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A. P. Siebesma
and
A. A. M. Holtslag

Abstract

A mass flux parameterization scheme for shallow cumulus convection is evaluated for a case based on observations and large eddy simulation (LES) results for the Barbados Oceanographic and Meteorological Experiment (BOMEX). The mass flux scheme is embedded in a one-column model with prescribed large-scale forcings. Comparing the findings of the latter with the LES results, it is found that the mass flux scheme is too active. As a result, the scheme is mixing too much heat and moisture between the cloud layer and the inversion layer, giving rise to erroneous moisture and temperature profiles for the trade wind region. This is due to an underestimation of the lateral exchange rates. LES results show that for shallow cumulus cloud ensembles (lateral) entrainment and detrainment rates are typically one order of magnitude larger than values used in most operational parameterization schemes and that the detrainment rate is systematically larger than the entrainment rate. When adapting them enhanced rates, the mass flux scheme produces realistic mass fluxes and cloud excess values for moisture and heat and is therefore capable of maintaining the stationary state as observed during BOMEX.

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R. A. J. Neggers
and
A. P. Siebesma

Abstract

This study explores the opportunities created by subjecting a system of interacting fast-acting parameterizations to long-term single-column model evaluation against multiple independent measurements at a permanent meteorological site. It is argued that constraining the system at multiple key points facilitates the tracing and identification of compensating errors between individual parametric components. The extended time range of the evaluation helps to enhance the statistical significance and representativeness of the single-column model result, which facilitates the attribution of model behavior as diagnosed in a general circulation model to its subgrid parameterizations. At the same time, the high model transparency and computational efficiency typical of single-column modeling is preserved.

The method is illustrated by investigating the impact of a model change in the Regional Atmospheric Climate Model (RACMO) on the representation of the coupled boundary layer–soil system at the Cabauw meteorological site in the Netherlands. A set of 12 relevant variables is defined that covers all involved processes, including cloud structure and amplitude, radiative transfer, the surface energy budget, and the thermodynamic state of the soil and various heights of the lower atmosphere. These variables are either routinely measured at the Cabauw site or are obtained from continuous large-eddy simulation at that site. This 12-point check proves effective in revealing the existence of a compensating error between cloud structure and radiative transfer, residing in the cloud overlap assumption. In this exercise, the application of conditional sampling proves a valuable tool in establishing which cloud regime exhibits the biggest impact.

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A. P. Siebesma
and
J. W. M. Cuijpers

Abstract

A large-eddy simulation (LES) model has been utilized to study nonprecipitating Shallow Convective clouds such as observed during the undisturbed BOMEX period in the trade wind areas. By choosing a realistic large-scale forcing the authors have been able to simulate shallow convective clouds under quasi-steady-state conditions over a long period of 7 hours. This is a necessary condition to investigate diagnostic cumulus parameterization schemes since such schemes usually assume steady-state conditions. The response of the model to the applied large-scale forcing compares well with budget study results of BOMEX. In addition, the LES model delivers detailed information concerning the dynamics of shallow convective clouds. This is used to verify basic parameterizations of turbulent fluxes and entrainment and detrainment rates used in large-scale models. The most important conclusions are (i) the fractional entrainment and detrainment rates used in present large-scale atmospheric models are one order of magnitude too small, confirming previous results obtained by Esbensen, and (ii) estimates of turbulent fluxes by bulk cloud updrafts and environmental downdrafts give an underestimation of 20% to 50% depending on the variable that is transported. Implications of these results for cumulus parameterizations will be discussed.

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R. A. J. Neggers
,
A. P. Siebesma
, and
T. Heus

Uncertainties in numerical predictions of weather and climate are often linked to the representation of unresolved processes that act relatively quickly compared to the resolved general circulation. These processes include turbulence, convection, clouds, and radiation. Single‐column model (SCM) simulation of idealized cases and the subsequent evaluation against large-eddy simulation (LES) results has become an often used and relied on method to obtain insight at process level into the behavior of such parameterization schemes; benefits of SCM simulation are the enhanced model transparency and the high computational efficiency. Although this approach has achieved demonstrable success, some shortcomings have been identified; among these, i) the statistical significance and relevance of single idealized case studies might be questioned and ii) the use of observational datasets has been relatively limited. A recently initiated project named the Royal Netherlands Meteorological Institute (KNMI) Parameterization Testbed (KPT) is part of a general move toward a more statistically significant process-level evaluation, with the purpose of optimizing the identification of problems in general circulation models that are related to parameterization schemes. The main strategy of KPT is to apply continuous long-term SCM simulation and LES at various permanent meteorological sites, in combination with comprehensive evaluation against observations at multiple time scales. We argue that this strategy enables the reproduction of typical long-term mean behavior of fast physics in large-scale models, but it still preserves the benefits of single-case studies (such as model transparency). This facilitates the tracing and understanding of errors in parameterization schemes, which should eventually lead to a reduction of related uncertainties in numerical predictions of weather and climate.

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R. A. J. Neggers
,
A. P. Siebesma
, and
H. J. J. Jonker

Abstract

A new parameterization for cumulus convection is formulated, that consists of an ensemble of small, rising parcels. Large eddy simulation (LES) results are used to parameterize the lateral mixing of such a parcel: for the mixing process a relaxation timescale is defined and its value is determined by investigating individual LES clouds. The timescale is found to be nearly independent of cloud depth, which implies that the entrainment rate is inversely proportional to the vertical velocity. As a consequence, a dynamical feedback mechanism is established: the parcel dynamics influence the mixing rate, which, together with the environmental properties, feeds back on the parcel properties and therefore on the parcel dynamics.

The multiparcel model is validated with LES fields. The characteristics of the buoyant part of the clouds are reproduced: the decreasing fractional cover and increasing liquid water content with height, the vertical dynamics and mass flux, and the conserved properties and the marginally buoyant state. The model also produces the variability typical for shallow cumulus.

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R. A. J. Neggers
,
H. J. J. Jonker
, and
A. P. Siebesma

Abstract

Cloud size distributions of shallow cumulus cloud populations are calculated using the large-eddy simulation (LES) approach. A range of different cases is simulated, and the results are compared to observations of real cloud populations. Accordingly, the same algorithm is applied as in observational studies using high-altitude photography or remote sensing.

The cloud size density of the simulated cloud populations is described well by a power law at the smaller sizes. This scaling covers roughly one order of magnitude of cloud sizes, with a power-law exponent of −1.70, which is comparable to exponents found in observational studies. A sensitivity test for the resolution suggests that the scaling continues at sizes smaller than the standard grid spacing. In contrast, on the other end, the scaling region is bounded by a distinct scale break. When the cloud size is nondimensionalized by the scale break size, the cloud size densities of all cases collapse. This corroborates the idea of a universal description for the whole cloud size density, with the scale break size as the only variable. The intermediate dominating size in the cloud fraction and mass flux decompositions is directly related to the presence of the scale break in the cloud size density. Despite their large number, the smallest clouds contribute very little to the total vertical mass transport. The intermediate size of the dominating clouds in the cloud fraction and mass flux is insensitive to the resolution of LES.

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J. J. van der Dussen
,
S. R. de Roode
, and
A. P. Siebesma

Abstract

The relationship between the inversion stability and the liquid water path (LWP) tendency of a vertically well-mixed, adiabatic stratocumulus cloud layer is investigated in this study through the analysis of the budget equation for the LWP. The LWP budget is mainly determined by the turbulent fluxes of heat and moisture at the top and the base of the cloud layer, as well as by the source terms due to radiation and precipitation. Through substitution of the inversion stability parameter κ into the budget equation, it immediately follows that the LWP tendency will become negative for increasing values of κ due to the entrainment of increasingly dry air. Large κ values are therefore associated with strong cloud thinning. Using the steady-state solution for the LWP, an equilibrium value κ eq is formulated, beyond which the stratocumulus cloud will thin. The Second Dynamics and Chemistry of Marine Stratocumulus field study (DYCOMS-II) is used to illustrate that, depending mainly on the magnitude of the moisture flux at cloud base, stratocumulus clouds can persist well within the buoyancy reversal regime.

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J. Teixeira
,
S. Cardoso
,
M. Bonazzola
,
J. Cole
,
A. DelGenio
,
C. DeMott
,
C. Franklin
,
C. Hannay
,
C. Jakob
,
Y. Jiao
,
J. Karlsson
,
H. Kitagawa
,
M. Köhler
,
A. Kuwano-Yoshida
,
C. LeDrian
,
J. Li
,
A. Lock
,
M. J. Miller
,
P. Marquet
,
J. Martins
,
C. R. Mechoso
,
E. v. Meijgaard
,
I. Meinke
,
P. M. A. Miranda
,
D. Mironov
,
R. Neggers
,
H. L. Pan
,
D. A. Randall
,
P. J. Rasch
,
B. Rockel
,
W. B. Rossow
,
B. Ritter
,
A. P. Siebesma
,
P. M. M. Soares
,
F. J. Turk
,
P. A. Vaillancourt
,
A. Von Engeln
, and
M. Zhao

Abstract

A model evaluation approach is proposed in which weather and climate prediction models are analyzed along a Pacific Ocean cross section, from the stratocumulus regions off the coast of California, across the shallow convection dominated trade winds, to the deep convection regions of the ITCZ—the Global Energy and Water Cycle Experiment Cloud System Study/Working Group on Numerical Experimentation (GCSS/WGNE) Pacific Cross-Section Intercomparison (GPCI). The main goal of GPCI is to evaluate and help understand and improve the representation of tropical and subtropical cloud processes in weather and climate prediction models. In this paper, a detailed analysis of cloud regime transitions along the cross section from the subtropics to the tropics for the season June–July–August of 1998 is presented. This GPCI study confirms many of the typical weather and climate prediction model problems in the representation of clouds: underestimation of clouds in the stratocumulus regime by most models with the corresponding consequences in terms of shortwave radiation biases; overestimation of clouds by the 40-yr ECMWF Re-Analysis (ERA-40) in the deep tropics (in particular) with the corresponding impact in the outgoing longwave radiation; large spread between the different models in terms of cloud cover, liquid water path and shortwave radiation; significant differences between the models in terms of vertical cross sections of cloud properties (in particular), vertical velocity, and relative humidity. An alternative analysis of cloud cover mean statistics is proposed where sharp gradients in cloud cover along the GPCI transect are taken into account. This analysis shows that the negative cloud bias of some models and ERA-40 in the stratocumulus regions [as compared to the first International Satellite Cloud Climatology Project (ISCCP)] is associated not only with lower values of cloud cover in these regimes, but also with a stratocumulus-to-cumulus transition that occurs too early along the trade wind Lagrangian trajectory. Histograms of cloud cover along the cross section differ significantly between models. Some models exhibit a quasi-bimodal structure with cloud cover being either very large (close to 100%) or very small, while other models show a more continuous transition. The ISCCP observations suggest that reality is in-between these two extreme examples. These different patterns reflect the diverse nature of the cloud, boundary layer, and convection parameterizations in the participating weather and climate prediction models.

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Robert M. Rauber
,
Harry T. Ochs III
,
L. Di Girolamo
,
S. Göke
,
E. Snodgrass
,
Bjorn Stevens
,
Charles Knight
,
J. B. Jensen
,
D. H. Lenschow
,
R. A. Rilling
,
D. C. Rogers
,
J. L. Stith
,
B. A. Albrecht
,
P. Zuidema
,
A. M. Blyth
,
C. W. Fairall
,
W. A. Brewer
,
S. Tucker
,
S. G. Lasher-Trapp
,
O. L. Mayol-Bracero
,
G. Vali
,
B. Geerts
,
J. R. Anderson
,
B. A. Baker
,
R. P. Lawson
,
A. R. Bandy
,
D. C. Thornton
,
E. Burnet
,
J-L. Brenguier
,
L. Gomes
,
P. R. A. Brown
,
P. Chuang
,
W. R. Cotton
,
H. Gerber
,
B. G. Heikes
,
J. G. Hudson
,
P. Kollias
,
S. K. Krueger
,
L. Nuijens
,
D. W. O'Sullivan
,
A. P. Siebesma
, and
C. H. Twohy
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Robert M. Rauber
,
Bjorn Stevens
,
Harry T. Ochs III
,
Charles Knight
,
B. A. Albrecht
,
A. M. Blyth
,
C. W. Fairall
,
J. B. Jensen
,
S. G. Lasher-Trapp
,
O. L. Mayol-Bracero
,
G. Vali
,
J. R. Anderson
,
B. A. Baker
,
A. R. Bandy
,
E. Burnet
,
J.-L. Brenguier
,
W. A. Brewer
,
P. R. A. Brown
,
R Chuang
,
W. R. Cotton
,
L. Di Girolamo
,
B. Geerts
,
H. Gerber
,
S. Göke
,
L. Gomes
,
B. G. Heikes
,
J. G. Hudson
,
P. Kollias
,
R. R Lawson
,
S. K. Krueger
,
D. H. Lenschow
,
L. Nuijens
,
D. W. O'Sullivan
,
R. A. Rilling
,
D. C. Rogers
,
A. P. Siebesma
,
E. Snodgrass
,
J. L. Stith
,
D. C. Thornton
,
S. Tucker
,
C. H. Twohy
, and
P. Zuidema

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.

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