Departures from convective equilibrium with a rapidly varying surface forcing

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 139; no. 676; pp. 1731 - 1746
• Main Authors: Davies, L., Plant, R. S., Derbyshire, S. H.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.10.2013; Wiley; Wiley Subscription Services, Inc
• Subjects: Breaking down; Clouds; cloud‐resolving modelling; Computer simulation; Convection; Convection modes; convective quasi‐equilibrium; Correlation; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Physics of the high neutral atmosphere; rapid forcing; Surface chemistry; models; convective quasi-equilibrium; rapid forcing; Convective equilibrium; cloud-resolving modelling; Forcing; clouds
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.2065

Convective equilibrium is a long‐standing and useful concept for understanding many aspects of the behaviour of deep moist convection. For example, it is often invoked in developing parametrizations for large‐scale models. However, the equilibrium assumption may begin to break down as models are increasingly used with shorter time steps and finer resolutions. Here we perform idealized cloud‐system resolving model simulations of deep convection with imposed time variations in the surface forcing. A range of rapid forcing time‐scales from 1 to 36 h are used, in order to induce systematic departures from equilibrium. For the longer forcing time‐scales, the equilibrium assumption remains valid, in at least the limited sense that cycle‐integrated measures of convective activity are very similar from cycle to cycle. For shorter forcing time‐scales, cycle‐integrated convection becomes more variable, with enhanced activity on one cycle being correlated with reduced activity on the next, suggesting a role for convective memory. Further investigation shows that the memory does not appear to be carried by the domain‐mean thermodynamic fields but rather by structures on horizontal scales of 5–20 km. Such structures are produced by the convective clouds and can persist beyond the lifetime of the cloud, even through to the next forcing cycle.