Nonlinear impacts upon frontal cyclones from dipole surface temperature anomalies

Summary

The effects of surface temperature anomalies (STAs) upon frontal cyclones are investigated with a nonlinear model. The model used is a modified version of the NCAR Community Climate Model (CCM 1). The experiments are run with hemispheric domain and R 30 (rhomboidal) truncation. The present study isolates the effects of sensible heating. Topography and latent heating are excluded from this model. The initial data are created from a solution (normal mode) to the linear eigenvalue problem. Six experiments use various locations and intensities for dipole-shaped STA; one control case is run without STA. The intensity is either ±5 or ±10°K and the anomalies ae located at 40°N, 50°N, or 35°N. The jet is centered at 40°N. All cases are run for 20 days. Nonlinear, time-dependent, growth rate and phase frequency are derived and compared to the linear (eigenvalue) amounts.

The resulting waves grow primarily by baroclinic instability. Perturbation fields at higher levels grow faster before they mature (“occlude”) and decay faster afterward, than do lower level fields. The baroclinic conversion of energy lessens as the perturbations mature. The principal hypothesis tested is that: the STA alters the static stability which in turn modulates the baroclinic instability. Over warm anomalies the static stability should be reduced, enhancing baroclinic instability. Over cold anomalies the opposite may happen. The nonlinear simulations confirm this hypothesis in part. In the present study, the intensity of the warm anomaly produces greater growth rate during and after the storm's mature state. Larger STA intensity increases the maximum amplitude of the perturbation in a roughly linear fashion. However, the STA effects are nonlinear after maximum amplitude is reached: during decay, the difference in amplitude between the control case and the 10°K STA case is more than twice the difference between the control and 5°K case. In contrast, little deviation from the control case is found for perturbations over the cold anomaly, indicating a nonlinear link between STA and wave growth. The latitudinal variation used of the surface temperature anomaly centers had no significant influence on the baroclinic growth. Secondary growths of storms after 10 days are more commonly seen in cases with STA.