The goal of this project is to use the digital cloud images, along with data on sensible and latent heating collected from a tower in the Catalina Mountains (by the University of Arizona/SAHRA) to explore the transition from shallow to deep convection.
The use of photogrammetric techniques to examine cloud behavior is not new. We plan to use digital image processing techniques to automatically extract information from the cloud images. This will allow us to process a large number of cases and examine the details of the cloud development in different large scale environments. We will also compare the development with the surface forcing to determine the relative importance of the surface forcing, modification of the temperature and moisture above the mountain, the the triggering of the ice phase.
All models, from those used for short range, operational numerical weather forecasts, to those used for long term climate predictions, contain assumptions about the rate at which energy (CAPE) is consumed by clouds. By determining the details of the cloud evolution in different environments, we will be able to refine the cloud schemes in these models, and improve the forecast skill.
A Typical Sequence
typical thunderstorm event begins at around 9am
each morning. Heating of the mountain surface
forces air to rise and shallow cumulus clouds
This sequence is from August 16, 2002. The top of Mount Lemmon is visible at in the center of the frame.
This process continues, with shallow clouds building and then evaporating. This picture is about an hour later, at 10am , on the same day.
About a half an hour later, the convection is more vigorous, and the turrets extend higher into the atmosphere.
At this point, the development becomes more rapid. About three minutes later, the turret on the left collapses and evaporates, while the one on the right grows. The top of that turret is at about 20,000 feet.
The turret continues to grow, and there is evidence of a "pinching off" about 1/4 of the way from the top. This is suggestive of dry air being entrained into turret.
The top of the turret detaches and begins to evaporate while the bottom collapses. This picture is roughly one minute from the frame above.
Shallow turrets again build over the top of the mountain and build into the remnants of the bubble that detached from the previous turret. This frame is about 7 minutes after the previous one.
About 10 minutes later deeper convection begins to build again.
10 minutes later, the convection builds vertically and expands in the horizontal.
10 minutes later, the development continues.
10 minutes later, there is the beginning of a deep cumulonimbus cloud over the top of the mountain.
What's happening here?
is evidence that the shallow convection the
develops early in the morning conditions the
atmosphere for further deep convection. Remember,
the atmosphere during the monsoon season in
Arizona tends to be marginally unstable, and
the thunderstorm development occurs primarily
because of the intense surface heating. The
marginal thunderstorm conditions are due, in
part, to the air aloft being relatively dry.
One possibility is that shallow convection develops and the thermals entrain dry air and evaporate. If the winds aloft are weak, the air can remain over the mountain, and subsequent turrets ascend through the remnants of the old ones. These entrain air that is less dry and evaporate less rapidly, which allows them to grow to a larger height. This process continues until a thunderstorm develops.
A second possibility is that as the surface heating intensifies during the morning, the convection is forced more strongly from below. The heating, along with evaporation from the mountain top, destabilizes the vertical temperature profile. There may be some modification of the temperature profile by the convection as well, which would decrease the convective inhibition , otherwise known as CIN (which is the vertically integrated negative buoyancy beneath the cloud base).
A third possibility is that the initial turrets condition the air for explosive development, but by leaving ice crystals behind. Subsequent turrets build and entrain ice crystals, which accelerates the freezing of cloud drops in the new convective cells. There is a larger latent heat release associated with water vapor being deposited onto ice crystals than condensing into water drops, as well as a heat released as the drops freeze, and the cloud buoyancy increases. Also, the saturation vapor pressure over ice is less than that over water, so ice crystals grow faster than water drops. This latter phenomenon is referred to as the Bergeron process.
In reality, there may be a little of all three of these processes going on. The initial development may be controlled by the surface forcing (solar heating) with the shallow convection moistening the column, and removing the CIN from the profile.
Later and explosive development may be due to the initiation of the ice phase in the clouds.