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Mountain-triggered cumulus evolving from shallow to deep convection.

February 24th, 2008 by cianca · 2 Comments

This post is a summary of “The Cumulus, Photogrammetric In Situ, And Doppler Observations Experiment 0f 2006″ published by R. Damiani, J. Zehnder, B. Geerts, J. Demko, S. Haimov, J. Petti, G.S. Poulos, A. Razdan, J. Huh, M. Leuthold, and J. French in the January 2008 issue of Bulletin of the American Meteorological Society (BAMS).

I have a sincere interest in this subject based on an old article that I read in National Geographic. The article concerned the “sky islands” or mountain ranges of Arizona and New Mexico, that erupted from the floor of the desert, into diverse and different ecosystems. What differentiated these areas from the normally arid portion of the country, was the elevation and the weather.

The Catalina Mountains, one of the “sky islands” of Arizona, became a perfect candidate to study cumulus clouds. The isolation of the mountains provided a unique place to observe the development of cumulus clouds based on shallow to deep convection by eliminating other variables. The Cumulus, Photogrammetric, In Situ, and Doppler Observations Experiment of 2006 (or CuPIDO) was created to achieve this goal. “Cumulus convection is the primary mechanism of boundary layer (BL)-troposphere exchange and precipitation in quasi-barotropic environments.” So here we have an isolated mountain range, being studying during the warm-season in the western United States. If you’ve seen a summer-time visible satellite picture of the western U.S. during the summer, you know that air-mass thunderstorms are almost a permanent fixture. Obviously there are a few cases otherwise, but generally that is the case. What makes the west so special is daily differential heating between the different elevations. The temperature difference between the tops of the “sky islands” and the desert floor below can be quite stunning, keeping the lapse rates steep and the propensity for convection very strong.

CuPIDO attacked this project with never-before-seen observation density and had three main hypothesis:

1. The onset of convection results from elevated heating, low-level convergence over the mountain, and local BL deepening to the level of free convection (LFC).

2. Subsequent convective development is affected by the mutual interaction between cumulus and the environment (environmental preconditioning).

3. The main growth mechanism of cumulus turrets consists of a succession of therms containing cloud-scale toroidal circulations, which govern both entrainment and cloud microphysical processes.

The experiment was performed from July 1st through August 31st, 2006, during Arizona’s monsoon season by means of several surface meteorology stations and observational cameras. Also, aircraft and sounding data were collected during on 15 “intense operation periods”. The surface meteorological stations were equipped with sonic anemometers and ultraviolet absorption hygrometers to take surface latent and sensible heat flux measurements. Soundings were taken by using radiosondes at short intervals. Finally, aircraft took additional measurements with a gust probe, an array of cloud particle probes, and high frequency temperature and humidity sensors. The airplane was also equipped with a 95-GHz polarimetric Doppler radar, which was used to look at the cumulus clouds as they grew.

CuPIDO encountered a couple of problems along the way, but mainly timing. It took time to retrieve data from all the sources which made real-time decision making difficult. Also, sometimes the convection was unpredictable and required instantaneous response, which is tough when you need to coordinate the takeoff of an aircraft. Even with these problems, however, the project did produce some interesting findings.

They were able to find examples backing up each of the hypotheses, though not all data supported them. More importantly, however, was the staggering amount of data that was collected. This data could help factor into forecast models and provide for a greater understanding of the interaction of the boundary layer over a mountain, cumulus convection, and the ambient atmosphere.

CuPIDO is considered a great success even though it faced some difficulties. It makes me wonder if the data can be supported in a more diverse mountain environment. And what of a non-mountainous environment. The mountains of th west aren’t the only place we see airmass thunderstorms over the summer months. What about a place with no topography whatsoever like portions of the southeast? What if the experiment was duplicated with a comparable observation density, yet over completely flat topography. I wonder if they’d achieve the same results.

Dann Cianca.

Tags: MTR3440

2 responses so far ↓

  • 1 Dann Cianca // Nov 16, 2008 at 12:44 pm

    Hi tech4geek, the original article appeared in the Bulletin of the American Meteorological Society (BAMS) magazine. I’m not sure if it is digitized and you may need to be a member of the AMS to access it. However, I recommend searching on the title of the article or authors in the first paragraph.

  • 2 tech4geek // Nov 18, 2008 at 11:12 am

    @ Dann : Thanks for your kindness,i’ll try your suggestion.

    Regards,
    Tech4geek

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