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Peter Bechtold and Pier Siebesma

Abstract

This study is intended to summarize and to simplify the complicated processes in boundary layer cloud regimes using a single parameter, Q 1, the normalized saturation deficit.

With the aid of large eddy simulation (LES) data from different boundary layer cloud regimes it is illustrated i) that the in-cloud buoyancy flux is maximized when the fractional cloudiness approaches zero; ii) that the ensemble average buoyancy flux possesses two maxima, one for the trade wind cumulus case and one for the stratocumulus case; and iii) that the preferred mode for boundary layer clouds is either small cumuli or high values of cloudiness, and that cloudiness transitions from one regime to the other are difficult to represent numerically as in the transition regime the cloud-water-related variables are very parameter sensitive.

In addition, the importance of the contribution of the liquid water flux to the in-cloud and total buoyancy flux is outlined, and simple analytical and empirical methods are presented to compute the liquid water flux as a function of the fluxes of conserved variables for different boundary layer cloud regimes.

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Jean-Pierre Chaboureau and Peter Bechtold

Abstract

A simple statistical parameterization of cloud water–related variables that has been originally developed for nonprecipitating boundary layer clouds is extended for all cloud types including deep precipitating convection. Based on three-dimensional cloud resolving model (CRM) simulations of observed tropical maritime and continental midlatitude convective periods, expressions for the partial cloudiness and the cloud water content are derived, which are a function of the normalized saturation deficit Q 1. It turns out that these relations are equivalent to boundary layer cloud relations described earlier, therefore allowing for a general description of subgrid-scale clouds.

The usefulness of the cloud relations is assessed by applying them diagnostically and prognostically in a mesoscale model for a midlatitude cyclone case and a subtropical case, and comparing the simulated cloud fields to satellite observations and to reference simulations with an explicit microphysical scheme. The comparison uses a model-to-satellite approach where synthetic radiances are computed from the meteorological fields and are compared to Meteosat satellite observations both in the visible and the thermal infrared spectral channels. The impact of the statistical cloud scheme is most pronounced for shallow and deep convective cloud fields (where Q 1 < 0), provided that the host models convection parameterization is able to correctly represent the ensemble average water vapor profile in the troposphere. The scheme significantly reduces the biases in the infrared and especially shortwave spectral range with respect to the explicit microphysical scheme. Furthermore, it produces more realistic (smooth) horizontal and vertical condensate distributions in both diagnostic or prognostic applications showing the potential use of this simple parameterization in larger-scale models.

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Peter Bechtold, Jean-Pierre Pinty, and Fatrick Mascart

Abstract

A two-dimensional mesoscale model is used to study the influence of large-scale background winds on sea-breeze- and inland- (vegetation) breeze-type circulations. It is found that the intensity (vertical velocity) of the sea breeze is at its maximum when the Propagation speed of the sea-breeze front is canceled out by the background wind speed. Using the gravity current theory, we get a fair prediction of this optimum background wind value.

The intensity and extent of the inland breeze, forming between a forecast and an adjacent crop area, do not vary over a large range of values for the large-scale wind. The location of the ascending branch of the inland breeze is stationary with respect to the interface between the two vegetation types. It is suggested that it is not friction drag but rather turbulent mixing that leads to a moon horizontally uniform boundary layer and which is responsible for the different behavior of the inland breeze, i.e., a weak and nonpropagating circulation.

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Jian Ling, Chidong Zhang, and Peter Bechtold

Abstract

In this study, the authors seek large-scale signals that may distinguish MJO from non-MJO convective events before they start over the Indian Ocean. Three such signals were found. Low-level easterly anomalies extend from the surface to the midtroposphere and move from the western to eastern Indian Ocean. Surface pressure anomalies exhibit a zonal structure of wavenumber 1 with an equatorial low-pressure surge penetrating eastward from Africa through the Indian Ocean and reaching the Maritime Continent. Negative temperature anomalies in the middle to upper troposphere start over the Indian Ocean and move eastward. All of them emerge 20 days before convective initiation of the MJO and move eastward at speeds close to that of the MJO without any direct connection to MJO convection. They are not obviously related to the extratropics in any discernible way or any preceding MJO events. They are absent in non-MJO convective events. These signals provide useful information for forecasting MJO initiation over the Indian Ocean. They can be signatures of a dry dynamics mode of the MJO, if it exists.

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Peter Bechtold, Jean Pierre Pinty, and Charles Fravalo

Abstract

A one-dimensional version of a multilevel mesoscale model is used to represent the cloud-topped boundary layer (CTBL). Turbulent exchanges are parameterized with a prognostic equation for the turbulent kinetic energy and an improved length-scale formulation. Furthermore, the scheme is extended to give a statistical description of the subgrid-scale condensation with a one-and-a-half-order closure.Several observed reference cases are simulated in order to test the model against observational data and results obtained with a higher-order turbulence model. The latter one is used as a powerful approach for testing the closure of the second-order moments involved in the partial cloudiness scheme. Two of the reference cases are extracted from stratocumulus (Sc) observations off the coast of the United Kingdom with a purely buoyancy-driven and a purely shear-driven CTBL, respectively. The third experiment tries to reproduce a case of Californian Sc clouds where both turbulent effects are important. Finally, the last numerical experiment concentrates on a cloudiness transition case observed during FIRE (First International Satellite Cloud Climatology Project Regional Experiment) with a series of soundings documenting the cloudiness transition from a solid Sc cloud deck over a partly covered region to a clear-sky region.The model results are shown to be in reasonable agreement with both observational data and numerical outputs from the higher-order turbulence model. Finally, it is shown that the partial cloudiness scheme does not only produce more realistic cloudiness and cloud water content than a simple “all or nothing” condensation scheme, but that it also assures model stability by producing a smooth onset of condensation.

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Peter Bechtold, Jean Pierre Pinty, and Patrick Mascart

Abstract

A method is proposed on how to handle the effects of partial cloudiness in a warm-rain microphysical scheme and how to generate subgrid-scale precipitation. The method is simple and concerns essentially two ideas: the use of the vertical distribution of the partial cloudiness and the use of environmental and cloud-scale values for the thermodynamic variables instead of their grid-mean values. It applies to any microphysical scheme.

Here, the method has been applied to a warm-rain parameterization scheme that has been implemented in a mesoscale model using a statistical partial cloudiness scheme. Numerical tests have been done for two one-dimensional cases of boundary-layer cloudiness: a cumulus case and a case of a decoupled stratocumulus layer.

The results show that the correct coupling of a partial cloudiness scheme and a microphysical scheme allows for a better description of the actual cloudiness and precipitation fields by ensuring a consistent computation of partial cloudiness, cloud water, and rainwater in partly cloudy regions.

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Michael J. Herman, Zeljka Fuchs, David J. Raymond, and Peter Bechtold

Abstract

The authors analyze composite structures of tropical convectively coupled Kelvin waves (CCKWs) in terms of the theory of Raymond and Fuchs using radiosonde data, 3D analysis and reanalysis model output, and annual integrations with the ECMWF model on the full planet and on an aquaplanet. Precipitation anomalies are estimated using the NOAA interpolated OLR and TRMM 3B42 datasets, as well as using model OLR and rainfall diagnostics. Derived variables from these datasets are used to examine assumptions of the theory. Large-scale characteristics of wave phenomena are robust in all datasets and models where Kelvin wave variance is large. Indices from the theory representing column moisture and convective inhibition are also robust. The results suggest that the CCKW is highly dependent on convective inhibition, while column moisture does not play an important role.

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Jun-Ichi Yano, Peter Bechtold, and Jean-Luc Redelsperger

Abstract

Physical processes in numerical modeling are currently handled by a dichotomy of either an explicit or a parameterization approach. Herein, an alternative approach is proposed, in which degrading explicit physics with decreasing resolutions are compensated by a “renormalization.” More specifically, a “renormalization” factor depending on the model resolution is multiplied on explicit evaluations so that the subgrid-scale contributions to a given grid scale are approximately recovered without a parameterization. The approach is analogous to the renormalization approach in statistical physics, but without rigorously relying on its mathematical basis. For this reason, this name is evoked with a quotation.

In order to demonstrate this idea, the domain-mean vertical fluxes of heat, moisture, and momentum from cloud-resolving model experiments, corresponding to the grid-box averages in the large-scale modeling, are examined. In order to mimic the effects of degrading horizontal resolution, data are filtered in wavelet space. The “renormalization” factors that recover the full vertical fluxes are found to be relatively stable with time, and the associated errors by “renormalization” are overall less than the order of the vertical variance of the fluxes, indicating a potential usefulness of this approach. An analogous approach is found to work more effectively using data compression by wavelets.

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Andrew Orr, Peter Bechtold, John Scinocca, Manfred Ern, and Marta Janiskova

Abstract

In model cycle 35r3 (Cy35r3) of the ECMWF Integrated Forecast System (IFS), the momentum deposition from small-scale nonorographic gravity waves is parameterized by the Scinocca scheme, which uses hydrostatic nonrotational wave dynamics to describe the vertical evolution of a broad, constant, and isotropic spectrum of gravity waves emanating from the troposphere. The Cy35r3 middle atmosphere climate shows the following: (i) an improved representation of the zonal-mean circulation and temperature structure; (ii) a realistic parameterized gravity wave drag; (iii) a reasonable stationary planetary wave structure and stationary wave driving in July and an underestimate of the generation of stationary wave activity in the troposphere and stationary wave driving in January; (iv) an improved representation of the tropical variability of the stratospheric circulation, although the westerly phase of the semiannual oscillation is missing; and (v) a realistic horizontal distribution of momentum flux in the stratosphere. By contrast, the middle atmosphere climate is much too close to radiative equilibrium when the Scinocca scheme is replaced by Rayleigh friction, which was the standard method of parameterizing the effects of nonorographic gravity waves in the IFS prior to Cy35r3. Finally, there is a reduction in Cy35r3 short-range high-resolution forecast error in the upper stratosphere.

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Jun-Ichi Yano, Peter Bechtold, Jean-Luc Redelsperger, and Francoise Guichard

Abstract

The capacity of wavelets to effectively represent atmospheric processes under compression is tested by a dataset generated by a cloud-resolving model simulation of deep convective events observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE).

Generally, more than 90% of the total variance is reproduced by retaining only the top 10% of the modes. The compressed data does not drastically deteriorate for graphic purposes, even when only the top 1% of modes are retained. The performance of compression is overall comparable for all the wavelets considered and also does not strongly depend on the type of physical variables.

Conventional quantitative measures do not distinguish the compression errors arising from different characters of individual wavelets well, although different wavelet modes filter out different structures as “noises” depending on their characteristics. The importance of choosing wavelets that represent the shape of signals physically expected is emphasized. Analytical discontinuities of wavelets are not necessarily undesirable for all the purposes, but must be consistent with our physical picture for the system. For this reason, the Haar wavelet may be acceptable because of its piecewise-constant structure, whereas the Daubechies wavelets of lower degrees are less appropriate because of their highly irregular structures.

Some preliminary analyses are performed for assessing the capacity of wavelets to represent the full physics of meteorological systems. It is suggested that the loss of magnitudes in vertical fluxes under high compression can be recovered by a kind of “renormalization” to a good extent. The mass continuity is found to be reasonably satisfied under the proposed compression method, although the latter is not explicitly constrained by the former.

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