Saturday, November 08, 2014

Cold Air Drainage Across The Tucson Area

First - a bit on recent minimum temperatures. Low here yesterday morning was 45 F and there was a light breeze blowing. But, down right along the Rillito only the tops of tallest trees were indicating wind, and it was calm and much cooler along the wash This morning it was calm both here at the house and down along the wash. Low at house was 37 F and it was markedly colder down along the wash - so much so that I wished I'd worn gloves.

Yesterday evening we drove out to Santa Rita Abbey (~ 5 miles north-northwest of the highways 82/83 intersection in Sonoita). I had noticed several years after the NWS started doing grid point, deterministic forecasts that the forecast low for the Abbey location was routinely too warm. Often times when we arrived out there just after dark, the temperature (from car's outside air thermometer reading) was often already colder than the forecast low for the next morning. So out of curiosity, re were the grid point forecasts improved now, I checked the forecast yesterday for the Abbey grid - it was for a low of 47 F. When we arrived up there (about 10 after 8:00 pm MST) the indicated outside air temperature was already down to 39 F - so things yesterday remained similar to a number of years ago. 

I also checked the grid point forecast for this morning's low at the Mt. Hopkins RAWS site: 46 F versus an observed low of 55 F. An east wind continued through the night there. It was calm at the Abbey, while a bit to the north-northeast at the Empire RAWS site the low was 38 F, with a light westerly drainage wind through the night. Point of all this is that the conditions related to observed low temperatures in this area are very much locally dependent upon wind, elevation and nearby land surface conditions (assuming clear skies, low humidity, and light winds at the larger scales).

All of this long-winded post was triggered by an NWS forecast discussion the other day that mentioned: THE WINDS MAY STAY UP IN SOME VALLEYS  OVERNIGHT...BUT AT THE VERY LEAST WINDS OVER THE HIGH TERRAIN WILL NOT ALLOW NORMAL DRAINAGE OF COOLER AIR INTO THE VALLEYS. This leads, of course, to the question: What is the "normal" drainage flow of cooler air into the valleys around Tucson? I feel that the answers to this question are not well documented or known. For example, the nighttime winds at the airport, under weak synoptic regimes, tend to be from the southeast. But what is not clear is at/from what elevations do these winds originate (for example, last night they certainly were not originating over the high terrain of the Santa Rita Mountains, where the potential temperature remained much warmer than those at the airport).

When I first moved here, I was puzzled by the obvious differences between drainage winds off the south faces of the Catalina Mountains versus those I had observed along the Colorado Front Range. Here at the house there seemed to be no drainage wind on nights with clear skies and light winds coming off the Catalinas; whereas, places I had experience with in Colorado had distinct drainage wind patterns from nearby high terrain into lower elevations. The foothills to the north of the Rillito wash are known locally to be a warm zone relative to lower elevations to the south. It was after a trip up to Summerhaven on Mt. Lemmon that the differences between here and mountains at higher latitudes began to sink in. I'll illustrate with some images snipped from Google.

The image above shows Pike's Peak and Colorado Springs, Colorado. The higher elevations west of Colorado Springs are dominated by rocks and barren terrain, above the tree line. The difference in elevation from the peak (14,114 ft MSL) to the airport (6,187 ft and ~19 miles from the peak at the southeast corner of image) is 7,927 ft.

In contrast, image below shows the Catalinas and Mt. Lemmon (9,159 ft MSL). Note that there is no high elevation tree line and that the Sky Island is covered by dense forest from lower elevations to the summits. The difference between the elevation of Mt. Lemmon and here at the house is 6,790 ft and the distance is about 14 miles. Thus, the elevation changes from the west side of Colorado Springs up to Pike's Peak and from here to Mt. Lemmon are fairly similar. 

The important difference is that the radiative surface characters are very much different (at least under clear, dry, quiescent conditions).
Cooler air that develops during the night at high elevations of the Catalinas can not drain very far down the slopes before compressional warming puts on the brakes. So, no routine cooling winds during the night/early morning here at house during the heat of May and June, at least not from the Catalinas to the north.

Above is an image showing the location of our house and nearby areas. There is some green apparent up in the foothills "banana belt". But a vast area to our east, along washes and toward Redington Pass, is very barren terrain that radiates very effectively. So my feeling is that the very cold temperatures observed along the Rillito develop in situ and over the barren terrain along the north and east slopes of the nearby washes. The image below emphasizes this by showing the area along and north of the Rillito wash within a few miles of the house. Probably the only way to study the local drainage regimes (say off the entire Catlinas Sky Island) would be to utilize a very-high resolution numerical model - one that could capture the terrain and surface character down at resolutions below 500 m.

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