ϱ → 4π in chirally symmetric models

The decays rho0 → 2π+2π− and rho0 → 2π0π+π− are studied using various effective Lagrangians for π and rho (and in some case a1) mesons, all of which respect the approximate chiral symmetry of the strong interaction. Partial widths of the order of 1 keV or less are found in all cases. These are an order of magnitude smaller than recent predictions based on non-chiral models

Generalized convective quasi-equilibrium principle

A generalization of Arakawa and Schubert's convective quasi-equilibrium principle is presented for a closure formulation of mass-flux convection parameterization. The original principle is based on the budget of the cloud work function. This principle is generalized by considering the budget for a vertical integral of an arbitrary convection-related quantity. The closure formulation includes Arakawa and Schubert's quasi-equilibrium, as well as both CAPE and moisture closures as special cases. The formulation also includes new possibilities for considering vertical integrals that are dependent on convective-scale variables, such as the moisture within convection. The generalized convective quasi-equilibrium is defined by a balance between large-scale forcing and convective response for a given vertically-integrated quantity. The latter takes the form of a convolution of a kernel matrix and a mass-flux spectrum, as in the original convective quasi-equilibrium. The kernel reduces to a scalar when either a bulk formulation is adopted, or only large-scale variables are considered within the vertical integral. Various physical implications of the generalized closure are discussed. These include the possibility that precipitation might be considered as a potentially-significant contribution to the large-scale forcing. Two dicta are proposed as guiding physical principles for the specifying a suitable vertically-integrated quantity

A dynamic extension of the pragmatic blending scheme for scale-dependent sub-grid mixing

The pragmatic blending approach of Boutle et al. (2014) treats sub-grid turbulent mixing using a weighted average of a 1D mesoscale-model and a 3D Smagorinsky formulation. Here the approach is modified and extended to incorporate a scale-dependent dynamic Smagorinsky scheme instead of a static Smagorinsky scheme. Results from simulating an evolving convective boundary layer show that the new scheme is able to improve the representation of turbulence statistics and potential temperature profiles at grey-zone resolutions during the transition from the shallow morning to the deep afternoon boundary layer. This is achieved mainly because the new scheme enables and controls an improved spin-up of resolved turbulence. The dynamic blending scheme is shown to be more adaptive to the evolving flow and somewhat less sensitive to the blending parameters. The new approach appears to offer a more robust and more flexible formulation of blending and the results are strongly encouraging of further assessment and development

Cloud trails past Bermuda: a five-year climatology from 2012-2016

Cloud trails are primarily thermally forced bands of cloud that extend down-wind of small islands. A novel algorithm to classify conventional geostationary visible-channel satellite images as Cloud Trail (CT), Non-Trail (NT), or Obscured (OB) is defined. The algorithm is then applied to the warm season months of five years at Bermuda comprising 16,400 images. Bermuda’s low elevation and location make this island ideal for isolating the role of the island thermal contrast on CT formation. CT are found to occur at Bermuda with an annual cycle, peaking in July, and a diurnal cycle that peaks in mid-afternoon. Composites of radiosonde observations and ERA-interim data suggest that a warm and humid low-level environment is conducive for CT development. From a Lagrangian perspective, wind direction modulates CT formation by maximizing low-level heating on local scales when winds are parallel to the long axis of the island. On larger scales, low-level wind direction also controls low-level humidity through advection

Resolution dependence of turbulent structures in convective boundary layer simulations

Large-eddy simulations are performed using the UK Met Office Large Eddy Model to study the effects of resolution on turbulent structures in a convective boundary layer. A standard Smagorinsky subgrid scheme is used. As the grid length is increased the diagnosed height of the boundary layer increases and the horizontally- and temporally-averaged temperature near the surface and in the inversion layer increase. At the highest resolution, quadrant analysis shows that the majority of events in the lower boundary layer are associated with cold descending air, followed by warm ascending air. The largest contribution to the total heat flux is made by warm ascending air, with associated strong thermals. At lower resolutions, the contribution to the heat flux from cold descending air is increased and that from cold ascending air is reduced in the lower boundary layer; around the inversion layer, however, the contribution from cold ascending air is increased. Calculations of the heating rate show that the differences in cold ascending air are responsible for the warm bias below the boundary layer top in the low resolution simulations. Correlation length and time scales for coherent resolved structures increase with increasing grid coarseness. The results overall suggest that differences in the simulations are due to weaker mixing between thermals and their environment at lower resolutions. Some simple numerical experiments are performed to increase the mixing in the lower-resolution simulations and to investigate backscatter. Such simulations are successful in reducing the contribution of cold ascending air to the heat flux just below the inversion, although the effects in the lower boundary layer are weaker

A stochastic framework for modeling the population dynamics of convective clouds

A stochastic prognostic framework for modeling the population dynamics of convective clouds and representing them in climate models is proposed. The framework follows the non-equilibrium statistical mechanical approach to constructing a master equation for representing the evolution of the number of convective cells of a specific size and their associated cloud-base mass flux, given a large-scale forcing. In this framework, referred to as STOchastic framework for Modeling Population dynamics of convective clouds (STOMP), the evolution of convective cell size is predicted from three key characteristics of convective cells: (i) the probability of growth, (ii) the probability of decay, and (iii) the cloud-base mass flux. STOMP models are constructed and evaluated against CPOL radar observations at Darwin and convection permitting model (CPM) simulations. Multiple models are constructed under various assumptions regarding these three key parameters and the realisms of these models are evaluated. It is shown that in a model where convective plumes prefer to aggregate spatially and the cloud-base mass flux is a non-linear function of convective cell area, then the mass flux manifests a recharge-discharge behavior under steady forcing. Such a model also produces observed behavior of convective cell populations and CPM simulated cloud-base mass flux variability under diurnally varying forcing. In addition to its use in developing understanding of convection processes and the controls on convective cell size distributions, this modeling framework is also designed to be capable of serving as a non-equilibrium closure formulations for spectral mass flux parameterizations

The implications of an idealised large-scale circulation for mechanical work done by tropical convection

A thermodynamic analysis is presented of an overturning circulation simulated by two cloud resolving models, coupled by a weak temperature gradient parametrisation. Taken together, they represent two separated regions over different sea surface temperatures, and the coupling represents an idealised large-scale circulation such as the Walker circulation. It is demonstrated that a thermodynamic budget linking net heat input to the generation of mechanical energy can be partitioned into contributions from the large-scale interaction between the two regions, as represented by the weak temperature gradient approximation, and from convective motions in the active warm region and the suppressed cool region. Model results imply that such thermodynamic diagnostics for the aggregate system are barely affected by the strength of the coupling, even its introduction, or by the SST contrast between the regions. This indicates that the weak temperature gradient parametrisation does not introduce anomalous thermodynamic behaviour. We find that the vertical kinetic energy associated with the large-scale circulation is more than three orders of magnitude smaller than the typical vertical kinetic energy in each region. However, even with very weak coupling circulations, the contrast between the thermodynamic budget terms for the suppressed and active regions is strong and is relatively insensitive to the degree of the coupling. Additionally, scaling arguments are developed for the relative values of the terms in the mechanical energy budget

Sensible heat fluxes control cloud trail strength

Convective cloud bands known as “cloud trails” (CTs), are commonly found downwind of small islands (<O(100) km2) throughout the world. They occur primarily in the afternoon, and are known to form in response to land-sea contrasts under the presence of background flow. A set of idealised numerical experiments with 100 m horizontal grid spacing are performed to quantify the relationship between the surface forcing produced by an island and the strength of the resulting CT circulation. These experiments are based on observed environmental conditions for which a CT occurred off Bermuda, a small subtropical island. For these simulations, the CT circulation is found to be controlled by the strength of the integrated excess heating of the flow as it passes over the island. This excess heating is in turn controlled by the strength of the island heat fluxes when the wind speed and the island geometry are kept constant. Our experiments show, all-else-equal, a linear relationship between CT circulation strength and the island surface heat flux

Composited structure of non-precipitating shallow cumulus clouds

The normalized distributions of thermodynamic and dynamical variables both within and outside shallow clouds are investigated through a composite algorithm using large eddy simulations of oceanic and continental cases. The normalized magnitude is maximum near cloud center and decreases outwards. While relative humidity (RH) and cloud liquid water ($q_l$) decrease smoothly to match the environment, the vertical velocity, virtual potential temperature ($\theta_v$) and potential temperature ($\theta$) perturbations have more complicated behaviour towards the cloud boundary. Below the inversion layer, $\theta_v^{'}$ becomes negative before the vertical velocity has turned from updraft to subsiding shell outside the cloud, indicating the presence of a transition zone where the updraft is negatively buoyant. Due to the downdraft outside the cloud and the enhanced horizontal turbulent mixing across the edge, the normalized turbulence kinetic energy (TKE) and horizontal turbulence kinetic energy (HTKE) decrease more slowly from the cloud center outwards than the thermodynamic variables. The distributions all present asymmetric structures in response to the vertical wind shear, with more negatively buoyant air, stronger downdrafts and larger TKE on the downshear side. We discuss several implications of the distributions for theoretical models and parameterizations. Positive buoyancy near cloud base is mostly due to the virtual effect of water vapor, emphasising the role of moisture in triggering. The mean vertical velocity is found to be approximately half the maximum vertical velocity within each cloud, providing a constraint to achieve possible power law distributions for some models. Finally, the normalized distributions for different variables are used to estimate the vertical heat and moisture fluxes within clouds. The results suggest the distributions near cloud edge and the variability of maximum perturbations need careful treatment. The fluxes are underestimated in the inversion layer because the cloud top downdrafts can not be well captured

A machine learning assisted development of a model for the populations of convective and stratiform clouds

Traditional parameterizations of the interaction between convection and the environment have relied on an assumption that the slowly-varying large-scale environment is in statistical equilibrium with a large number of small and short-lived convective clouds. They fail to capture non-equilibrium transitions such as the diurnal cycle and theformation of meso-scale convective systems as well asobserved precipitation statisticsand extremes. Informed by analysis of radar observations, cloud-permitting model simulation, theory and machine learning, this work presents a new stochastic cloud population dynamics model for characterizing the interactions between convective and stratiform clouds,with the ultimate goal of informing the representation ofthese interactions in global climate models. 15 wet seasons of precipitating cloud observations by a C-band radar at Darwin, Australia are fed into a machine learning algorithm to obtain transition functions that close a set of coupled equation relating large-scale forcing, mass flux, the convective cell size distribution and the stratiform area. Under realistic large-scale forcing, the derived transition functions show that, on the one hand, interactions with stratiform clouds act to dampen the variability in the size and number of convective cells and therefore in the convective mass flux. On the other hand, for a given convective area fraction, a larger number of smaller cells is more favorable for the growth of stratiform area than a smaller number of larger cells. The combination of these two factors gives rise to solutions with a number of convective cells embedded in a large stratiform area, reminiscent of mesoscale convective systems