Influences of Environmental Relative Humidity and Horizontal Scale of Subcloud Ascent on Deep Convective Initiation

Journal Article · 31 January 2022 · Journal of the Atmospheric Sciences

DOI: https://doi.org/10.1175/jas-d-21-0056.1 · OSTI ID:1980916

Morrison, Hugh ^[1]; Peters, John M. ^[2]; Chandrakar, Kamal Kant ^[3]; Sherwood, Steven C. ^[4]

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

This study examines two factors impacting initiation of moist deep convection: free-tropospheric environmental relative humidity (φ_E) and horizontal scale of subcloud ascent (R_sub), the latter exerting a dominant control on cumulus cloud width. A simple theoretical model is used to formulate a “scale selection” hypothesis: that a minimum R_sub is required for moist convection to go deep, and that this minimum scale decreases with increasing φ_E. Specifically, the ratio of R²_sub to saturation deficit (1 - φ_E) must exceed a certain threshold value that depends on cloud-layer environmental lapse rate. Idealized, large-eddy simulations of moist convection forced by horizontally varying surface fluxes show strong sensitivity of maximum cumulus height to both φ_E and R_sub consistent with the hypothesis. Increasing R_sub by only 300–400 m can lead to a large increase (>5 km) in cloud height. A passive tracer analysis shows that the bulk fractional entrainment rate decreases rapidly with R_sub but depends little on φ_E. However, buoyancy dilution increases as either R_sub or φ_E decreases; buoyancy above the level of free convection is rapidly depleted in dry environments when R_sub is small. While deep convective initiation occurs with an increase in relative humidity of the near environment from moistening by earlier convection, the importance of this moisture preconditioning is inconclusive as it is accompanied by an increase in R_sub. Overall, it is concluded that small changes to R_sub driven by external forcing or by convection itself could be a dominant regulator of deep convective initiation.

• Research Organization: University Corporation for Atmospheric Research, Boulder, CO (United States)
• Sponsoring Organization: Australian Research Council (ARC); National Science Foundation (NSF); USDOE Office of Science (SC)
• Grant/Contract Number: SC0020104
• OSTI ID: 1980916
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Journal Issue: 2 Vol. 79; ISSN 0022-4928
• Publisher: American Meteorological SocietyCopyright Statement
• Country of Publication: United States
• Language: English