Statistical properties of cloud lifecycles in cloud-resolving models

Abstract

Abstract.A new technique is described for the analysis of cloud-resolving model simulations, which allows one to investigate the statistics of the lifecycles of cumulus clouds. Clouds are tracked from timestep to timestep within the model run. This allows for a very simple method of tracking, but one which is both comprehensive and robust. An approach for handling cloud splits and mergers is described which allows clouds with simple and complicated time histories to be compared within a single framework. This is found to be important for the analysis of an idealized simulation of radiative-convective equilibrium, in which the moist, buoyant updrafts (i.e., the convective cores) were tracked. Around half of all such cores were subject to splits and mergers during their lifecycles. For cores without any such events, the average lifetime is 30 min, but events can lengthen the typical lifetime considerably.

Lagrangian Cloud Tracking and the Precipitation-Column Humidity Relationship

“…Many others have tracked clouds in models [11][12][13]28], from radar data [9,10], or from photography [38]. Most of these previously employed methods share computational and conceptual characteristics with one another and with the method we discuss below.…”

“…Additionally, 6% of clouds split and then remerge at some later time. Similarly, Plant [28] and Lochbihler et al [63] found that approximately half of all convective events are associated with a split or merger but Dawe and Austin [13] found far fewer shallow clouds are. Convection is clustered in the RCE simulation in which clouds are tracked so it is impossible to know whether this split/merger frequency represents a natural one.…”

“…High-resolution temporal output is required to accurately link-in-time rapidly evolving clouds. While it is possible to avoid the need for finely spaced temporal output by using an online tracking method [28], offline tracking presents several benefits such as adaptability and simplicity. For the purposes of tracking, clouds are defined to be made up of two parts, "cores" and "shells".…”

“…It is obvious from everyday experience that many clouds do not follow a simple, scripted lifecycle. Many isolated clouds are the result of splits from larger clouds or the merger of two smaller clouds at some previous time [28][29][30][31]. These types of complex evolutions lack a sufficient conceptual place within the standard evolutionary pathway.…”

Finite departure from convective quasi‐equilibrium: periodic cycle and discharge–recharge mechanism

“…It opens a possible route towards statistical cumulus dynamics, a methodology proposed by Arakawa and Schubert (1974) as a systematic approach to the closure problem. At present, only semi-phenomenological descriptions of some aspects of equilibrium statistical cumulus dynamics exist (e.g., Craig, 2004, 2006;Craig and Cohen, 2006;Plant and Craig, 2008;Plant, 2009). In order to address time-varying applications, such investigations should also be compared with suitable CRM analyses of plume statistics, such as those produced by Xu and Randall (2001).…”