Interactions between Moist Heating and Dynamics in Atmospheric Predictability

Abstract

The predictability properties of a fixed heating version of a GCM in which the moist heating is specified beforehand are studied in a series of identical twin experiments. Comparison is made to an identical set of experiments using the control GCM, a five-level R30 version of the COLA GCM. The experiments each contain six ensembles, with a single ensemble consisting of six 30-day integrations starting from slightly perturbed Northern Hemisphere wintertime initial conditions. The moist heating from each integration within a single control ensemble was averaged over the ensemble. This averaged heating (a function of three spatial dimensions and time) was used as the prespecified heating in each member of the corresponding fixed heating ensemble.The errors grow less rapidly in the fixed heating case. The most rapidly growing scales at small times (global wavenumber 6) have doubling times of 3.2 days compared to 2.4 days for the control experiments. The predictability times for the most energetic scales (global wavenumbers 9?12) are about two weeks for the fixed heating experiments, compared to 9 days for the control. The ratio of error energy in the fixed heating to the control case falls below 0.5 by day 8, and then gradually increases as the error growth slows in the control case.The growth of errors is described in terms of budgets of error kinetic energy (EKE) and error available potential energy (EAPE) developed in terms of global wavenumber n. The diabatic generation of EAPE (GAPE) is positive in the control case and is dominated by midlatitude heating errors after day 2. The fixed heating GAPE is negative at all times due to longwave radiative cooling.The linearized interactions of the errors with the mean flow in the thermodynamic equation lead to the creation of EAPE via the term CAPE, but the corresponding term in the momentum equation CEKE leads to the loss of EKE, in analogy to the life cycle of baroclinic eddies. (The choice of the mean flow has a noticeable impact only at short times.) The nonlinear terms, consisting of explicit nonlinearities in the error plus residual quasi-linear terms, create both EKE and EAPE. At the earliest forecast times the creation of total error energy is dominated by CAPE at large scales, and by the nonlinear terms at small scales. By day 8 in the control case (later in the fixed heating case) CAPE dominates both the nonlinearities and the diabatic generation at all scales. The ratioCAPE(control/CAPE(fixed heating) ranges between 2 and 3.The larger GAPE and CAPE in the control case allow for a much stronger conversion of EAPE to EKE than in the fixed heating case, by a factor of 4. This conversion is the major source of EKE in both sets of experiments, but dominates over the nonlinear creation of EKE more completely in the control case. The nonlinearities create EKE preferentially at the larger scales.