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doi: 10.1002/qj.49712354102pmid: N/A
The theory of the mass‐flux approach to parametrization of convective momentum‐transport is presented. A cloud‐resolving model is used to simulate momentum transport by ensembles of deep convective clouds in two very different regimes: a mid‐latitude cold‐air outbreak, and tropical convection forced by convergence. Idealized, unidirectional, wind‐profiles are used to simplify interpretation of the results. Diagnostics relevant to the parametrization problem are presented, and it is shown that, for both regimes, the approximations inherent in the parametrization equations are reasonable. the pressure gradients inside clouds play an important role in determining horizontal velocities in the clouds. For the cold‐air outbreak with linear shear, the results suggest that these pressure gradients are proportional to the shear and to the up/downdraught mass‐fluxes. Sensitivity studies suggest that the results are not very sensitive to resolution or parametrization of subgrid‐scale processes, giving some confidence that the results are reasonably accurate. For the tropical case with a low‐level jet, the pressure gradients change sign with the wind shear. In part II of this paper, these results will be used to develop and validate a mass‐flux parametrization of convective momentum‐transport which is tested in single column and global versions of the Meteorological Office Unified Model.
Gregory, D.; Kershaw, R.; Inness, P. M.
doi: 10.1002/qj.49712354103pmid: N/A
Diagnostics derived from cloud‐resolving‐model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum‐transports for deep convection based upon the mass‐flux convection‐scheme discussed by Gregory and Rowntree. the importance of cloud pressure‐gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud‐resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum‐transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.
doi: 10.1002/qj.49712354104pmid: N/A
Analysis of field observations has yielded the conclusion that the Hallett‐Mossop process (H‐M) of secondary ice production plays a major role in the glaciation of summertime cumulus clouds over New Mexico. Other studies have revealed that these clouds possess a characteristic multi‐thermal structure.
doi: 10.1002/qj.49712354105pmid: N/A
Decoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface‐based mixed layer have frequently been observed and modelled. However, little is known of the details of the interaction between the two cloud types, how this affects the microphysical and radiative properties of the stratocumulus, and how the cloud‐topped marine boundary layer evolves in such cases. Recent observational studies suggest that the microphysical and geometrical characteristics of the stratocumulus layer can be modified significantly by interaction with cumulus clouds. In this paper, a preliminary assessment of the relative contribution of different processes and initial boundary‐layer conditions, as suggested by the observations, to the outcome of the interaction between cumulus and stratocumulus clouds is carried out using a one‐dimensional entraining parcel model, which has been modified to simulate a cumulus cloud which penetrates a stratocumulus layer. the results suggest that the change in droplet size at the stratocumulus top as a result of mixing with the penetrating cumulus clouds is particularly sensitive both to the amount of pollution in the air mass and to the vertical structure of the boundary layer, and that these may be more important in determining the outcome of the interaction than the local temperature. A simple coalescence calculation is then used to show that the introduction of cumulus cloud droplets into a stratocumulus layer may alter the potential for drizzle formation in the cloud layer by introducing droplets which are of a different size from those already existing in the stratocumulus. However, it is clear that more detailed dynamical, microphysical and radiative‐transfer models are required, in conjunction with further observations, if these effects are to be parametrized for use in large‐scale numerical models.
doi: 10.1002/qj.49712354106pmid: N/A
A stratiform‐cloud and precipitation scheme, incorporating prognostic variables for cloud liquid water and cloud ice, has been developed for the CSIRO global climate model (GCM). the scheme includes physically based treatments of key microphysical processes, turbulent mixing and semi‐Lagrangian advection of cloud‐water species and interactive cloud radiative properties. Objectives in the development of the scheme were to improve upon the physical realism of parametrizations used in earlier schemes, whilst also trying to provide a scheme with moderate computational overheads.
Haddad, Ziad S.; Rosenfeld, Daniel
doi: 10.1002/qj.49712354107pmid: N/A
This paper attempts to justify mathematically the two empirical approaches to the problem of deriving Z‐R relations from (Z, R) measurements, namely the power‐law regression and the ‘probability matching method’. the basic mathematical assumptions that apply in each case are explicitly identified. In the first case, the appropriate assumption is that the scatter in the (Z, R) measurements reflects exactly the randomness in the connection between Z and R due to a lack of sufficient a priori information about either of them. In the second case, the assumption is that the measurements have been classified into categories a priori, in a way that allows one to expect a nearly one‐to‐one correspondence between Z and R in each catergory, the scatter in the measurements being due to residual noise. the paper then shows how the assmuptions naturally lead, in the first case, to a ‘conditional‐mean’ Z‐R relation of which the power laws are regression‐based approximations, and, in the second case, to a probability‐matchedr relation.
Kamra, A. K.; Deshpande, C. G.; Gopalakrishnan, V.
doi: 10.1002/qj.49712354108pmid: N/A
Measurements of the atmospheric electric conductivity made in the equatorial Indian Ocean and Arabian Sea in August and September 1991 show that the value of conductivity decreases from 2.3 × 10−14 mho m−1 in the equatorial Indian Ocean, where relative humidity of the surface air is 70‐80%, to 1.1 × 10−14 mho m−1 in the region of the Somali current, where relative humidity of the surface air increases to 80‐90%. the inverse relationship observed between conductivity and relative humidity, in spite of showing a large scatter, is stronger in the case of negative than positive conductivity. the sharp increase in the sizes of ions and marine aerosol particles when the relative humidity exceeds 75‐80% is proposed as the cause of the observed decrease in conductivity in the region of the Somali current.
doi: 10.1002/qj.49712354109pmid: N/A
A three‐layer shallow‐water model with a convection parametrization is used to study the track deflections of slow‐moving hurricanes on a β‐plane as they approach straight coastlines of different orientations with respect to north, or as they cross to a cooler or warmer body of water. In the landfall experiments, the land surface is level and is characterized by an abrupt increase of the drag coefficient and zero evaporation. the tracks of a 72 h integration on a β‐plane are compared with an ocean control experiment.
Ferranti, L.; Slingo, J. M.; Palmer, T. N.; Hoskins, B. J.
doi: 10.1002/qj.49712354110pmid: N/A
Monsoon variability on intraseasonal and interannual time‐scales is analysed using data from five 10‐year European Centre for Medium‐Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. the results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. the spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20°N to 5°N, covering most of the land areas, with the other band, of opposite sign, lying between 5°N and 10°S, mostly over the Indian Ocean. This mode therefore describes both the active/break monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India.
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