UCLA

This dissertation focuses on two projects related to tropical cyclone (TC) structure. The first project is about understanding how and why the cloud-radiative forcing (CRF) modulates the structure of TCs. The second project is about the influence of planetary boundary layer (PBL) mixing on TC structure.

The first project demonstrates that CRF, the interaction of hydrometeors with longwave and shortwave radiation, can significantly modulate the TC structure through the employment of “semi- idealized” integrations of the Hurricane Weather Research and Forecasting model (HWRF) and an axisymmetric cloud model. Averaged through a diurnal cycle, CRF consists of pronounced cooling along the anvil top and weak warming through the cloudy air, which locally reverses the large net cooling that occurs in the troposphere under clear-sky conditions. CRF itself depends on the microphysics parameterization and represents one of the major reasons why simulations can be sensitive to microphysical assumptions.

By itself, CRF enhances convective activity in the TC’s outer core, leading to a wide eye, a broader tangential wind field, and a stronger secondary circulation. This forcing also functions as a positive feedback, assisting in the development of a thicker and more radially extensive anvil than would otherwise have formed. These simulations clearly show that the weak (primarily longwave) warming within the cloud anvil is the major component of CRF, directly forcing stronger upper- tropospheric radial outflow as well as slow, yet sustained, ascent throughout the outer core. In particular, this ascent leads to enhanced convective heating, which in turn broadens the wind field, as demonstrated with dry simulations using realistic heat sources.

The second project examines the TC response to the different PBL-generated mixing magnitudes in the GFS PBL scheme that is used in the operational HWRF model, in which the vertical eddy mixing is tuned by a parameter α. This parameter can strongly modify TC structure when the environment is favorable. With large α values, the GFS PBL scheme can produce momentum diffusion that is larger than the magnitudes suggested by observations, which can dramatically broaden the TC horizontal extent. However, this sensitivity is case-dependent, and decreases as the environment becomes less favorable for convective activity.

The influence of PBL mixing on storm structure is dramatic, but also complex, because it emerged that the vertical mixing of moisture and momentum represent competing influences. Decreasing moisture diffusion causes the storms to be weaker and much narrower, because to acts to limit moisture availability in the hurricane outer core and thus reduce peripheral convective activity. On the other hand, decreasing momentum diffusion acts to strengthen the wind field at all radii via enhancing the radial inflow and reducing the momentum sink. The TC structure is also found to be sensitive to the vertical shape of the eddy mixing profile, specifically the height of where the mixing magnitude is at maximum.

From this second project, we learned that imposing a constant α does not guarantee that the eddy diffusivity will not exceed the observed values in some cases. Further, the value that may be optimal for the inner core may not be best for the outer region or the surrounding environment. Therefore, a new computational algorithm to limit vertical mixing in the TC’s inner core region without artificially constraining the scheme in the surrounding environment or changing the vertical shape of the eddy diffusivity profile was proposed and tested.

In the model, CRF and PBL mixing determine the TC structure via controlling the convective activity in the TC’s outer region. These two processes can cooperate and compete, making these influences difficult to deconvolve and complicating the implementation and evaluation of model physics improvements. In the case of the HWRF model, we demonstrate that inadequacies in the representation of CRF was largely masked via apparently excessive vertical mixing in the PBL scheme. Future work should concentrate on the outsized influence that the PBL parameterization is shown to have on important storm aspects such as intensity, motion and radial extent.