An investigation into the effects of CCN on the timing of convective cold pool initiation

Ross, Tobias Innes David

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https://hdl.handle.net/2142/115609 Copy

Description

Title

An investigation into the effects of CCN on the timing of convective cold pool initiation

Author(s)

Ross, Tobias Innes David

Issue Date

2022-04-27

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

Lasher-Trapp, Sonia

Committee Member(s)

• Trapp, Robert
• Nesbitt , Stephen

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Deep Convection
• Cold Pools
• Microphysics
• Aerosols

Abstract

Convectively generated cold pools exert controlling influences on the lifecycle of deep convection. Constraining parameterizations of these influences in larger-scale weather and climate models is challenged by poor consensus as to their most important physical drivers. Despite originating from latent cooling within convective downdrafts, little attention has been paid to the microphysical influences on cold pool characteristics. This study exploited the unique observational dataset of cold pools obtained during the Clouds, Aerosols, And Complex Terrain Interactions (CACTI) field campaign, along with idealized numerical modeling of select cases, to investigate the hypothesis that cold pools are generated sooner in the lifecycle of their parent convection in low CCN environments, characterized by a faster warm rain process that expedites the first instance convective rainfall. CCN concentrations, measured beneath developing cumulus, were compared with in cloud measurements of hydrometeor size distributions made during nine CACTI research flights. Cloud fields ingesting fewer CCN consistently featured drizzle concentrations approaching an order of magnitude greater than for cloud fields ingesting more CCN, justifying the use of CCN concentrations as a robust predictor for the speed of the warm rain process in subsequent deeper convection. The initiation and evolution of three cold pools, observed on three of these days, was estimated using the CACTI / RELAMPAGO radar and surface observation network. Contrary to the study hypothesis, no clear relationship emerged between ambient CCN concentrations and cold pool onset time. Convective environments and morphologies, purported to modulate CCN effects on convective precipitation in previous modelling studies, varied as much as CCN concentrations among the three cases, increasing the likelihood that any CCN effects on the speed of cold pool onset may have been overwhelmed by differences in convective dynamics and thermodynamics. Idealized simulations, using Cloud Model 1 (CM1), of two of the events, one of which supported an ordinary thunderstorm and the other a splitting supercell, were used to test the sensitivity of cold pool onset to CCN within different convective environments. Time-integrated microphysical budgets were also calculated to quantify the contribution of each precipitating hydrometeor phase change to initiating the surface cold pool. The hypothesized positive correlation between CCN concentrations and cold pool onset was produced in both environments through delayed and weakened surface precipitation, but this was realized through the influence of CCN on warm rain which in turn influenced graupel amounts, and through its melting, surface rainfall. Cold pool onset was most sensitive to initial CCN concentrations in both environments for values below those observed, and the supercell environment produced much weaker sensitivities than the ordinary thunderstorm. The magnitude and vertical extent latent cooling maxima decreased by an order of magnitude with increasing CCN for the ordinary thunderstorm, resulting in cold pools that were weaker, more slowly expanding, and shallower. The magnitude and vertical extent of latent cooling maxima were much less affected by CCN across the supercell simulations, resulting in only minor changes to cold pool properties. Although the graupel sublimation rate was often the dominant contributor to latent cooling that produced the cold pool in both environments, cold pool onset time showed the best correlation with rain evaporation. Cold pools across all but one of the simulations in both environments were found to initiate once time-integrated surface total latent cooling, dominate by rain evaporation, had surpassed a threshold unique to each environment, and the potential utility of such a threshold in future parameterizations is discussed.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Tobias Ross

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