Publishing a paper as the sole author seems to be a rite of passage for any respectable scientist, and I’ve finally crossed the threshold!
Hannah, W. M., 2017: Entrainment vs. Dilution in Tropical Deep Convection. J. Atmos. Sci., 74, 3725–3747.
I’ve been stewing on an idea to build a convective parameterization for a couple years, but there are a lot of issues to work out so I’ve been trying to study up on how other parameterizations work. The biggest issue with a convective parameterization always seems to revolve around entrainment. In reading about recent parameterization developments, like Chikira and Sugiyama (2010), many people seem to like the idea of relating entrainment to the vertical velocity of the cloud updraft. This is not a new idea, but it has taken awhile to be utilized in weather and climate models. The paper discussed below provides a great discussion of this relationship, and I was surprised to learn that it is based on an inaccurate assumption about how important entrainment is for cloud momentum.
I still struggle with using potential vorticity (PV) to think about atmospheric phenomena, but many extremely smart people find it very useful, so it’s a good thing to study. PV is perhaps most often used to discuss circulations on very large scales, but there has been a lot of work to make “PV thinking” work for mesoscale circulations as well. This paper describes such a theory to utilize PV to understand the maintenance of long-lived mesoscale systems in which convection plays a substantial role.
Raymond, D. J., and H. Jiang, 1990: A Theory for Long-Lived Mesoscale Convective Systems. J. Atmos. Sci., 47, 3067–3077.
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At a recent conference I was talking to a colleague about research ideas, and we kept coming back to the conclusion that we don’t know as much as we should about how clouds are treated in the various models used around the world. There are many different schemes in use today and most of them are closely related to one another, but it’s hard to keep track of the subtle differences between the approaches. Sometimes it’s hard to find any information about the particular configuration of the convective schemes in certain models. I decided to compile a list of the different types of parameterization schemes, and gather some notes on the key papers that developed these schemes. Continue reading
I find the concept of inertial instability somewhat difficult to grasp. However, it seems to explain some interesting and important atmospheric phenomena, so I’m committed to wrapping my head around it. The current paper is an example of how inertial instability can be used to explain the seasonal behavior of the African monsoon system.
Cook, K. H., 2015: Role of inertial instability in the West African monsoon jump. J. Geophys. Res. Atmos., 120, 3085–3102. Continue reading
A recent post on RealClimate.org touched on the question of what is the best simple mental model of the greenhouse effect that should be used to educate the public. Specifically, the author mentions that it is important to consider how convection affects the optical depth, in addition to the obvious role of radiative transfer. This led me to read the more thorough paper by Rasmus Benestad.
A lot of interesting weather phenomena are associated with various types of atmospheric waves. In most cases, we have a good understanding of wave dynamics when the atmosphere is dry and there’s no condensation or evaporation, but we lack a solid understanding of how things change when moist convection gets involved (i.e. clouds). A good example is how convectively coupled Kelvin waves move at a fraction of the speed compared to their dry counter parts (Kiladis 2009). I recently read the paper below by Paul O’Gorman that discusses a simple way to modify dry theory such that the effects of heating by convection can be implicitly included.
The Sahel is a particularly sensitive region with respect to the effects of global warming. The Sahel has been plagued by decadal-ish periods of drought, such as in the 1980’s. It’s generally understood that the variations in Sahel rainfall are mostly attributable to ocean variability (Giannini et al. 2003), but there are still many questions about the importance of other natural and anthropogenic factors. These questions are hard to answer with current climate models, because ocean temperature biases can lead to large differences in the distribution of rainfall. In spite of the lack of agreement between models, this paper was able to identify a robust result across a group of models from the CMIP3 data archive.
In a recent discussion with some colleagues someone asked the simple question, “Do you know why the African easterly jet occurs at 600mb?” and I realized didn’t actually know the whole story. The usual explanation is related thermal wind balance and the meridional temperature gradient at the surface. However, this explanation misses some subtle aspects of the vertical structure of wind over Africa. This paper appears to be the first to provide a thorough explanation of the processes which maintain the African easterly jet (AEJ).
African easterly waves (AEW) have a large influence on weather in the African Sahel region, and are known to provide seed disturbances for many Atlantic tropical cyclones (TC). Because of all the factors that can affect the development of a TC (SST, shear, etc.), it is difficult to know how any given change in AEW activity will affect the climatology of Atlantic TCs. This paper is one the few that has tried to characterize future AEW activity from the model projections (CMIP5).