Reexamining the Vertical Structure of Tangential Winds in Tropical Cyclones: Observations and Theory

Abstract

A few commonly held beliefs regarding the vertical structure of tropical cyclones drawn from prior studies, both observational and theoretical, are examined in this study. One of these beliefs is that the outward slope of the radius of maximum winds (RMW) is a function of the size of the RMW. Another belief is that the outward slope of the RMW is also a function of the intensity of the storm. Specifically, Shea and Gray found that the RMW becomes increasingly vertical with increasing intensity and decreasing radius. The third belief evaluated here is that the RMW is a surface of constant absolute angular momentum M. These three conventional wisdoms of vertical structure are revisited with a dataset of three-dimensional Doppler wind analyses, comprising seven hurricanes on 17 different days. Azimuthal mean tangential winds are calculated for each storm, and the slopes of the RMW and M surfaces are objectively determined. The outward slope of the RMW is shown to increase with radius, which supports prior studies. In contrast to prior results, no relationship is found between the slope of the RMW and intensity. It is shown that the RMW is indeed closely approximated by an M surface for the majority of storms. However, there is a small but systematic tendency for M to decrease upward along the RMW. Utilizing Emanuel's analytical hurricane model, a new equation is derived for the slope of the RMW in radius-pressure space. This predicts a linear increase of slope with radius and essentially no dependence of slope on intensity. An exactly analogous equation can be derived in log-pressure height coordinates, and a numerical solution yields the same conclusions in geometric height coordinates. These conclusions are further supported by the results of simulations utilizing Emanuel's simple, timedependent, axisymmetric hurricane model. As both the model and the analytical theory are governed by the dual constraints of thermal wind balance and slantwise moist neutrality, it is demonstrated that it is these two assumptions that require the slope of the RMW to be a function of its size but not of the intensity of the storm. Finally, it is shown that within the context of Emanuel's theory, the RMW must very closely approximate an M surface through most of the depth of the vortex.Corresponding author address: Daniel P. Stern, RSMAS/MPO, 1 Although he then confuses his own terminology in the following sentence, writing that ''Shea and Gray also showed that the slope of the radius of maximum winds decrease for stronger storms''; by his convention, the slope actually increases for stronger storms.

Assimilation of GBVTD‐retrieved winds from single‐Doppler radar for short‐term forecasting of super typhoon Saomai (0608) at landfall

“…In ExpGV (Figure 7(c)), the vortex has a smaller RMW of about 20 km, a maximum mean wind speed of about 55 m s −1 , and a maximum temperature anomaly of about 20 K. It is also noted that a secondary wind maximum is found at a radius of about 55 km in ExpGV (Figure 7(c)), which is associated with the outer eyewall; such a secondary maximum is not found in ExpVr. Although the vertical structure cannot be directly verified by observations, the steepest slope of the RMW line associated with the smallest RMW in ExpGV is consistent with the study of Stern and Nolan (2009), where the slope of the RMW is found to be inversely proportional to the RMW. The more realistic vortex structure obtained in ExpGV can be attributed to a better representation of the vortex inner core circulation by the V GBVTD wind data.…”

Evolution of Eyewall Convective Events as Indicated by Intracloud and Cloud-to-Ground Lightning Activity during the Rapid Intensification of Hurricanes Rita and Katrina

“…size, the eyewall of Katrina produced less lightning in total than Rita: LASA detected 1430 eyewall lightning flashes (ICs 1 CGs) for Rita, compared to 684 for Katrina. A recent study from Squires and Businger (2008), based solely on CG detection, hypothesized that the larger, and therefore more tilted, eyewall (Stern and Nolan 2009) of Katrina caused the main charge regions to be radially displaced from each other in the vertical, in turn reducing the area of strong electric field and hence lightning activity in the eyewall. This hypothesis is in line with Black and Hallett's (1999) seminal work showing that in hurricane eyewalls the ratio of ice to supercooled water, a key ingredient for noninductive collisional charge separation with riming graupel (e.g., Takahashi 1978;Saunders and Peck 1998), increases radially outward.…”

Sensitivity of Tropical Cyclone Feedback on the Intensity of the Western Pacific Subtropical High to Microphysics Schemes

“…The microphysical heating is mostly concentrated in the area of eyewall convections and extends outward with height following the eyewall slope. Stern and Nolan (2009) showed that the outward slope of the eyewall with height is directly proportional to the radius of the eyewall. As expected, the WSM6 case is characterized by a less upright (i.e., larger slope) eyewall in terms of the latent heat release because of the larger eyewall radius when compared with the WSM3 case.…”