Tropical Cyclone - Environment interaction in the UTLS

A detailed energetic analysis of the cloud permitting simulation of several idealized and real tropical cyclones reveals critical energy exchanges with the Upper Troposphere and Lower Stratosphere (UTLS). TC energy deposited in the UTLS takes the form of large positive potential energy anomalies, large negative internal energy anomalies and negative potential vorticity anomalies. These anomalies appear as high Montgomery Stream function and low potential vorticity anomalies at the outflow level in isentropic coordinates. Unlike in the lower troposphere, the UTLS is above the zero net radiation level, meaning that there is not a net radiation cooling to space, and in fact there may be a net warming. The low thermal anomaly of the outflow exasperates this condition, making the complete Carnot cycle dependent on radiative loss to space, described by Emanuel and Rotunno unlikely in the near environment of the TC. Instead, the TC must couple to existing vertical circulations of the environment or build its own regional vertical circulation using the free energy expressed by the MSF anomalies. It will be shown that the MSF anomalies have a critical impact on both the TC lifecycle and how the TC interacts with its environment. The local environment in which the TC is embedded can be composed of the previous outflow of the TC, and as such will have a favorable, “neutral” level of resistance to TC development, but these environments take a long time to become established and storm relative environmental flow can continuously move the storm's own outflow environment away replacing it with a less favorable external environment. Under some circumstances, the high MSF/ low PV environments of TC outflow can strongly contrast with an environment of distinctly lower MSF. This results in the production of high levels of “free energy” that can be used to grow the regional TC outflow circulation. This would typically result from baroclinicity in the UTLS. Understanding outflow-environment interactions is shown to be critical to the prediction of storm lifecycle, intensity variability and even track.