Oceanic stochastic parametrizations in a seasonal forecast system

We study the impact of three stochastic parametrizations in the ocean component of a coupled model, on forecast reliability over seasonal timescales. The relative impacts of these schemes upon the ocean mean state and ensemble spread are analyzed. The oceanic variability induced by the atmospheric forcing of the coupled system is, in most regions, the major source of ensemble spread. The largest impact on spread and bias came from the Stochastically Perturbed Parametrization Tendency (SPPT) scheme - which has proven particularly effective in the atmosphere. The key regions affected are eddy-active regions, namely the western boundary currents and the Southern Ocean. However, unlike its impact in the atmosphere, SPPT in the ocean did not result in a significant decrease in forecast error. Whilst there are good grounds for implementing stochastic schemes in ocean models, our results suggest that they will have to be more sophisticated. Some suggestions for next-generation stochastic schemes are made.Comment: 24 pages, 3 figure

Oceanic stochastic parametrizations in a seasonal forecast system

We study the impact of three stochastic parametrizations in the ocean component of a coupled model, on forecast reliability over seasonal timescales. The relative impacts of these schemes upon the ocean mean state and ensemble spread are analyzed. The oceanic variability induced by the atmospheric forcing of the coupled system is, in most regions, the major source of ensemble spread. The largest impact on spread and bias came from the Stochastically Perturbed Parametrization Tendency (SPPT) scheme - which has proven particularly effective in the atmosphere. The key regions affected are eddy-active regions, namely the western boundary currents and the Southern Ocean. However, unlike its impact in the atmosphere, SPPT in the ocean did not result in a significant decrease in forecast error. Whilst there are good grounds for implementing stochastic schemes in ocean models, our results suggest that they will have to be more sophisticated. Some suggestions for next-generation stochastic schemes are made.Comment: 24 pages, 3 figure

Cloud microphysical effects of turbulent mixing and entrainment

Turbulent mixing and entrainment at the boundary of a cloud is studied by means of direct numerical simulations that couple the Eulerian description of the turbulent velocity and water vapor fields with a Lagrangian ensemble of cloud water droplets that can grow and shrink by condensation and evaporation, respectively. The focus is on detailed analysis of the relaxation process of the droplet ensemble during the entrainment of subsaturated air, in particular the dependence on turbulence time scales, droplet number density, initial droplet radius and particle inertia. We find that the droplet evolution during the entrainment process is captured best by a phase relaxation time that is based on the droplet number density with respect to the entire simulation domain and the initial droplet radius. Even under conditions favoring homogeneous mixing, the probability density function of supersaturation at droplet locations exhibits initially strong negative skewness, consistent with droplets near the cloud boundary being suddenly mixed into clear air, but rapidly approaches a narrower, symmetric shape. The droplet size distribution, which is initialized as perfectly monodisperse, broadens and also becomes somewhat negatively skewed. Particle inertia and gravitational settling lead to a more rapid initial evaporation, but ultimately only to slight depletion of both tails of the droplet size distribution. The Reynolds number dependence of the mixing process remained weak over the parameter range studied, most probably due to the fact that the inhomogeneous mixing regime could not be fully accessed when phase relaxation times based on global number density are considered.Comment: 17 pages, 10 Postscript figures (figures 3,4,6,7,8 and 10 are in reduced quality), to appear in Theoretical Computational Fluid Dynamic

Limited-are a modelling of stratocumulus over South-Eastern Pacific

This paper presents application of the Weather Research and Forecasting (WRF) model to limited-area modeling of atmospheric processes over the subtropical south-eastern Pacific, with the emphasis on the stratocumulus-topped boundary layer. The simulations cover a domain from the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field project conducted in the subtropical south-eastern Pacific in October and November 2008. We focus on a day where the UK's BAe-146 research aircraft encountered Pockets of Open Cells (POCs) at the very western edge of its flight track, rather than on the entire campaign as investigated in previous limited-area modeling studies. Model results are compared to aircraft observations with the main conclusion that the simulated stratocumulus-topped boundary layer is significantly too shallow. This appears to be a combination of an already too shallow boundary layer in the dataset used to provide initial and lateral boundary conditions, and the inability of the WRF model to increase the boundary-layer height. Several sensitivity simulations, applying different subgrid-scale parameterizations available in the model, a larger computational domain and longer simulations, as well as a different dataset providing initial and lateral boundary conditions were all tried to improve the simulation. These changes appeared to have a rather small effect on the results. The model does simulate the formation of mesoscale cloud-free regions that one might consider similar to Pockets of Open Cells observed in nature. However, formation of these regions does not seem to be related to drizzle-induced transition from open- to closed-cell circulations as simulated by LES models. Instead, the cloud-free regions appear to result from mesoscale variations of the lower-tropspheric vertical velocity. Areas of negative vertical velocity with minima (a few cm s<sup>−1</sup>) near the boundary layer top seem to induce direct evaporation of the cloud layer. It remains to be seen in LES studies whether the mechanism seen in the model is realistic or if it is simply an artifact of interactions between resolved and parameterized processes

Numerical simulations of stratocumulus cloud response to aerosol perturbation

In this paper results from the 2D numerical model with Lagrangian representation of microphysics are used to investigate the response of the radiative properties of stratocumulus as a result of adding aerosol within the boundary layer. Three different cases characterized by low, moderate and high cloud droplet number and for 3 sizes of additional aerosol 0.01. μm, 0.1. μm and 0.5. μm are discussed. The model setup is an idealization of one of the proposed Solar Radiation Management methods to mitigate global warming by increasing albedo of stratocumulus clouds. Analysis of the model results shows that: the albedo may increase directly in response to additional aerosol in the boundary layer; the magnitude of the increase depends on the microphysical properties of the existing cloud and is larger for cloud characterized by low cloud droplet number; for some cases for clouds characterized by high cloud droplet number seeding may lead to the decrease in albedo when too large radius of seeding aerosol is used

Implementation of U.K. Earth system models for CMIP6

We describe the scientific and technical implementation of two models for a core set of experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The models used are the physical atmosphere-land-ocean-sea ice model HadGEM3-GC3.1 and the Earth system model UKESM1 which adds a carbon-nitrogen cycle and atmospheric chemistry to HadGEM3-GC3.1. The model results are constrained by the external boundary conditions (forcing data) and initial conditions.We outline the scientific rationale and assumptions made in specifying these. Notable details of the implementation include an ozone redistribution scheme for prescribed ozone simulations (HadGEM3-GC3.1) to avoid inconsistencies with the model's thermal tropopause, and land use change in dynamic vegetation simulations (UKESM1) whose influence will be subject to potential biases in the simulation of background natural vegetation.We discuss the implications of these decisions for interpretation of the simulation results. These simulations are expensive in terms of human and CPU resources and will underpin many further experiments; we describe some of the technical steps taken to ensure their scientific robustness and reproducibility

Another look at stochastic condensation in clouds: Exact solutions, fokker-planck approximations and adiabatic evolution

In this manuscript - which closely follows Jeffery et al. (2006) - we have taken "another look" at stochastic condensation in the hope of clarifying the earlier derivations and fully exploring the implications of this theory. In contrast to the derivations of Levin and Sedunov (1966a, b) and Manton (1979), we begin with a simple model of stochastic condensation - independent, Gaussian supersaturation fluctuations (S′) renewed after a time τ - that is exactly solvable. This model is trivial to simulate on a computer and can be used to compare and contrast Lagrangian and Eulerian approaches for modeling droplet spectra (Andrejczuk et al. 2006). The Fokker-Planck approximation to this exact solution follows by replacing the discrete sampling of S′ with its continuous surrogate. The Fokker-Planck diffusivity and operator are thus seen to be the natural smooth-in-time approximation to a discrete-in-time process. We have also taken another - look at the equation for the mean supersaturation, S̄FP, in the presence of S′ fluctuations modeled using the Levin-Sedunov-Mazin Fokker-Planck operator. While this problem is treated in an approximate fashion (and with little transparency) in Voloshchukand Sedunov (1977), we derive the expression for (S′|r)FP without approximation and show how this expression "closes" the S̄FP-equation self-consistently, thereby ensuring that total water mass is exactly conserved. Using the quasi-stationary (QS) evaluation of S̄FP, we derive the exact correction term to SFP, QS (i.e. the S′ contribution corresponding to the Levin-Sedunov-Mazin model). The correction term is negative definite, peaks in magnitude when (r)r is near the accommodation length (≈ 2 μm), and decays as (r-1)r as the droplet spectrum grows to large sizes. This exact result has a direct correspondence to the analysis of Cooper (1989). Using our self-consistent equation for S̄FP, we evaluate spectral broadening in an adiabatic parcel and find some broadening to larger sizes (consistent with earlier estimates, e.g. Manton (1979)), but a more significant decrease in 〈r2〉 r at fixed liquid water content which may have implications for modeled cloud reflectivity. While the proceeding discussion is largely a clarification and elucidation of previous work, most notably Voloshchuk and Sedunov (1977), we have also extended the theory of stochastic condensation by deriving the non-dimensional number, ND, that determines the relative impact of S′-fluctuations on droplet spectral evolution in an adiabatic volume and in the QS limit. For constant updraft velocity and Fokker-Planck diffusivity, ND is also a constant, ranging from 10-2 to 102 for typical atmospheric conditions and model grid sizes when the assumed S′-standard deviation is 1%. We find significant spectral broadening, and in particular decreasing 〈r2〉r, for ND > 1, and discover that S̄FP, QS can be negative in a rising adiabatic parcel when ND > 6.5 for droplets of zero initial size. Using in-situ droplet spectra from cumulus cloud fields observed during the RICO and SCMS field campaigns, we have verified a seminal prediction of the theory of stochastic condensation - increasing broadening with increasing spatial scale - by averaging the observed spectra over segments containing one or more clouds. In addition, scale-dependent values of ND retrieved from the segment-averaged spectra using our adiabatic model show good consistency with the previously discussed theoretical estimates. We believe this encouraging result to be the first observational confirmation of the stochastic condensation mechanism and the decades-old, pioneering work of Levin-Sedunov-Mazin. Moreover, these results suggest that the parameterization of unresolved S′-fluctuations using Fokker-Planck theory or other means will become increasingly important as explicit (bin) microphysics schemes are applied at larger scales (Lynn et al. 2005), where an increasing fraction of individual clouds are, themselves, unresolved. However, important differences between the observed and modeled droplet spectra are also observed. In particular, the observed spectra suggest non-Gaussian S′ fluctuations and the inhomogeneous mixing process of Baker et al. (1980). Further work is needed to assess the impact of non-Gaussian S′-fluctuations and large renewal times on droplet spectral broadening and to derive differential operators that can model their ensemble effect in the equations of cloud physics