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Alan K. Betts

Abstract

Soundings from a land tropical experiment (VIMHEX II) are classified according to local area rainfall into four regimes depicting the transition from dry to highly disturbed days. The thermodynamic structure for each of the four regimes is presented, and the implications for convective parameterization theories and the prediction problem are discussed in terms of the potential energy available for parcel convection for different entrainment rates.

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Alan K. Betts

Abstract

A lagged mixing parameterization is proposed for the dry convective boundary layer in which the entrainment rate is controlled by the adjustment time chosen. Data from a growing boundary layer over land are used as illustration.

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Alan K. Betts

Abstract

A vector representation of the BOMEX thermodynamic budget data is presented which shows graphically the relationship of the fluxes and the mean layer structure.

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Alan K. Betts

Abstract

The diagnostic study of the thermodynamic structure of nonprecipitating clouds and cloudy boundary layers is formulated using a mixing line and saturation point approach. A parametric model for the mean structure is developed as a tool for diagnostic and prognostic modeling. Cloud-scale mixing process are analyzed in the same framework, together with the energetics of the evaporative instability in cumulus clouds. A velocity scale emerges for this evaporative instability. The statistical study of saturation level distribution in partially cloudy boundary layers is proposed to related cloud fraction to the mean thermodynamic mixing processes.

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Alan K. Betts

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No abstract available.

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Alan K. Betts

Abstract

The transformation by precipitation of the well-mixed subcloud layer into a new structure which is nearly wet adiabatic and has a lower moist static energy is presented. A simple two-layer model is used to show that the precipitating convection appears to strip off the subcloud layer which ascends in updrafts, and to replace it with an equal layer of air from just above cloud base, which descends in downdrafts associated with the evaporation of falling rain. The mean transformation is presented, and the incorporation of the results into a parametric model of the transformation of the subcloud layer is discussed.

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Alan K. Betts

Abstract

A new analysis method based on air parcel saturation point is used to intercompare data sampled by different systems (aircraft, rawinsondes, and a surface mesonet) during the passage of one severe storm in the 1981 Cooperative Convective Precipitation Experiment. The low-level thermodynamic structure simplifies into two distinct branches on a thermodynamic diagram. The lower tropospheric environment ahead of the severe storm has a characteristic mixing line structure while the storm low-level outflow has a distinct evaporation line structure.

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Alan K. Betts

The land surface coupling, a crucial element of the climate system, is explored in the recent 40-yr European Centre for Medium–Range Forecasts (ECMWF) reanalysis (ERA-40) model. In seasonal forecasts for the Northern Hemisphere summer, initialized with idealized soil moisture fields, the ERA-40 model has a large evaporation–precipitation feedback over the continents, and the memory of initial soil moisture is longest at high northern latitudes. Thirty years of hourly data from the ERA-40 reanalysis are averaged over the Madeira, Red–Arkansas, and Athabasca River basins. Although the model fully resolves the diurnal cycle and has an interactive prognostic cloud field, the transitions in the boundary layer climate over land can be mapped with remarkable precision by the daily mean state and daily flux averages. The coupling to cloud processes plays an essential role in the surface and boundary layer equilibrium. Soil moisture, cloud base, cloud cover, radiation fields, and evaporative fraction are coupled quite tightly on daily time scales. The long wave flux control by cloud-base height and cloud cover is particularly strong across all basins. Evaporation can be regarded as being determined somewhat indirectly by the dependence of net radiation on cloud cover and cloud base, and sensible heat flux on subcloud-layer processes. Cloud and boundary layer processes and the land surface components of a model must be evaluated as a tightly coupled system, not as independent components. This analysis provides a new framework for comparing global models with each other, and for evaluating them against observations.

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Alan K. Betts

Abstract

This paper interprets the diagnostic budget studies of the undisturbed BOMEX period, 22-26 June 1969, using a parametric cloud model with continuous detrainment as well as entrainment. Compensating environmental motion, lateral detrainment and cloud transience are discussed in relation to the model results. The model shows the close coupling of the thermodynamic fluxes, and suggests that they can be represented well for some purposes, by a single cloud mass flux profile, rather than by a spectral cloud model. The extension of the methodology to precipitating convection is indicated briefly.

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Alan K. Betts

Abstract

A one-dimensional cumulus model is used to interrelate cloud radius, stratification and cloud height. It is suggested that if only certain ranges of cloud height and radius are permitted, then from the stratification, one can predict, using a model, allowed cloud radii and a corresponding depth to the convective layer.

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