Climatological Storm Track Research Articles

The Usefulness of MSU3 Analyses as a Forecasting Aid: A Statistical Study

A statistical analysis is performed on a 6-month global dataset consisting of satellite-derived channel 3 Microwave Sounding Unit (MSU3) brightness temperature and various conventionally derived fields to quantify the potential usefulness of MSU3 analyses in the nowcasting and forecasting of baroclinic waves. High positive spatial and temporal correlations are obtained between the MSU3 brightness temperature and 400‐100-mb thickness fields over all wavelengths in the data. Slightly lesser positive correlations are found between the MSU3 and the 200-mb temperature. The MSU3‐500-mb and MSU3‐50-mb height correlation results indicate a scale dependence in the hydrostatic spreading of thickness anomalies in the vertical. Most significantly, relatively high negative MSU3‐500-mb height correlations for the short (# synoptic scale) wavelength portion of the data suggest that upper-level thermal anomalies are reflected downward and that MSU3 analyses can be used to track midlevel synoptic-scale baroclinic waves. This conclusion is also supported by relatively high negative MSU3‐500-mb vorticity and MSU3‐dynamic tropopause correlations along the climatological storm tracks.

Linear Modes and Storm Tracks in a Two-Level Primitive Equation Model

Linear stability analyses of three-dimensional time-mean flows in a two-level primitive equation model are presented. The model's stationary wave is generated by implementing idealized mountains at the lower boundary. Two sets of experiments are performed: the first with one mountain and the second with three mountains at equal distance from each other. Structures of streamfunctions and heat fluxes from the linearly unstable normal mode are compared with their bandpass transient counterparts in the nonlinear model simulation. The three-dimensional time-mean flow, about which the equations are linearized, is convectively unstable for both the one and three mountain cases. For the three mountain case, there is reasonable agreement between the linear mode and the bandpass transient eddies in terms of both the longitudinal location of the largest eddy amplitudes and the potential enstrophy budget, suggesting that the global mode can capture the correct structure of the climatological storm tracks for different reasons. For the one mountain case, however, the largest eddy amplitude of the linear mode extends farther downstream than that of the bandpass transient eddies. The reasonable correspondence between the linear modes and the bandpass eddies for the three mountain case appears to be due to the relative proximity of successive unstable regions. Between these successive unstable regions are diffluent flows, resulting in increased deformation and enhanced horizontal diffusion, which plays an important role in dissipating transient eddy enstrophy. It is suspected that this locally enhanced dissipation represents strained eddies by the deformation field.

Organization of Extratropical Transients during El Niño

Abstract Four observed El Nino-Southern Oscillation (ENSO) events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter. A parallel analysis of a GCM's response to El Nino is performed in order to assess if similar mechanisms are operative in the model atmosphere. The observed stationary wave anomalies over the Pacific/North American (PNA) region are found to he similar during the four winters despite appreciable differences in sea surface temperatures. The anomalous transient vorticity fluxes are remarkably robust over the North Pacific during each event, with an eastward extension of the climatological storm track leading to strong cyclonic forcing near 40°N, 150°W. This forcing is in phase with the seasonal mean Aleutian trough anomaly suggesting the importance of eddy-mean flow interactions. By comparison, the intersample variability of the GCM response over the PNA region is found to exceed the observed inter-El Nino variabi...

North atlantic blocking during January 1979: Linear theory

AbstractThe second half of January 1979 was a time of persistent and large‐scale blocking in the North Atlantic region. In this study we examine the instability properties of the flow during this period and relate the growing modes to the development that occurs. We have analysed the growth rates, phase frequencies and structures of the ten fastest‐growing modes, within a five‐level quasi‐geostrophic model incorporating spherical geometry, on each day for the period between 10 and 30 January 1979.Large‐scale equivalent barotropic dipole or multipole instability modes are prevalent in the North Atlantic during the second half of January. These modes are stationary or slowly propagating and grow rapidly with efolding times as short as 1.7 days in some cases. We find that there is a close connection between these modes and the dynamical developments observed subsequently a few days later. Associated with the early stages of block formation, there are large‐scale eastward propagating westward tilting onset‐of‐blocking wave‐train modes. the onset‐of‐blocking modes also appear to be associated with rapid cyclogenesis off the east coast of North America.The formation of the North Atlantic blocking system and diffluent flow also produces corresponding changes in the storm tracks. During the early parts of this period the fastest‐growing monopole cyciogenesis instability modes have regions of preferential development in the climatological storm tracks over the Pacific, Atlantic and Siberian regions. Subsequently, however, the Atlantic cyclogenesis modes split into wave trains poleward and equatorward of the block.In the developments during this period, we also examine the roles of the adjoint eigenmodes and of optimal perturbations which produce the largest initial tendency of the L2 norm squared of the streamfunction.