Simulations of Trade Wind Cumuli under a Strong Inversion

Abstract

The fifth intercomparison of the Global Water and Energy Experiment Cloud System Studies Working Group 1 is used as a vehicle for better understanding the dynamics of trade wind cumuli capped by a strong inversion. The basis of the intercomparison is 10 simulations by 7 groups. These simulations are supplemented by many further sensitivity studies, including some with very refined grid meshes. The simulations help illustrate the turbulent dynamics of trade cumuli in such a regime. In many respects the dynamics are similar to those found in many previous simulations of trade cumuli capped by weaker inversions. The principal differences are the extent to which the cloud layer is quasi-steady in the current simulations, evidence of weak countergradient momentum transport within the cloud layer, and the development and influence of an incipient stratiform cloud layer at the top of the cloud layer. Although many elements of the turbulent structure (including the wind profiles, the evolution of cloud-base height, the statistics of the subcloud layer, and the nature of mixing in the lower and middle parts of the cloud layer) are robustly predicted, the representation of the stratiform cloud amount by the different simulations is remarkably sensitive to a number of factors. Chief among these are differences between numerical algorithms. These sensitivities persist even among simulations on relatively refined grid meshes. Part of this sensitivity is attributed to a physically realistic positive radiative feedback, whereby a propensity toward higher cloud fractions in any given simulation is amplified by longwave radiative cooling. The simulations also provide new insight into the dynamics of the transition layer at cloud base. In accord with observations, the simulations predict that this layer is most identifiable in terms of moisture variances and gradients. The simulations help illustrate the highly variable (in both height and thickness) nature of the transition layer, and we speculate that this variability helps regulate convection. Lastly the simulations are used to help evaluate simple models of trade wind boundary layers. In accord with previous studies, mass-flux models well represent the dynamics of the cloud layer, while mixing-length models well represent the subcloud layer. The development of the stratiform cloud layer is not, however, captured by the mass-flux models. The simulations indicate that future theoretical research needs to focus on interface rules, whereby the cloud layer is coupled to the subcloud layer below and the free atmosphere above. Future observational studies of this regime would be of most benefit if they could provide robust cloud statistics as a function of mean environmental conditions.

Analytical expressions for entrainment and detrainment in cumulus convection

“…In that case many LES studies showed that the cloud fraction varies strongly with height (e.g. Siebesma and Cuijpers, 1995;Stevens et al, 2001;Brown et al, 2002). The turbulent flux across the cloud interface can be well approximated by an eddy diffusivity approach (Asai and Kasahara, 1967;Kuo, 1962):…”

A Bayesian algorithm for the retrieval of liquid water cloud properties from microwave radiometer and millimeter radar data

“…[73] To simulate maritime trade-wind cumulus, we adapt an idealization from Stevens et al [2001] of measurements averaged over the first 5 days of the Atlantic Trade-Wind Experiment (ATEX). The model is idealized with surface conditions, and soundings from the upstream ship in the ATEX flotilla.…”

“…Following Stevens et al [2001], we parameterize these forcings to fade linearly from a maximum at the surface to zero at the trade inversion. The surface drying tendency we use (1.3 g/kg/d) is taken directly from Stevens et al [2001].…”

“…Following Stevens et al [2001], we parameterize these forcings to fade linearly from a maximum at the surface to zero at the trade inversion. The surface drying tendency we use (1.3 g/kg/d) is taken directly from Stevens et al [2001]. We double the surface cooling tendency to 2 K/d, which compensates for a clear sky cooling rate that is half the value imposed by Stevens et al [2001].…”

“…The surface drying tendency we use (1.3 g/kg/d) is taken directly from Stevens et al [2001]. We double the surface cooling tendency to 2 K/d, which compensates for a clear sky cooling rate that is half the value imposed by Stevens et al [2001].…”