The Quarterly Journal of the Royal Meteorological Society

Alexander, M. J.; Geller, M.; McLandress, C.; Polavarapu, S.; Preusse, P.; Sassi, F.; Sato, K.; Eckermann, S.; Ern, M.; Hertzog, A.; Kawatani, Y.; Pulido, M.; Shaw, T. A.; Sigmond, M.; Vincent, R.; Watanabe, S.

doi: 10.1002/qj.637pmid: N/A

Recent observational and theoretical studies of the global properties of small‐scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large‐scale circulation have long been treated via parametrizations in both climate and weather‐forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity‐wave momentum flux, wavelengths and frequencies. Until recently, global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global datasets with improved resolution, along with innovative analysis methods, are now providing constraints for the parametrizations that can improve the treatment of these waves in climate‐prediction models. Research using very‐high‐resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects, and when tested against observations these models show some very realistic properties. Here we review recent studies on gravity‐wave effects in stratosphere‐resolving climate models, recent observations and analysis methods that reveal global patterns in gravity‐wave momentum fluxes and results of very‐high‐resolution model studies, and we outline some future research requirements to improve the treatment of these waves in climate simulations. Copyright © 2010 Royal Meteorological Society and Crown in the right of Canada

Tuck, A. F.

doi: 10.1002/qj.644pmid: N/A

This review attempts to interpret the generalized scale invariance observed in common atmospheric variables—wind, temperature, humidity, ozone and some trace species—in terms of the computed emergence of ring currents (vortices) in simulations of populations of Maxwellian molecules subject to an anisotropy in the form of a flux. The data are taken from ‘horizontal’ tracks of research aircraft and from ‘vertical’ trajectories of research dropsondes. It is argued that any attempt to represent the energy distribution in the atmosphere quantitatively must have a proper basis in molecular physics, a prerequisite to accommodate the observed long‐tailed velocity probability distributions and the implied effects on radiative transfer, atmospheric chemistry, turbulent structure and the definition of temperature itself. The relationship between fluctuations and dissipation is discussed in a framework of non‐equilibrium statistical mechanics, and a link between maximization of entropy production and scale invariance is hypothesized. Copyright © 2010 Royal Meteorological Society

Jung, T.; Balsamo, G.; Bechtold, P.; Beljaars, A. C. M.; Köhler, M.; Miller, M. J.; Morcrette, J.‐J.; Orr, A.; Rodwell, M. J.; Tompkins, A. M.

doi: 10.1002/qj.634pmid: N/A

The progress achieved since 2005 in simulating today's climate with the European Centre for Medium‐Range Weather Forecasts (ECMWF) model through improved physical parametrizations is described. Results are based on climate integrations at an intermediate horizontal resolution (TL159) using major model versions employed operationally at ECMWF since June 2005. Recent improvements to the physical parametrization package are shown to substantially reduce long‐standing systematic model deficiencies in the tropical precipitation, convectively coupled tropical waves, and circulation features in the Northern Hemisphere Extratropics including synoptic‐scale variability and Euro‐Atlantic blocking. The climate integrations are augmented by a set of monthly forecast experiments. By considering the atmospheric response in a seamless sense, i.e. from time‐scales of hours to many months, an attempt is made to understand the impact of changes to the convection and radiation schemes. Overall, the largest and mostly beneficial impact results from the introduction of a major revision to the convection scheme made in November 2007. This is true for systematic errors in the Tropics and Extratropics over a wide range of time‐scales as well as for the short‐range and medium‐range deterministic forecast skill over the Northern Hemisphere. Copyright © 2010 Royal Meteorological Society

Nicolis, C.; Nicolis, G.

doi: 10.1002/qj.642pmid: N/A

The formalism of irreversible thermodynamics is extended to include the effect of random perturbations and applied to representative systems giving rise to instabilities and to complex nonlinear behaviours. The extent to which dissipation as measured by the entropy production exhibits variational properties that can be linked to key indicators of the dynamical behaviour is explored with emphasis on the conjecture of the climate system as a system of maximum dissipation. Copyright © 2010 Royal Meteorological Society

Jiang, Zhina; Wang, Donghai

doi: 10.1002/qj.630pmid: N/A

This work explores the inverse problem of what are the precursors to a given blocking anomaly in climatological flow over the Atlantic and Pacific Oceans, respectively. Blocking anomaly in geopotential height field is specified as a dipole structure which is dominated by a strong positive anomaly centred at about 60°N and a weak negative anomaly south of it. The method of conditional nonlinear optimal perturbation (CNOP) is applied to investigate the above problem by using a T21L3 quasi‐geostrophic model and its tangent linear and adjoint versions. Results show that both for the Atlantic and Pacific blockings, the precursors are baroclinic synoptic‐scale wave train disturbances, whose maximum amplitudes are located upstream of the corresponding blocking regions. The disturbances, which mostly focus on the northward flanks of the corresponding Atlantic and Pacific upper‐level jet, take on a northeast‐southwest trend. However, the leftover parts located in the southward flanks of the corresponding upper‐level jet take on a northwest‐southeast trend. This structure is favourable for the precursors to gain more kinetic energy from the horizontal shear of the basic flow. Further energy analysis reveals that the available potential energy contributes more to the initial precursors, and with time, the kinetic energy dominates the structures. With the help of potential vorticity analysis, it is shown that the planetary‐scale eddy feedback seems to advect anticyclonic (cyclonic) vorticity to the high pressure (low trough) region of blocking, and thus it promotes the onset of blocking. In addition, more specific differences for the precursors to blocking anomalies over the two oceans are compared. Copyright © 2010 Royal Meteorological Society

Bell, Christopher J.; Gray, Lesley J.; Kettleborough, Jamie

doi: 10.1002/qj.633pmid: N/A

The robustness of stratospheric circulation changes under increased concentrations of carbon dioxide are investigated using the Met Office HadSM3‐L64 model. Equilibrium climate change simulations employing forcing of two and four times pre‐industrial CO2 are presented, with particular focus on the temperature response of the Arctic lower stratosphere during Northern Hemisphere winter. High CO2 loading provides the ability to attain the statistical significance of any response, typically a problem given the large component of interannual variability common to the region. In response to CO2, the expected global stratospheric cooling is modified by an anomalous dynamical warming of the Arctic winter lower stratosphere. This warming is shown to be associated with an increase in frequency of stratospheric sudden warming (SSW) events. At four times pre‐industrial CO2, the frequency of SSW events per year is doubled with respect to the control simulation. Further, by comparing winters with and without SSW events, it is shown that the warming of the lower stratosphere cannot be achieved without the presence of a frequency modulation of SSW events. Copyright © 2010 Royal Meteorological Society and Crown Copyright.

Shonk, Jonathan K. P.; Hogan, Robin J.; Edwards, John M.; Mace, Gerald G.

doi: 10.1002/qj.647pmid: N/A

A poor representation of cloud structure in a general circulation model (GCM) is widely recognised as a potential source of error in the radiation budget. Here, we develop a new way of representing both horizontal and vertical cloud structure in a radiation scheme. This combines the ‘Tripleclouds’ parametrization, which introduces inhomogeneity by using two cloudy regions in each layer as opposed to one, each with different water content values, with ‘exponential‐random’ overlap, in which clouds in adjacent layers are not overlapped maximally, but according to a vertical decorrelation scale. This paper, Part I of two, aims to parametrize the two effects such that they can be used in a GCM. To achieve this, we first review a number of studies for a globally applicable value of fractional standard deviation of water content for use in Tripleclouds. We obtain a value of 0.75 ± 0.18 from a variety of different types of observations, with no apparent dependence on cloud type or gridbox size. Then, through a second short review, we create a parametrization of decorrelation scale for use in exponential‐random overlap, which varies the scale linearly with latitude from 2.9 km at the Equator to 0.4 km at the poles. When applied to radar data, both components are found to have radiative impacts capable of offsetting biases caused by cloud misrepresentation. Part II of this paper implements Tripleclouds and exponential‐random overlap into a radiation code and examines both their individual and combined impacts on the global radiation budget using re‐analysis data. Copyright © 2010 Royal Meteorological Society and Crown Copyright

Shonk, Jonathan K. P.; Hogan, Robin J.

doi: 10.1002/qj.646pmid: N/A

Reliably representing both horizontal cloud inhomogeneity and vertical cloud overlap is fundamentally important for the radiation budget of a general circulation model. Here, we build on the work of Part I of this two‐part paper by applying a pair of parametrizations that account for horizontal inhomogeneity and vertical overlap to global re‐analysis data. These are applied both together and separately in an attempt to quantify the effects of poor representation of the two components on radiation budget. Horizontal inhomogeneity is accounted for using the ‘Tripleclouds’ scheme, which uses two regions of cloud in each layer of a gridbox as opposed to one; vertical overlap is accounted for using ‘exponential‐random’ overlap, which aligns vertically continuous cloud according to a decorrelation height. These are applied to a sample of scenes from a year of ERA‐40 data. The largest radiative effect of horizontal inhomogeneity is found to be in areas of marine stratocumulus; the effect of vertical overlap is found to be fairly uniform, but with larger individual short‐wave and long‐wave effects in areas of deep, tropical convection. The combined effect of the two parametrizations is found to reduce the magnitude of the net top‐of‐atmosphere (TOA) cloud radiative forcing by 2.25 W m−2, with shifts of up to 10 W m−2 in areas of marine stratocumulus. The effects on radiation budget of the uncertainty in our parametrizations is also investigated. It is found that the uncertainty in the impact of horizontal inhomogeneity is of order ±60%, while the uncertainty in the impact of vertical overlap is much smaller. This suggests an insensitivity of the radiation budget to the exact nature of the global decorrelation height distribution derived in Part I. Copyright © 2010 Royal Meteorological Society

de Rooy, Wim C.; Pier Siebesma, A.

doi: 10.1002/qj.640pmid: N/A

Analytical expressions for entrainment and detrainment are derived based on general total water specific humidity and mass budget equations. From these expressions, containing a small‐scale turbulent and a larger‐scale organized term, a physical picture emerges for a shallow cumulus cloud ensemble in which the individual clouds have a massive entrainment at the bottom, lateral turbulent mixing with constant mass flux between bottom and top, and massive detrainment at the top. Combining these results with the general budget equation for vertical velocity, new formulae for entrainment and detrainment rates can be expressed in terms of buoyancy, vertical velocity and cloud fraction. For a variety of shallow convection cases, results from large‐eddy simulations show a good correspondence of these new formulae with more traditional methods to diagnose entrainment and detrainment rates. Moreover, the formulae give insight into the behaviour and the physical nature of the mixing coefficients. They explain the observed large variability of the detrainment. The formulae cannot be directly applied as a parametrization. However, it is demonstrated how they can be used to evaluate existing parametrization approaches and as a sound physical base for future parametrization developments. Copyright © 2010 Royal Meteorological Society

Pinsky, Mark; Khain, Alexander; Magaritz, Leehi

doi: 10.1002/qj.624pmid: N/A

In many explicit microphysics cloud models the description of turbulent mixing of droplet size distributions (DSDs) treats them as conservative quantities, which leads to changes in DSDs for adiabatic profiles of liquid water. A new approach representing turbulent diffusion of DSD in Eulerian and Lagrangian cloud models is proposed. The approach is an extension of the classical K‐theory (used for calculation of turbulent fluxes) to the mixing of non‐conservative quantities. The proposed approach takes into account the growth/evaporation of drops as well as nucleation/denucleation in the course of the turbulent mixing. Implementation of the method is illustrated by the analysis of mixing between several different pairs of adjacent Lagrangian parcels: two cloudy parcels, a cloudy parcel with a drop‐free one, and two drop‐free parcels. The initial values of DSD and other parameters of the parcels are taken from simulations of stratocumulus clouds using the Lagrangian trajectory ensemble model. It is shown that taking into account the non‐conservativity of DSD makes the rate and the results of mixing strongly dependent on the mutual locations of the parcels. The effects of mixing increase with the deviation of the vertical profile of liquid water content (LWC) from the adiabatic one. If the upper parcel contains a larger LWC, the effect of mixing on the DSD is significantly weaker than for lower LWC in the upper parcel. The mixing near cloud base and cloud top is more intense. The standard method underestimates the rate of mixing near cloud top and overestimates it near cloud base. The proposed approach can serve as an efficient tool for investigating the role of mixing in Eulerian and Lagrangian models of stratocumulus and cumulus clouds. Copyright © 2010 Royal Meteorological Society