Basic research in atmospheric dynamics aims to develop a better fundamental understand of weather systems across a wide range of scales. The problems we explore involve the stability properties of the jet stream, weather systems in the jet stream, and the interactions of weather systems across a wide range of scales and with topography. We also develop new theories that are then tested on observed data (see synoptic meteorology).

Graduate student Steven Cavallo is working to understand the formation and intensity changes of high-latitude sub-synoptic scale vortices. While it is quite well understood that upper level disturbances play a major role in the formation of extratropical surface cyclones, there is much less understanding of the mechanisms controlling the strength of the upper level disturbances. These upper level disturbances often originate over polar regions, well removed from the mid-latitude jet stream, and often move into the mid-latitudes where surface cyclogenesis can result. Further, because they commonly originate well poleward of the mid-latitude jet stream, they are generally vortices and their dynamics is nonlinear. Due to these characteristics they are termed tropopause polar vortices (TPVs). Graduate student Steven Cavallo is examining the physics behind what causes changes in the intensity of TPVs using a potential vorticity diagnosis method. In a case study of one particular TPV, Steven determined that cloud-top radiational cooling can cause significant increases in the intensity of a tropopause cyclone. Figure 1 shows a MODIS visible satellite image of a polar low in association with a tropopause polar cyclone, located near an ice edge in Hudson Bay. Tropopause pressure, winds, and sea level pressures in a corresponding numerical simulation are shown in Figure 2 near the same time as the MODIS image in Figure 1. Future work extends these ideas in a more idealized setting in order to determine the more complicated mechanisms behind the weakening of TPVs.

Figure 1: Moderate Resolution Imaging Spectroradiometer (MODIS) visible satellite image of a polar low in association with a tropopause polar cyclone over Hudson Bay.

Figure 2: Tropopause pressure (colors), winds (barbs), and sea level pressure (contours) from a WRF model simulation valid on November 23, 2005 at 18 UTC.

Graduate student Lucas Harris is working on the formation of vortices in the wakes of mountainous islands. These delicate lee vortices are occasionally seen in clouds downstream of mountainous islands. They are formed when vorticity is baroclinically generated in the lees of mountains during times of strong static stability and moderate winds. These vortices often shed downstream to create a vortex street. Typically, these vortices all form with approximately the same strength, but events where one vortex is stronger than the other has been observed. Observations and numerical modeling have both shown that directional wind shear, or a turning of the wind with height, can cause such asymmetric vortex shedding. Research is being carried out to determine the link between the directional shear and the appearance of asymmetric vortices. In particular, this project seeks to determine whether the asymmetry is caused by the tilting of horizontal vorticity caused by the ambient directional shear, or whether it is the result of the mountain wave being altered in some manner by the shear.

Figure 1: MODIS image of lee vortices off of the Cape Verde islands. Note that all of the vortices show a counter-clockwise orientation.

Figure 2: Plot of potential vorticity from a numerical model with directional shear. The cyclones appear much stronger than the anticyclones.

Recent Papers:

Chen, C.-C., G. J. Hakim, and D. R. Durran, 2006: Transient mountain waves and their interaction with large scales. J. Atmos. Sci 63, accepted. (pdf)

Torn, R., D., G. J. Hakim, and C. Snyder, 2006: Boundary conditions for limited-area ensemble Kalman filters. Mon. Wea. Rev. 134,  2490--2502. (pdf)

Chen, C.-C., D. R. Durran, and G. J. Hakim, 2005: Mountain wave momentum flux in an evolving synoptic-scale flow. J. Atmos. Sci 62,  3213--3231. (pdf)

Snyder, C., and G. J. Hakim, 2005: Cyclogenetic perturbations and analysis errors decomposed into singular vectors J. Atmos. Sci. 62, 2234--2247. (pdf)

Stevens, M. R., and G. J. Hakim, 2005: Perturbation growth in baroclinic waves. J. Atmos. Sci. 62,  2847--2863. (pdf)

Patoux, J., G. J. Hakim, and R. A. Brown, 2005: Diagnosis of frontal instabilities over the Southern Ocean. Mon. Wea. Rev. 133,  863--875. (pdf)

Hakim, G. J., and A. Canavan, 2005: Observed cyclone--anticyclone tropopause vortex asymmetries. J. Atmos. Sci. 62,  231--240. (pdf)

Hakim, G. J., 2003: Developing wave packets in the North Pacific storm track. Mon. Wea. Rev. 131,  2824--2837.  (pdf)

Hakim, G. J., C. Snyder, and D. J. Muraki, 2002: A new surface model for cyclone--anticyclone asymmetry. J. Atmos. Sci. 59,  2405--2420.  (pdf)