Introduction:

On December 14th and 15th, 1997, rapid cyclogenesis occurred off the northeast coast of Florida. Surface pressures fell from 1007 mb to 996 mb over the 30-hour period ending at 18Z on December 15th. A rapid increase in the pressure gradient on the northwest side of the storm resulted in strong, north to northeast winds and heavy surf along the northeast coast of Florida. A section of the Flagler Beach Pier was destroyed on the morning of December 15th due to a combination of high waves and winds.

Question:

Can Potential Vorticity (PV) analysis be used to diagnose the development and evolution of the Flagler Beach Storm?

What is PV?:

In isentropic coordinates, potential vorticity can be expressed as:

In this form, isentropic potential vorticity is represented as gravity, multiplied by absolute vorticity, multiplied by static stability.

The units of isentropic potential vorticity are:

PV values will be high where either static stability or vorticity is high. Due to the high static stability of the stratosphere, PV values greater than 1.0 usually indicate air of stratospheric origin.

If PV is calculated on an isentropic surface near the tropopause, high PV values (PV anomalies) can indicate locations where stratospheric air has extended downward into the troposphere. These downward penetrations of stratospheric air are often refered to as "tropopause undulations" or "tropopause folds".

Hypothesis:

According to Hirschberg et al. (1991), tropopause undulations, especially their associated temperature changes, can act synergistically with lower tropospheric processes to initiate, maintain, and enhance the development of extratropical cyclones.

Specifically, if hydrostatic balance is assumed and if an isobaric lid exists, then the juxtaposition of high-level warmth associated with a developing tropopause undulation and low-level warmth associated with tropospheric baroclinic regions creates the potential for the production of relatively low pressure at the ground.

The production of low pressure near the ground has the potential to produce increased low level convergence and upward vertical motion.

Upward vertical motion of a moist, unstable airmass can produce condensation and latent heat release, leading to further warming of the airmass and lowering of the pressures near the ground.

I. Six-Hourly PV Overview of the Development of Flagler Beach Storm

II. The Role of Upper Tropospheric Warm Air Advection

III. Upper Tropospheric Isentropic Analysis: The Delicate Balance Between Warm Air Advection and Rising Motion

IV. The Role of Static Stability and Latent Heat Release in the Development of the Surface Cyclone

V. Conclusions

VI. References