Walter Hannah

Ph.D. Candidate
Maloney Research Group

Department of Atmospheric Science
Colorado State University
1371 Campus Delivery
Fort Collins, CO 80523-1371

(970) 491-1487

whannah (at) atmos.colostate.edu

Curriculum Vitae



Current Research

My research aims to understand how best to represent the macro-scale effect of clouds in climate models, primarily in the Tropics. While the real world climate system is a continuum of scales from planetary to microscopic, the current state of climate models cannot resolve circulations on the convective scale due to computational expense.

There are many unsolved problems that are hinged on the interaction between large-scale fields and convective processes that are unresolvable in a global model. A good example of this is the Madden-Julian Oscillation (MJO). Investigating new approaches to cloud parameterization will not only help address these issues but will also lead us closer to a unified theory of convection that can account for the dynamics of all cloud types.

Recently I’ve been developing a modification to the Relaxed Arakawa-Schubert scheme (Moorthi and Suarez, 1992) that mimics the effects of convective organization. Most convection schemes do not have any memory of past conditions. Mapes and Neale (2011) modified the Zhang-McFarlane scheme so that past precipitation affects the strength of new updrafts and showed that it leads to improvements in a global model. I hope to build on their work by analyzing explicitly simulated convection in the SAM cloud-resolving model to guide the development of a new cumulus scheme. Once the final scheme is implemented I plan to look into several phenomena such as the MJO, equatorial waves and the timing of diurnal precipitation.


DYNAMO

I was lucky to be part of a field project called DYNAMO, whose main goal was to study the initiation of the MJO. Data was collected in the Indian Ocean at four main sites, which included two islands and two ships. The project took place in late 2011 through early 2012. I worked for the sounding operations on Diego Garcia, part of British Indian Ocean Territory, at 7S 72E.

I had an unforgettable time on Diego Garcia. We were fortunate to capture a strong MJO event during the period where we were launching eight sondes per day. Going to the island and seeing the wildlife was a once-in-a-lifetime experience. I also got to take a ride on the NOAA P3 and help with the drop sondes. In addition to getting an amazing photo opportunity, the pilots were nice enough to let me and my friend Stephanie actually take the controls and fly the plane!


Looking down on DG Inside the plane Me at the wheel! Southern DG

I also posted some time-lapse videos of clouds from the Diego Garcia sounding site as well as some other videos from the trip here on YouTube.

As soon as this data is processed quality-controlled I plan to use the forcing dataset to test some of my ideas for improving cumulus parameterization. If you're curious about what working on a tropical island is like, or want to know more about what went on during DYNAMO, check out the earlier postings on my blog,

A Grain of Walt

Here are two other great blogs about DYNAMO that are worth reading as well.

Sans Continent

The Madden Julian Conversation  


EAPSI Fellowship

During the summer of 2011 I participated in the East Asia and Pacific Summer Institutes (EAPSI) program in South Korea. I worked with Professor Myong-In Lee at the Ulsan National Institute of Science and Technology (UNIST). I met a lot of great people and ate some amazing food! I would strongly encourage any fellow PhD students to consider applying for EAPSI.


Raw whale meat! Pot bing su Prof. Myong-In Lee Bon chon Bulgogi

The aim of my project was to consider the claim made by several papers that convective entrainment should be large in the lowest part of a cloud and decrease with height. Del Genio and Wu (2009) found evidence for this and showed how entrainment may help regulate the timing of diurnal precipitation. I was able to impose this condition in a convection scheme and tested several variations in a single column atmospheric model. Although I was unable to demostrate the effect of this modification in the single column model, after returning from Korea I implemented this same scheme in a global atmospheric model and produced some interesting results.

Specifically, the nocturnal precip maximimum that is observed over the US Great Plains during boreal summer (Jun-Aug) was captured in the model only when entrainment was biased towards the lower levels. The figure to the left illustrates this result. The terrain used by the model is contoured in the upper panel, and the hashing indicates the area over which the composite diurnal cycle is calculated in the lower panel. The solid curve shows the control simulation with no changes to the cumulus scheme. The dashed curve (i.e. exp1) shows the effect of the modification with the highest precip occuring after midnight (local time). The dotted curve shows a different modification designed to validate that the behavior seen in exp1 is in fact due to entrainment being highest in the lower portion of each cloud.

I am currently extending this analysis to look at the diurnal variability in other parts of the globe, such as over the amazon basin during austral summer. I have also used these simulations to analyze how the simulation of the MJO is affected. Preliminary analysis has shown that this same modification produces a more focused spectral peak in the intraseasonal band where the MJO should reside, but also reduces spurious low-frequency variable outside the intraseasonal band that is not observed in nature.