Cumulus Initialization in a Global Model for Numerical Weather Prediction

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  • 1 Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

A procedure for adjusting temperature and humidity analyses used as initial conditions for numerical weather prediction models so that diagnosed distributions of cumulus convection exist during the initial stages of the forecast is applied in a global atmospheric model. This cumulus initialization procedure is designed to ameliorate the problem of numerical weather prediction spinup, the departure of diabatic forcing from diagnosis and observation, which is characteristic of the early portions of integrations in numerical weather prediction. Formally, cumulus initialization consists of a minimization of the adjustments to the original analyses of temperature and humidity, subject to nonlinear constraints imposed by the cumulus parameterization in the numerical weather prediction model, if cumulus heating is taken as known by diagnostic methods as an initial condition. Experiments with a global model exhibiting severe spinup when initialized with an analysis subject only to diabatic normal-mode initialization show that cumulus initialization can recover initial horizontal and vertical distributions of latent heat release (produced synthetically by spinning up the same global model through an independent integration using an earlier analysis to provide initial conditions) quite successfully. This recovery depends on the simultaneous initialization of the divergence, temperature, and humidity fields. The magnitudes of the adjustments in the temperature and humidity fields produced by cumulus initialization are smaller than the changes in those fields produced by the global model itself as it is integrated forward from an analysis without cumulus initialization through spinup. The cumulus initialization procedure can be modified to allow for uncertainties in the diagnosis of initial heating rates. Despite the successfully initial recovery of cumulus heating, further adjustment occurs as the global model is integrated forward from a cumulus-initialized analysis; this adjustment is characterized by an overshoot in the intensity of both latent heat release and divergence. A severe imbalance between globally averaged precipitation and evaporation that occurred without cumulus initialization is considerably ameliorated in integrations with cumulus initialization.

Abstract

A procedure for adjusting temperature and humidity analyses used as initial conditions for numerical weather prediction models so that diagnosed distributions of cumulus convection exist during the initial stages of the forecast is applied in a global atmospheric model. This cumulus initialization procedure is designed to ameliorate the problem of numerical weather prediction spinup, the departure of diabatic forcing from diagnosis and observation, which is characteristic of the early portions of integrations in numerical weather prediction. Formally, cumulus initialization consists of a minimization of the adjustments to the original analyses of temperature and humidity, subject to nonlinear constraints imposed by the cumulus parameterization in the numerical weather prediction model, if cumulus heating is taken as known by diagnostic methods as an initial condition. Experiments with a global model exhibiting severe spinup when initialized with an analysis subject only to diabatic normal-mode initialization show that cumulus initialization can recover initial horizontal and vertical distributions of latent heat release (produced synthetically by spinning up the same global model through an independent integration using an earlier analysis to provide initial conditions) quite successfully. This recovery depends on the simultaneous initialization of the divergence, temperature, and humidity fields. The magnitudes of the adjustments in the temperature and humidity fields produced by cumulus initialization are smaller than the changes in those fields produced by the global model itself as it is integrated forward from an analysis without cumulus initialization through spinup. The cumulus initialization procedure can be modified to allow for uncertainties in the diagnosis of initial heating rates. Despite the successfully initial recovery of cumulus heating, further adjustment occurs as the global model is integrated forward from a cumulus-initialized analysis; this adjustment is characterized by an overshoot in the intensity of both latent heat release and divergence. A severe imbalance between globally averaged precipitation and evaporation that occurred without cumulus initialization is considerably ameliorated in integrations with cumulus initialization.

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