Research under this grant will refine and extend our understanding of the thermodynamic control of tropical convective clouds and precipitation over a broader range of tropical weather systems including tropical cyclones (hurricanes), convectively coupled Kelvin waves, tropical easterly waves (which can develop into hurricanes), and the Madden-Julian oscillation. The research uses a combination of simple models, observations from field campaigns, and a cloud resolving model (CRM) to identify the most important physical processes which govern tropical convection and its relationship to the large-scale atmospheric circulation. Specific tasks of the project would include 1) Extension of the theory of covective quasi-equilibrium. The quasi-equilibrium state for tropical convection which is assumed in much of the theoretical literature on tropical circulation dynamics does not develop fully in CRM simulations. This research will use simple models to understand the implied limitations of quasi-equilibrium theory; 2) Extension of the Raymond-Fuchs model of convectively coupled tropical waves. The Raymond-Fuchs model has been successful in reproducing convectively coupled waves, and the proposed extensions would be used to study the effects of top-heavy, botttom-heavy, and tilted heating profiles associated with observed tropical convection; 3) Examination of the role of convection in the development of tropical cyclones. The research would address the nature of the vertical convective mass flux in tropical cyclone development, paticularly the extent to which the mass flux happens in a few intense "vortical hot towers" rather than a large number of unexceptional clouds; 4) Examination of the structure and dynamics of easterly waves, particularly the extent to which their development is controlled by surface wind speed, temperature, humidity, wind shear, and surface fluxes of heat and moisture.
The project will support and train two graduate students and one undergraduate, thereby training the next generation of scientists in the field of tropical convection. The graduate students will gain experience with large field campaigns through participation in the NSF PREDICT project. The work will support international collaboration in science through the collaboration of a scientist in Croatia, and international scientific outreach will be promoted through a workshop held in Split. The work has broader scientific impact due to its relevance to the task of developing cumulus parameterization schemes for use in weather and climate models.
This project studied how localized convective rainstorms and squalls interact with large-scale tropical weather systems. Such systems include hurricanes and typhoons and the other grand flows in the tropics that are responsible for organizing the patterns of rain and wind there. We are interested in this problem because we don't know how these large-scale systems control local weather, nor do we understand how this weather feeds back on the larger systems. Global weather prediction models aren't detailed enough to predict local weather, so some shortcuts need to be taken which so far are generally unsatisfactory. Our main contribution in this project has been to show how localized rainsqualls work together with the large-scale circulations of developing tropical cyclones (hurricanes or typhoons) to cause these violent phenomena to strengthen. This also tells us why they might not intensify under various circumstances. Forming tropical cyclones first develop strong rotation at the middle levels of the troposphere, typically near an elevation of 5 km. The modifications of the temperature and humidity patterns in the atmosphere that go along with this giant swirling motion aloft produce the kind of convection that draws air into the system at lower levels. Under the influence of the earth's rotation, this inflow itself begins to swirl, resulting in the formation of an infant tropical cyclone. If the resulting column of moist, rotating air is sufficiently protected from its surroundings, the intensification continues until a full-fledged hurricane or typhoon develops. Figuring out how the mid-level circulation aids in this process is the main contribution we have made in this project. However, we could not have made this progress without the help of high-technology field programs in which extensive measurements were made on developing tropical cyclones. These programs, also funded by the National Science Foundation, were undertaken outside the scope of this particular grant, but this grant allowed us to participate in and help guide them. The knowledge we have gained about convective rainstorms and how they interact with developing tropical cyclones is applicable to most other tropical weather systems. We are therefore trying to leverage this knowledge into better ways of treating such localized weather systems in global weather prediction models. This work is still under way.
