Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: M. Tiedtke x
  • Refine by Access: All Content x
Clear All Modify Search
M. Tiedtke

Abstract

Current climate and forecast models treat clouds as plane-uniform, ignoring subgrid-scale variations of cloud water content in radiative transfer calculations. The plane-uniform assumption is relaxed in a new cloud-radiation parameterization that considers cloud inhomogeneities and heterogeneities associated with convective and stratiform cloudiness. Convective cloudiness is diagnosed from the assumption that the ensemble of convective clouds within a model grid column is in a quasi-steady state. Global forecast experiments with the new parameterization show that the introduction of cloud inhomogeneities and heterogeneities leads to substantially increased and more realistic net downward shortwave radiative fluxes at the top of the atmosphere; over the tropical and subtropical oceans (30°S–30°N) the increase by inhomogeneities and heterogeneities is on average 9 and 7 W m−2, respectively.

Full access
M. Tiedtke

Abstract

Observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large-scale models, but this approach, through the introduction of a spectral cloud ensemble, leads normally to rather complex schemes. In this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions. This is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection (GATE, Marshall Islands), tradewind cumuli (ATEX, BOMEX) and extratropical organized convection (SESAME-79) and then in a NWP model.

The scheme considers a population of clouds where the cloud ensemble is described by a one-dimensional bulk model as earlier applied by Yanai et al. in a diagnostic study of tropical convection. Cumulus scale downdrafts are included. Various types of convection are represented, i.e., penetrative convection in connection with large-scale convergent flow, shallow convection in suppressed conditions like tradewind cumuli and midlevel convection like extratropical organized convection associated with potentially unstable air above the boundary layer and large-scale ascent. The closure assumptions for determining the bulk cloud mass flux are: penetrative convection and midlevel convection are maintained by large-scale moisture convergence and shallow convection by supply of moisture due to surface evaporation.

The parameterization produces realistic fields of convective heating and appears to be in fair balance with real data for NWP as it does not initiate strong adjustment processes (spinup) in global form.

Full access
M. Tiedtke

Abstract

A prognostic scheme for stratiform and convective clouds is developed for large-scale models. The time evolution of clouds is defined through the large-scale budget equations for cloud water content and cloud air (which is converted into a prognostic equation for fractional cloud cover). The scheme considers the formation of clouds in connection with large-scale ascent diabatic cooling, boundary-layer turbulence, and horizontal transport of cloud water from convective updrafts. Clouds dissipate through adiabatic and diabatic heating, turbulent mixing of cloud air with unsaturated environmental air, and depletion of cloud water by precipitation.

The scheme differs from conventional schemes in its approach, which is fully prognostic and model consistent, and in the larger degree of complexity as the formation of anvil and circus clouds originating by cumulus updrafts and boundary-layer clouds is included.

The scheme has been tested in the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast model and compared with the ECMWF operational cloud scheme. The results show that realistic cloud fields are produced when compared to observed values of cloud cover and cloud water content. The representation of cloud process in connection with anvil clouds is shown to have a strong effect on the hydrological cycle and the maintenance of the tropospheric water vapor content.

Full access
A. Hollingsworth
,
K. Arpe
,
M. Tiedtke
,
M. Capaldo
, and
H. Savijärvi

Abstract

We present the results of a series of forecasts on seven weather situations from February 1976 using two models which differ only in their physical parameterizations.

One set of parameterizations was developed at the Geophysical Fluid Dynamics Laboratory (GFDL) some years ago, the other more recently at the European Centre for Medium Range Forcasts (ECMWF). The resolution of the model (N48, 15 levels) was that which ECMWF has used in the first phase of operations, which began in August 1979. The particular aim of the experiments was to study the importance of the differences in the parameterization schemes for the model; in addition, we obtained a general view of the forecast results that might be available in the first phase of operations.

Both sets of parameterizations gave similar results in terms of forecast quality. When measured by the standard objective methods, the range of predictability was 5–6 days. A study of the systematic errors in the forecasts showed that these were mainly associated with a loss of energy in the largest waves; the evolution of these systematic errors appeared to be roughly linear in time. (This is not to say that the systematic errors arise due to linear mechanisms.) A study of the energetics showed that the major part of the loss of energy in the long waves was due to a failure to maintain the stationary part of the long-wave energy. Regarding transient phenomena, the downstream intensification of baroclinic waves appeared sometimes to be predictable beyond 6 days.

Full access
Julia M. Slingo
,
U. C. Mohanty
,
M. Tiedtke
, and
R. P. Pearce

Abstract

This paper describes the impact on the tropical simulation in the ECMWF model of various changes to the treatment of physical processes, with particular emphasis on the onset of the Asian summer monsoon. A series of 10-day forecasts were carried out, each integration starting from 1200 UTC 11 June 1979 and covering the rapid intensification of the monsoon over the Arabian Sea and Southern India. The changes to the physical processes involved modifications to the radiation and Kuo penetrative convective schemes and the introduction of a shallow convection scheme.

The changes to the treatment of convection, particularly the introduction of the shallow convection scheme, are found to have a large impact on the tropical circulation and precipitation. In addition to an overall beneficial effect on the simulated large-scale flow (e.g., tradewind boundary layer structure, tradewinds, ITCZ) there is a significant improvement of the monsoon simulation. It is found that only when the radiation changes are combined with the convection changes is there a marked improvement in the monsoon region. The intensification of the low level flow over the Arabian Sea is then much improved as are the onset of the rains over Southern India and the establishment of the upper level cross equatorial return Bow.

Full access