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Knut von Salzen
and
Norman A. McFarlane

Abstract

A parameterization of shallow cumulus clouds for use in atmospheric general circulation models is proposed. The parameterization uses a bulk representation of an ensemble of transient clouds. Entrainment of environmental air occurs at the ascending top of the cumulus cloud and also at the lateral boundaries of the region below the top of the cloud. Complete detrainment of the air in the cloud occurs when the top of the cloud reaches its maximum height, chosen to be the level of neutral buoyancy. The parameterization is calibrated using results from the undisturbed period of the Barbados Oceanographic and Meteorological Experiment (BOMEX). Vertical profiles of in-cloud properties and mass fluxes obtained from large eddy simulations (LES) for the undisturbed BOMEX period are successfully reproduced by the parameterization. Good agreement is also found in comparisons with large-scale heat and moisture budgets diagnosed from observations during the same period of the experiment. However, this is achieved with a different choice for the parameters of the scheme.

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Guang Jun Zhang
and
Norman A. McFarlane

Abstract

The upper-air sounding data from PRE-STORM are used to investigate the convective stabilization effect on the large-scale atmosphere. To facilitate comparison between different stages of cumulus convection, the data are divided into four categories: environment, presystem, insystem, and postsystem. It is found that the convective available potential energy of the atmosphere is reduced substantially after cumulus convection, most of which is consumed during the transition from presystem to insystem. Examination of the temperature and moisture changes during cumulus convection suggests that cooling and drying in the subcloud layer are the most important factors in stabilizing the atmosphere. In general, virtual potential temperature profiles in all categories are close to reversible moist adiabats below the 600-mb level and nearly parallel to moist pseudoadiabats above it.

The effect of entrainment on parcel buoyancy is also studied. It is found that a small amount of entrainment of ambient air can lead to a pronounced decrease of parcel buoyancy. Furthermore, for diluted parcel ascent, the convective available potential energy is greater for the insystem category than for the postsystem one, whereas the opposite is true for undiluted parcel ascent.

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John F. Scinocca
and
Norman A. McFarlane

Abstract

This paper investigates the temporal properties of tropical precipitation in the Canadian Centre for Climate Modelling and Analysis (CCCma) third-generation atmospheric general circulation model (AGCM3). AGCM3 employs the penetrative mass-flux (PMF) scheme of Zhang and McFarlane (ZM) for the parameterization of deep cumulus convection. It is found that the temporal variability of the ZM scheme is sensitive to a number of its internal parameters as well to the use of a prognostic, rather than diagnostic, closure condition for the cloud-base mass flux. Sensitivity experiments suggest that the ZM scheme can produce realistic amounts of variability when compared to direct radar observations of deep cumulus convection in the Tropics.

A central finding of this study is that the resolved large-scale stratiform precipitation (LSP) in the model can participate in the modeling of deep latent heating and so compete with the ZM scheme in the Tropics. In modeling deep latent heating the LSP is found to mimic the behavior of a moist-convective adjustment scheme. In AGCM3 it is found that typical parameter settings of the ZM scheme place it in a regime in which the temporal variability of tropical precipitation is dominated by this behavior of the LSP, while the temporal mean is dominated by the ZM scheme. In such circumstances it is the LSP, and not the ZM scheme, that provides the primary source of resolved tropical Kelvin and mixed Rossby–gravity waves in the GCM. Such competition between LSP and the parameterization of deep convection appears to be active in other modeling studies. Consequently, it has the potential to complicate efforts to understand the nature of resolved tropical waves in GCMs and their role in the forcing of the quasi-biennial and semiannual oscillations.

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Ulrike Lohmann
,
Norman McFarlane
,
Lubomir Levkov
,
Kenzu Abdella
, and
Frank Albers

Abstract

Different cloud schemes are compared using the single column model (SCM) version of the general circulation model of the Canadian Centre for Climate Modelling and Analysis. Emphasis is placed on the differences between a statistical cloud scheme and an explicit one, two approaches commonly used in GCMs. The microphysical processes are identical in both schemes so that the differences can be attributed to cloud formation and dissipation only. Two case studies are chosen, one for a day during the European Cloud and Radiation Experiment (EUCREX) and one for a day during the North Atlantic Regional Experiment (NARE). During the EUCREX case study the SCM is forced by advection from the mesoscale model GESIMA (Geesthacht Simulation Model of the Atmosphere). The comparison of ice water content as a function of height shows that the SCM cannot reproduce the observed nearly linear decrease with height as well as GESIMA does above 8.5 km. If temperature, specific humidity, and cloud ice advection are used to force the SCM, the explicit scheme simulates a coherent thick cirrus cloud, which is in better agreement with observations than the separate cloud layers simulated with the statistical scheme. Sensitivity studies show that cloud ice advection is crucial for the formation of the cirrus deck in this case study, but omitting specific humidity advection improves the agreement with observations. During the NARE case study four sequential vertical profiles are available so that wind, temperature, and moisture of the SCM can be nudged toward their observed values. The observed lifting of the boundary layer cloud with time is captured best by the statistical scheme when adjusted toward observations with a relaxation timescale of one hour or less.

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Adam H. Monahan
,
Tim Rees
,
Yanping He
, and
Norman McFarlane

Abstract

A long time series of temporally high-resolution wind and potential temperature data from the 213-m tower at Cabauw in the Netherlands demonstrates the existence of two distinct regimes of the stably stratified nocturnal boundary layer at this location. Hidden Markov model (HMM) analysis is used to objectively characterize these regimes and classify individual observed states. The first regime is characterized by strongly stable stratification, large wind speed differences between 10 and 200 m, and relatively weak turbulence. The second is associated with near-neutral stratification, weaker wind speed differences between 10 and 200 m, and relatively strong turbulence. In this second regime, the state of the boundary layer is similar to that during the day. The occupation statistics of these regimes are shown to covary with the large-scale pressure gradient force and cloud cover such that the first regime predominates under clear skies with weak geostrophic wind speed and the second regime predominates under conditions of extensive cloud cover or large geostrophic wind speed. These regimes are not distinguished by standard measures of stability, such as the Obukhov length or the bulk Richardson number. Evidence is presented that the mechanism generating these distinct regimes is associated with a previously documented feedback resulting from the existence of an upper limit on the maximum downward heat flux that can be sustained for a given near-surface wind speed.

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Yanping He
,
Norman A. McFarlane
, and
Adam H. Monahan

Abstract

Knowledge of the diurnally varying land surface wind speed probability distribution is essential for surface flux estimation and wind power management. Global observations indicate that the surface wind speed probability density function (PDF) is characterized by a Weibull-like PDF during the day and a nighttime PDF with considerably greater skewness. Consideration of long-term tower observations at Cabauw, the Netherlands, indicates that this nighttime skewness is a shallow feature connected to the formation of a stably stratified nocturnal boundary layer. The observed diurnally varying vertical structure of the leading three climatological moments of near-surface wind speed (mean, standard deviation, and skewness) and the wind power density at the Cabauw site can be successfully simulated using the single-column version of the Canadian Centre for Climate Modelling and Analysis (CCCma) fourth-generation atmospheric general circulation model (CanAM4) with a new semiempirical diagnostic turbulent kinetic energy (TKE) scheme representing downgradient turbulent transfer processes for cloud-free conditions. This model also includes a simple stochastic representation of intermittent turbulence at the boundary layer inversion. It is found that the mean and the standard deviation of wind speed are most influenced by large-scale “weather” variability, while the shape of the PDF is influenced by the intermittent mixing process. This effect is quantitatively dependent on the asymptotic flux Richardson number, which determines the Prandtl number in stable flows. High vertical resolution near the land surface is also necessary for realistic simulation of the observed fine vertical structure of wind speed distribution.

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Adam H. Monahan
,
Yanping He
,
Norman McFarlane
, and
Aiguo Dai

Abstract

The probability density function (pdf) of land surface wind speeds is characterized using a global network of observations. Daytime surface wind speeds are shown to be broadly consistent with the Weibull distribution, while nighttime surface wind speeds are generally more positively skewed than the corresponding Weibull distribution (particularly in summer). In the midlatitudes, these strongly positive skewnesses are shown to be generally associated with conditions of strong surface stability and weak lower-tropospheric wind shear. Long-term tower observations from Cabauw, the Netherlands, and Los Alamos, New Mexico, demonstrate that lower-tropospheric wind speeds become more positively skewed than the corresponding Weibull distribution only in the shallow (~50 m) nocturnal boundary layer. This skewness is associated with two populations of nighttime winds: (i) strongly stably stratified with strong wind shear and (ii) weakly stably or unstably stratified with weak wind shear. Using an idealized two-layer model of the boundary layer momentum budget, it is shown that the observed variability of the daytime and nighttime surface wind speeds can be accounted for through a stochastic representation of intermittent turbulent mixing at the nocturnal boundary layer inversion.

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Adam H. Monahan
,
Boualem Khouider
,
Norman McFarlane
,
John Scinocca
, and
Knut von Salzen

Abstract

No Abstract available.

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Boualem Khouider
,
Adam H. Monahan
,
Norman McFarlane
,
John Scinocca
, and
Knut von Salzen

Abstract

No Abstract available.

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