Pressure perturbations in and below trade wind cumulus clouds: Forcing patterns

The marine boundary layer with fair-weather cumulus clouds is normally thought of as a fairly benign environment. Nevertheless, the organization of clouds is not normally random; rather there are systematic forcings that often tend to organize cumulus clouds in cloud bands and 'lumps'. The organization may be dependent on e.g. wind shear, boundaries between cold and warm air, etc. Where organization is present, one would expect that to be reflected in deviations from a mean buoyancy state.

One way of describing this deviation from the mean state is to examine the pressure perturbations. From aircraft the measurement of pressure pertubations are done by finding the difference between the aircraft altitude and the pressure altitude as measured by the aircraft. In early studies of cumulus cloud pressure pertubations, Lemone and Tarleton (19XX) used an INS derived aircraft altitude, but this is notoriously difficult due to INS drift.

In the present study we examine pressure pertubations in RICO cumulus clouds by utilizing measurements from the NSF/NCAR C130 aircraft. The aircraft was instrumented with a dual wavelength GPS unit (Novatel OEM-4) which was referenced to a similar unit that was positioned on the ground in Antigua's St. Johns Airport. The accuracy of this differential GPS system was found to be 0.25 m aircraft altitude, a measure far superior to previous measurements. The accuracy of 0.25 m actual altitude corresponds to a pressure value of about 0.025 in the cumulus cloud environment.

Flights below cloud base show pressure pertubations of about 0.3 mb for 'normal' trade wind cumulus clouds (cloud top typically below 2 km). In precipitation regions, with evaporatively cooled 'cold pools' below cumuli, the pressure perturbations may typically be 0.5 mb. For more sizeable cumuli extending a few kilometers above the trade inversion and containing strong precipitation, the pressure pertubations may approach 1 mb.

These pressure pertubations are important because they lead to significant changes to the horizontal pressure gradients below cloud base. The result was wind speed changes (in the direction of flight) of 4-8 m/s when significant pressure pertubations were observed. These 4-8 m/s velocity perturbations approach the typical mean trade wind speeds of 8 m/s near sea surface, and that may lead to forcing and new cloud formation above.