Sensitivity of climate simulations to the parameterization of cumulus convection in the Canadian climate centre general circulation model

Abstract

A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective‐scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub‐cloud layer, acts to remove CAPE at an exponential rate with a specified adjustment time scale.

The performance of the scheme and its sensitivity to choices of disposable parameters is illustrated by presenting results from a series of idealized single‐column model tests. These tests demonstrate that the scheme permits establishment of a quasi‐equilibrium between large‐scale forcing and convective response. However, it is also shown that the strength of convective downdrafts is an important factor in determining the nature of the equilibrium state. Relatively strong down‐drafts give rise to an unsteady irregularly fluctuating state characterized by alternate periods of deep and shallow convection.

The effect of using the scheme for GCM climate simulations is illustrated by presenting selected results of a multi‐year simulation carried out with the Canadian Climate Centre GCM using the new parameterization (the CONV simulation). Comparison of these results with those for a climate simulation made with the standard model (the CONTROL simulation, as documented by McFarlane et al., 1992) reveals the importance of other parameterized processes in determining the ultimate effect of introducing the new convective scheme. The radiative response to changes in the cloudiness regime is particularly important in this regard.     

Gypsies in the palace: experimentalist's view on the use of 3‐D physics‐based simulation of hillslope hydrological response

As a fundamental unit of the landscape, hillslopes are studied for their retention and release of water and nutrients across a wide range of ecosystems. The understanding of these near‐surface processes is relevant to issues of runoff generation, groundwater–surface water interactions, catchment export of nutrients, dissolved organic carbon, contaminants (e.g. mercury) and ultimately surface water health. We develop a 3‐D physics‐based representation of the Panola Mountain Research Watershed experimental hillslope using the TOUGH2 sub‐surface flow and transport simulator. A recent investigation of sub‐surface flow within this experimental hillslope has generated important knowledge of threshold rainfall‐runoff response and its relation to patterns of transient water table development. This work has identified components of the 3‐D sub‐surface, such as bedrock topography, that contribute to changing connectivity in saturated zones and the generation of sub‐surface stormflow. Here, we test the ability of a 3‐D hillslope model (both calibrated and uncalibrated) to simulate forested hillslope rainfall‐runoff response and internal transient sub‐surface stormflow dynamics. We also provide a transparent illustration of physics‐based model development, issues of parameterization, examples of model rejection and usefulness of data types (e.g. runoff, mean soil moisture and transient water table depth) to the model enterprise. Our simulations show the inability of an uncalibrated model based on laboratory and field characterization of soil properties and topography to successfully simulate the integrated hydrological response or the distributed water table within the soil profile. Although not an uncommon result, the failure of the field‐based characterized model to represent system behaviour is an important challenge that continues to vex scientists at many scales. We focus our attention particularly on examining the influence of bedrock permeability, soil anisotropy and drainable porosity on the development of patterns of transient groundwater and sub‐surface flow. Internal dynamics of transient water table development prove to be essential in determining appropriate model parameterization. Copyright © 2010 John Wiley & Sons, Ltd.     

MHD simulations of rayleigh-taylor instability in young supernova remnants

Radio observations shows that young supernova remnants such as Tycho and Cas A generally exhibit a circular clumpy shell. This shell shows a radial magnetic field whose equipartition strength is 2 to 3 orders of magnitude higher than the interstellar field. A simple compression of the ambient field by the shock can explain neither of these observations. We show that the Rayleigh-Taylor instability which occurs at the ejecta/ISM interface can explain these observations. We have done MHD simulations of the instability in the shell of Type-I supernova remnants for the first time by utilizing moving grid technique. Our simulation shows that Rayleigh-Taylor and Kelvin-Helmholtz instabilities amplify ambient magnetic fields locally and produce the clumpy radio shell. Strong magnetic field lines draped around the Rayleigh-Taylor fingers produce the radial B-vector polarization, whereas thermal bremsstrahlung from the dense fingers themselves produce the clumpy X-ray emission.     

Polysynthetic feldspar twin-width distributions: Models for inversion twinning

The lamella-width distribution of the transformation polysynthetic twins produced in volcanic anorthoclases during cooling reflects certain aspects of twin nucleation and growth kinetics. An analysis of simple hypothetical statistical models for the kinetics of the transformation process outlines the range and type of twin-width distributions possible. The general characteristics of these distributions, particularly at small twin widths, are sensitive to whether twin boundaries or twin centers nucleate. The rates of twin nucleation and growth during transformation also influence the general form, particularly the skewness, of the final twin-width distribution. The generally exponential nature of the upper tails to the distributions studied is apparently quite insensitive to kinetic details.     

Tangential discontinuities in the solar wind

This paper considers six discontinuity surfaces which were observed by magnetometers on 3 spacecraft in the solar wind. It is shown that the actual surface orientations, determined from the measured time delays and solar wind speed, are consistent with the theoretical orientations which were computed from the relation , where is the normal to the surface of a hydromagnetic tangential discontinuity across which the magnetic field direction changes from to . The plasma and magnetic field data for these discontinuities are consistent with the pressure balance condition, and the magnetic field vectors in the associated current sheets are parallel to the discontinuity surface, as required theoretically. The 6 discontinuity surfaces extended without much distortion over ∼ 0.002 AU. A seventh surface is discussed which satisfies the condition but which extended without much distortion over 0.01 AU. This latter is not a typical surface, however, and its curvature is larger than average. Most of the surfaces tended to lie along the spiral direction, but one was nearly perpendicular to the spiral direction.     

On the detectability of turbulence and bulk flows in X-ray clusters

Coolingflows, cluster mergers, and the motions of galaxies through cluster gas with supersonic and sonic velocities must lead to large scale motions of the intracluster medium (ICM). A high-resolution numerical simulation of X-ray cluster formation by Norman and Bryan (1999) predicts cluster-wide turbulence with ν_turb ≈300–600 km s^?1 and eddy scales l_outer ≈100–500 kpc, the larger numbers being characteristic of turbulence near the virial radius, while the smaller numbers pertain to the core. The simulation also predicts the existence of ordered bulk flows in the core with v≈400 km s^?1 on scales of several hundred kpc. In this paper, we consider the observability of such fluid motions via the distortions they induce in the CMB via the kinematic SZ effect, as well as via Doppler broadening and shifting of metal lines in the X-ray spectrum. We estimate |ΔT/T|_kinematic?6—at or below current limits of detectability. However, we find that an energy resolution of a few eV is sufficient to detect several Doppler shifted components in the 6.7 keV Fe line in the cluster core.     

Interactions of isolated vortices I: Modons colliding with modons

Abstract

Collisions between isolated(i.e. localized) dipolar vortex states, called modons, are examined in various numerical solutions of the quasigeostrophic, equivalent barotropic equation. For a range of parameters, the collision interactions are soliton-like in that the vorticity maxima are displaced (phase-shifted) with only small speed changes and little excitation of internal degrees of freedom. For other parameters, new “inelastic” effects are observed, including speed changes due to vorticity rearrangement, vorticity filamentation, modon “capture” or “fusion” in an overtaking collision, and the “fission” of a modon into its component vorticity monopoles in a head-on collision.     

Organohalogens and metals in marine fish and mussels and some relationships to biological variables at reference localities in Norway

The main goals of this study were to define, for the Norwegian coast, "high background" levels of micropollutants routinely monitored within the the Oslo and Paris Commission Joint Assessment and Monitoring Programme, and to reassess the present Norwegian reference levels used as the basis for a classification of environmental quality. The indicator organisms analysed as bulked/individual samples were cod (Gadus morhua, n approximately 1170; n approximately 240 for organochlorines), dab (Limanda limanda, n approximately 75), plaice (Pleuronectes platessa, n approximately 55) lemon sole (Microstomus kitt, n approximately 13). and blue mussels (Mytilus edulis, n approximately 200) collected between 1991-2000. Based on the 90-95 percentiles of measurements at localities far from point sources, we propose for liver of cod a reference level for SigmaPCB(7) (CBs 28, 52, 101, 118, 138, 153 and 180) of 500 microg kg(-1) w.w. and further for DDE 200 microg kg(-1), SigmaHCHs 30 microg kg(-1), HCB 20 and OCS 5 microgkg(-1) w.w. In the same order the proposed reference levels for liver of dab are: 150, 60, 15, 5, and 2 microgkg(-1) w.w.; and for the soft body of mussels: 3, 1, 1, 0.1 and 0.1 microg kg(-1) wet weight. Reference levels of metals are 0.1 mgHg kg(-1) w.w. in fish fillet, and for Hg, Cd, Pb, Zn, Cu in mussels 0.2, 2, 3, 200 and 10 mg kg(-1) d.w., respectively. Hg in fillet of cod and dab were positively correlated with length and weight. PCB levels in liver of cod and dab showed a positive but weak correlation with length, whereas no co-variation was found between PCBs and liver fat. The results of introductory recordings of polychlorinated dibenzo-p-dioxins/dibenzofurans, polychlorinated naphtalenes, Toxaphene and brominated flame retardants in liver of cod from reference sites are also presented.