Convective cold pools are important modulators of the onset and evolution of deep convection in the tropics. Cold pools are generated by downdrafts and can outlive the storms they originate from to spark new convection. However, most of our understanding of cold pool mechanics comes from high-resolution simulations and a relatively small number of in situ observational studies. This thesis brings novel observational approaches together with climate model data to understand the behavior of cold pools on a global scale and how a mesoscale weather behavior can be accounted for in a climate-scale simulation.
First, we leverage a dataset derived from the Advanced Scatterometer (ASCAT) satellite instrument by Garg et al. (2020) to quantify seasonal variations in cold pool activity and their relationship to deep convection across tropical ocean basins. The dataset identifies gradient features (GFs) in the surface wind field, which have been shown to serve as reliable proxies for the boundaries of atmospheric cold pools. We examine the relationship between GFs and climatologies of precipitation, column relative humidity (CRH), and bulk vertical wind shear. We also collocate GFs with precipitation and CRH. High GF frequency, precipitation, and CRH coincide in many regions of the tropics, consistent with our understanding of the physical connections between precipitation and cold pool generation. On the other hand, climatological bulk wind shear is often low in convective regions, and there is a weak inverse correlation between GF frequency and bulk wind shear, while our prior expectation might have been that shear promotes cold pool formation. Compared to GF frequency, GF size shows a weaker relationship with the convective environment, with some of the largest GFs occurring at lower CRH values for a given rainfall rate. In a few exceptional regions and seasons, such as the Indian Ocean in northern hemisphere summer, the region of greatest precipitation does not coincide with the region of greatest GF frequency. These cases also have very high seasonal mean CRH, suggesting that in these regions cold pool formation is suppressed by reduced evaporation of precipitation.
Following that, we apply the GF data set to the task of evaluating the realism of the cold pool parameterization in the GISS E3 earth model originally designed by Del Genio et al. (2015). We compare the GF data set to model results from six versions of the GISS model with perturbed parameters. Cold pools generated by the model have significantly different geographic distribution to satellite-observed GFs, particularly in critical convective regions. They also appear to be much less common than GFs, though they have a broadly similar dependence on column water vapor (CWV), especially in terms of size.
Finally, we seek to understand the mechanics of the model cold pool parameterization on its own. A subset of high-time resolution model versions is used to deconstruct the behavior of the model parameterization at the scale of individual time steps. Our aim is to see what level of physical realism is associated with the emergent trends seen in the climatological statistics. We find that the model generates cold pool temperature and moisture depressions of similar magnitude to cold pools measured from ships, but tend to dissipate too quickly. Model cold pools also appear to spark increased precipitation, as they are designed to do, but that precipitation appears to come from the stratiform model parameterization, not the moist convection one.
Together, these results provide a first opportunity to empirically evaluate a model parameterization originally developed using theory.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/8hfp-2q26 |
| Date | January 2024 |
| Creators | Orenstein, Patrick Dunn |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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