Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: A. P. Siebesma x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
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.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access