Tropical oceanic mesoscale cold pools in observations and models

Garg, Piyush

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https://hdl.handle.net/2142/113127 Copy

Description

Title

Tropical oceanic mesoscale cold pools in observations and models

Author(s)

Garg, Piyush

Issue Date

2021-06-24

Director of Research (if dissertation) or Advisor (if thesis)

Nesbitt, Stephen W.

Doctoral Committee Chair(s)

Nesbitt, Stephen W.

Committee Member(s)

• Lang, Timothy J.
• Trapp, Robert J.
• Hence, Deanna A.

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Tropical Convection
• Cold Pools, Satellite Remote Sensing, High Resolution Cloud Resolving Model

Abstract

Convective cold pools over oceans contribute to the atmosphere-ocean exchange of heat and moisture across the globe. Beyond cold pools’ physical properties, cold pools and their intersections are important mechanisms controlling the life cycle of tropical convective systems. However, few observational techniques are available to routinely observe the properties of cold pools and their association with convection. The primary goal of the first part of this study is to demonstrate a new technique using gradient in remotely-sensed ocean vector winds as a proxy for the structure and characteristics of tropical oceanic cold pools. Using Advanced Scatterometer (ASCAT) vector wind retrievals with field of view of 12.5 km, regions of enhanced gradients in surface winds are identified as gradient features (or GFs). A 3-km Weather Research and Forecasting (WRF) model simulation was carried out to illustrate the technique over the Indian Ocean during the 2011 DYNAmics of the Madden-Julian Oscillation (DYNAMO) field campaign. A forward operator was applied to the model wind fields, allowing direct comparisons of wind gradients and virtual temperature-defined cold pool thermodynamic and kinematic properties. In situ buoy measurements of air temperature were also used to validate ASCAT-identified cold pools. Quantitative skill metrics demonstrate that the technique has a low false alarm rate (<10%), high critical success index (>85%), and a low bias (~1). The GF technique was then applied to ASCAT-retrieved ocean vector winds, revealing global variations in cold pool properties. These properties are shown to be related to the properties of global convective systems observed by the Tropical Rainfall Measurement Mission (TRMM). The results from the first objective of this study are explained in Chapter 2 of this dissertation. Once the GF technique was well-validated against independent buoy observations and a model simulation, it was applied on NASA’s RapidScat scatterometer in the second objective. Compared to ASCAT (sun-synchronous orbit), RapidScat was in a non-sun-synchronous orbit onboard the International Space Station (ISS) from 2014-2016 and thus was able to provide the diurnal cycle of ocean vector winds. Since the field of view and underlying scatterometry principles of both the scatterometers were similar, ASCAT GF methodology was effectively replicated on RapidScat ocean vector winds for the two-year period to identify diurnally-resolved cold pool properties. This similarity also allowed us to compare the tropical oceanic cold pool properties across the basins and thus helped identify how different wavelengths, swath width, and rain-flag algorithms would change the cold pool properties. Buoy data over the Pacific, Atlantic, and Indian Ocean was also used to validate RapidScat-observed diurnal cycle of cold pool number and precipitation. GF- and buoy-observed cold pool number and precipitation exhibits a similar bimodal diurnal variability with a dominant morning and a secondary afternoon maxima, thus establishing confidence in using GF as a proxy to observe cold pools over tropical oceans. NASA globally-merged (MERGIR) infrared brightness temperature (TB) and ERA5 reanalysis’ total column water vapor (TCWV) is then used to obtain a conceptual understanding of the bimodal diurnal behavior of tropical oceanic mesoscale cold pools. Results from this objective are explained in Chapter 3 of this dissertation. In the recent years, global kilometer-scale convection-permitting models have shown promising results in producing realistic convection and precipitation. Cold pools are identified as one of the major mesoscale processes responsible for modulating the life cycle of mesoscale organized convection in these models. Once an observational perspective of diurnally-resolved global tropical oceanic cold pool properties was obtained in the first two objectives, the third and final objective of this study is to find out if the global high-resolution cloud-resolving model simulations produce realistic cold pool properties across tropical oceanic basins. In this objective, a 2.5 km global Icosahedral Nonhydrostatic (ICON) model simulation run for 40 days (06 UTC 01 Aug – 23 UTC 10 Aug 2016) from the Dynamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains (DYAMOND) initiative is used to identify thermal cold pools (using Tv) over the tropical oceans. Model-simulated cold pools are compared against RapidScat-observed diurnal climatology of GFs and NASA’s Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG) precipitation. Overall, the model-simulated global cold pool climatology is similar to RapidScat across oceanic basins, but ICON portrays a near-unimodal distribution in cold pool diurnal cycle. Environmental properties such as column water vapor, surface fluxes, vertical wind shear, and convective available potential energy (CAPE) are analyzed to identify controls on cold pool strength, density, and size. A random forest regression model was then applied to identify statistically significant features important in modulating cold pool properties across the global tropics. Regional difference in relationship between environment and cold pool properties is explored and it was observed that EPAC and AO cold pool activity is strongly controlled by synoptic-scale waves while WPAC and AO have strong mesoscale controls on cold pools. Results from the third objective are depicted in the Chapter 4 of this dissertation.

Graduation Semester

2021-08

Type of Resource

Thesis

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http://hdl.handle.net/2142/113127

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