Comparing Growth Rates of Simulated Moist and Dry Convective Thermals

Journal Article · Wed Feb 24 00:00:00 EST 2021 · Journal of the Atmospheric Sciences

DOI:https://doi.org/10.1175/jas-d-20-0166.1· OSTI ID:1787001

Morrison, Hugh ^[1]; Peters, John M. ^[2]; Sherwood, Steven C. ^[3]

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1. National Center for Atmospheric Research, Boulder, CO (United States); Univ. of New South Wales, Sydney, NSW (Australia); NCAR
2. Naval Postgraduate School, Monterey, CA (United States)
3. Univ. of New South Wales, Sydney, NSW (Australia)

The spreading rates of convective thermals are linked to their net entrainment, and previous literature has suggested differences in spreading rates between moist and dry thermals. Here, growth rates of idealized numerically simulated axisymmetric dry and moist convective thermals are directly compared. In an environment with dry-neutral stratification, the increase of thermal radius with height dR/dz is a larger by a factor of 1.7 for dry thermals relative to moist thermals. The fractional change in thermal volume ε is also greater for dry thermals within a distance of ~4 radii from the initial thermal height. Values of dR/dz are nearly constant with height for both moist and dry thermals consistent with classical theory based on dimensional analysis. The simulations are also consistent with theory relating impulse, circulation, and spreading rate for dry thermals proposed in previous papers and extended here to moist thermals, predicting they will spread less than dry thermals. Tests adding heating to dry thermals, either spread uniformly across the thermal volume or concentrated in the inner core, indicate that dR/dz and ε are smaller for moist thermals because latent heating is confined mostly to their cores. Additional axisymmetric moist simulations using modified lapse rates and large-eddy simulations support this analysis. Overall, these results indicate that slow spreading rates are a fundamental feature of moist thermals caused by latent heating that alters the spatial distribution of buoyancy within them relative to dry thermals.

• Research Organization: National Center for Atmospheric Research, Boulder, CO (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: SC0016476
• OSTI ID: 1787001
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Journal Issue: 3 Vol. 78; ISSN 0022-4928
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English

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