Search Results
You are looking at 1 - 2 of 2 items for :
- Author or Editor: P. Malanotte-Rizzoli x
- Journal of the Atmospheric Sciences x
- Refine by Access: All Content x
Abstract
A weakly nonlinear theory is presented that may explain the persistence of the two main types of low-frequency anomalies observed in the midlatitude jet stream by Dole and Gordon (1983). The theory describes how nonlinearity can balance dispersion effects for both split jet stream anomalies (which resemble blocking flows) and for jet intensification anomalies. It is shown that the variation of the potential or refractive-index function ≡ dq/dψ across the jet stream is crucial for determining which types of anomaly will tend to persist. Although the theory is only weakly nonlinear it is argued that the same dynamical mechanisms will remain important in the high-amplitude regime particularly for the intense-jet anomalies. In the split anomalies the potential vorticity contours can easily become closed at high amplitude hence trapping air parcels (this is the origin of the strongly nonlinear modon solutions). However, even for very strong intense-jet anomalies the potential vorticity contours may remain open and then no air trapping occurs, thus, the variations in the cross jet stream potential function remain important. Initial value numerical experiments are presented to demonstrate that both types of anomaly are close to persistent states of the full barotropic vorticity equation, even at amplitudes that are beyond the strict range of validity of the weakly nonlinear theory. Some discussion and investigation of the possible role of critical lines in preventing dispersion into equatorial latitudes is also presented. Finally, the possibility of testing this theory by making appropriate diagnostic measurements is considered.
Abstract
A weakly nonlinear theory is presented that may explain the persistence of the two main types of low-frequency anomalies observed in the midlatitude jet stream by Dole and Gordon (1983). The theory describes how nonlinearity can balance dispersion effects for both split jet stream anomalies (which resemble blocking flows) and for jet intensification anomalies. It is shown that the variation of the potential or refractive-index function ≡ dq/dψ across the jet stream is crucial for determining which types of anomaly will tend to persist. Although the theory is only weakly nonlinear it is argued that the same dynamical mechanisms will remain important in the high-amplitude regime particularly for the intense-jet anomalies. In the split anomalies the potential vorticity contours can easily become closed at high amplitude hence trapping air parcels (this is the origin of the strongly nonlinear modon solutions). However, even for very strong intense-jet anomalies the potential vorticity contours may remain open and then no air trapping occurs, thus, the variations in the cross jet stream potential function remain important. Initial value numerical experiments are presented to demonstrate that both types of anomaly are close to persistent states of the full barotropic vorticity equation, even at amplitudes that are beyond the strict range of validity of the weakly nonlinear theory. Some discussion and investigation of the possible role of critical lines in preventing dispersion into equatorial latitudes is also presented. Finally, the possibility of testing this theory by making appropriate diagnostic measurements is considered.
Abstract
A diagnostic study of persistent intense jet stream events in the Pacific has been carried out with a 15 winter NMC dataset to assess the relevance of the weakly nonlinear model recently proposed by Haines and Malanotte-Rizzoli. Composited data from 14 episodes of persistent intensification anomalies in the central and eastern Pacific have been analyzed with scatter diagrams of potential vorticity q plotted against geopotential Φ on the 300-mb surface. The slope of the functional relationship gives a measure of the wavelength independent component of the refractive index (n 2 = −Λ0 = −dq 0/dψ 0). The theoretical model suggests that if dq 0/dψ 0 is more negative on the northern and southern flanks, a local intense region within the jet stream may be abnormally persistent. The composited dataset shows that this condition is satisfied during the postonset period as defined by Dole. In contrast, the climatology and the mean flow before onset does not show much variation in Λ0 across the jet. Results are encouraging, but higher-resolution data is needed to draw firm conclusions.
Abstract
A diagnostic study of persistent intense jet stream events in the Pacific has been carried out with a 15 winter NMC dataset to assess the relevance of the weakly nonlinear model recently proposed by Haines and Malanotte-Rizzoli. Composited data from 14 episodes of persistent intensification anomalies in the central and eastern Pacific have been analyzed with scatter diagrams of potential vorticity q plotted against geopotential Φ on the 300-mb surface. The slope of the functional relationship gives a measure of the wavelength independent component of the refractive index (n 2 = −Λ0 = −dq 0/dψ 0). The theoretical model suggests that if dq 0/dψ 0 is more negative on the northern and southern flanks, a local intense region within the jet stream may be abnormally persistent. The composited dataset shows that this condition is satisfied during the postonset period as defined by Dole. In contrast, the climatology and the mean flow before onset does not show much variation in Λ0 across the jet. Results are encouraging, but higher-resolution data is needed to draw firm conclusions.
