METR 5413: Advanced Synoptic Meteorology (University of Oklahoma)

Lazear and Morgan (2005) illustrate tropical cyclones, including Hurricane Rita, and their evolution with potential tropopause temperature plotted on a PV surface in Figure 3. An upper-level trough is present in Figure 3a, which is the same trough of interest mentioned above in the synoptic overview. It becomes cut-off in Figure 3b, during which time Rita becomes a tropical depression. Meanwhile, another major hurricane named Ophelia dissipates into anticyclonic outflow into the Atlantic. Figure 3c jumps ahead a few days later to depict Hurricane Rita at Category 4 strength as a positive PV anomaly off the Texas and Louisiana coast. It is noticeable that this anomaly seems to appear ahead of where the heavier rainfall in Figure 2 was, confirming diabatic heating most likely accommodated it. Then, Figure 3d shows an interesting phenomenon where the anticyclonic PV near the tropopause becomes stretched as the positive PV anomaly near the surface weakens. Wu and Kurihara 1996 illustrated that this negative PV anomaly associated with the hurricane outflow could linger for days after the event. All this information gives a clearer picture of how PV anomalies behave when the atmosphere is not adiabatic. So what are other factors that can relate Hurricane Rita to this IPV thinking?


First off, for positive (a.k.a cyclonic) IPV to form near the surface, static stability occurs because isotherms become tightly packed near the surface, where the maximum intensity is just slightly above the ground. This means positive PV anomalies near the surface indicate a warm core system. As expected, hurricanes are also warm core systems that create low surface pressures in the eye, so this theory checks out. Secondly, being a warm core system also means the PV inversion dictates that the winds that swirl about the center decrease with height due to the presence of anticyclonic thermal vorticity aloft. In the upper-troposphere, negative (a.k.a anticyclonic) PV anomalies develop when isotherms are more spread apart, giving way to low static stability. The upper-level anticyclonic PV can be attributed to the hurricane’s outflow, as seen from Figure 3d. Generally, tropical cyclone outflow will start to expand away from the center once the cyclone intensifies (Rappin 2011). One can also show that IPV thinking ties closely to the quasi-geostrophic relation for Hurricane Rita. There will be height falls in a region where a cyclonic PV is present, which is associated with convergence and the upper level trough meaning everything connects as it should. Quasi-geostrophic components can, therefore, still serve as helpful even when the focus is on IPV thinking.