Was Donald Duck right (but wrong)? What does it take to make rain in the dessert?

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Background: Over subtropical areas the subsiding branch of the Hadley sirkulasjon strongly supress precipitation due to the formation of a strong capping inversion above the planetary boundary layer (PBL). Observations and models show that the PBL itself can be quite moist due to regional evaporation from ocean areas. This allows for the build-up of Convective Available Potential Energy (CAPE, see Wallace & Hobbs page 345) over the Persian Gulf (as shown here). However, the energy requirements to break through the capping inversion barrier is generally too large to allow for formation of convective cells and precipitation.

Standard methods (or rather therories) to generate precipitaion have focused on seeding clouds with ice nucleation particles to speed up the precipitation formation process (the BWF-process, see Wallace & Hobbs, section 6.6) as shown in the figure (http://www.donaldisme.dk/weather.htm)

A very different approach would be to enhance the energy content of an air parcel in the PBL (by warming and adding water vapor) to allow it to break through the energy barrier of the capping inversion.

The Project: In this project, the student will use the Weather and Research Forecast model (WRF) and perform high-resolution simulations (in convection permitting mode) for selected cases over the Persian Gulf and adjacent land areas. The initial thermodynamic properties of the air in a small number of grid cells located in the PBL will be manipulated in the model to study how much energy is needed to actually break through the CI barrier. The simulations will further show if this is sufficient to allow for a deep convective cell to form and calculate the amount of precipitation that is actually formed and eventually reaching the ground (some will evaporate as the raindrops falls through drier air). 

Although the project focus on a (so far) purely theoretical case, the project will give the student good knowledge of how thermodynamics and cloud microphysics is parameterized in a state-of-the-art numerical weather prediction model (WRF). The project will also give the student a practical knowledge of how to make simulations on a supercomputer and how to handle model results and assess the uncertainties in the results

Published Oct. 17, 2019 11:56 AM - Last modified Oct. 17, 2019 11:59 AM

Scope (credits)

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