s clouds with some commentary.
These clouds came in two separate segments, the first batch were at Altocumulus levels, some 12,000 feet above the ground according to the TUS balloon sounding that morning at 5 AM AST. Those were the clouds that produced the sprinkles around 5:30 AM. Poor snowflakes melting into drops had to fall such a long way!
After a brief clearing, a surge of lower Altocumulus and Stratocumulus came in. For a time, they looked awful threatening, and appreciable rain could be seen falling from them to the north. They produced a few sprinkles here in the late morning and early afternoon about the time the clouds had lowered (as predicted by the UA model, to the tops of Samaniego Ridge to the east).
Why didn’t they rain more?
The answer, as always here, is that the tops were much shallower, and therefore warmer, than those early Altocumulus clouds sporting considerable ice at times. You can be sure that those Stratocumulus clouds over us had tops warmer than -10 C (14 F), a general threshold for ice formation around these here parts. (Over the oceans, where the drops inside the clouds are larger, the threshold temperature for ice formation is higher.)
Just to the north of us, where rain was occurring, you can be sure that the tops sloped upward in that direction, becoming colder than -10 C.
That second batch of lower clouds looked dark and threatening, but lots of times with lower clouds its because they have higher concentrations of drops in them, not because they’re especially thick as you might guess at first. The droplet concentrations in those dark Stratocumulus might have been twice as high as in those early higher, Altocumulus clouds.
Drops in clouds with higher droplet concentrations, say due to smog, reflect more of the sun’s light off the top. That makes them darker on the bottom, and because they are then also harder to get precip out of, they last longer.
This is a real problem, BTW, for climate models, since longer lasting clouds reflect more light back into space and in that sense, and help counter the global warming expected from trace gases like CO2. But, would you rather have ugly clouds and smog infested skies and a cooler planet, or clean skies and clouds and a warmer planet?
The weather ahead
No rain in sight. But a big heat wave, probably temps around 100 F now looming toward mid-may. May is our driest month, BTW, averaging only a quarter of an inch.