Greg Hakim
- Greg Hakim :
- home :
- research :
- teaching :
- publications :
- talks :
- tropopause :
- EnKF :
- weather :
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)