It has been well documented that the National Hurricane Center (NHC) has made significant improvements in Atlantic basin tropical cyclone (TC) track forecasting during the past half century. In contrast, NHC’s TC intensity forecast errors changed little from the 1970s to the early 2000s. Recently, however, there has been a notable decrease in TC intensity forecast error and an increase in intensity forecast skill. This study documents these trends and discusses the advancements in TC intensity guidance that have led to the improvements in NHC’s intensity forecasts in the Atlantic basin. We conclude with a brief projection of future capabilities.

The High-Definition Sounding System (HDSS) is an automated system deploying the expendable digital dropsonde (XDD) designed to measure wind and pressure–temperature–humidity (PTH) profiles, and skin sea surface temperature (SST) within and around tropical cyclones (TCs) and other high-impact weather events needing high sampling density. Three experiments were conducted to validate the XDD. On two successive days off the California coast, 10 XDDs and 14 Vaisala RD-94s were deployed from the navy’s Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft over offshore buoys. The Twin Otter made spiral descents from 4 km to 60 m at the same descent rate as the sondes. Differences between successive XDD and RD-94 profiles due to true meteorological variability were on the same order as the profile differences between the spirals, XDDs, and RD-94s. XDD SST measured via infrared microradiometer, referred to as infrared skin SST (SSTir), and surface wind measurements were within 0.5°C and 1.5 m s−1, respectively, of buoy and Twin Otter values. A NASA DC-8 flight launched six XDDs from 12 km between ex-TC Cosme and the Baja California coast. Repeatability was shown with good agreement between features in successive profiles. XDD SSTir measurements from 18° to 28°C and surface winds agreed well with drifting buoy- and satellite-derived estimates. Excellent agreement was found between PTH and wind profiles measured by XDDs deployed from a NASA WB-57 at 18-km altitude offshore from the Texas coast and NWS radiosonde profiles from Brownsville and Corpus Christi, Texas. Successful XDD profiles were obtained in the clear and within precipitation over an offshore squall line.

Large uncertainty still remains in determining whether a tropical cloud cluster will develop into a tropical cyclone. During The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment, over 50 tropical cloud clusters were monitored for development, but only 4 developed into a tropical cyclone. One nondeveloping tropical disturbance (TCS025) was closely observed for potential formation during five aircraft research missions, which provided an unprecedented set of observations pertaining to the large-scale and convective environments of a nondeveloping system. The TCS025 disturbance was comprised of episodic convection that occurred in relation to the diurnal cycle along the eastern extent of a broad low-level trough. The upper-level environment was dominated by two cyclonic cells in the tropical upper-tropospheric trough (TUTT) north of the low-level trough in which the TCS025 circulation was embedded. An in-depth examination of in situ observations revealed that the nondeveloping circulation was asymmetric and vertically misaligned, which led to larger system-relative flow on the mesoscale. Persistent environmental vertical wind shear and horizontal shearing deformation near the circulation kept the system from becoming better organized and appears to have allowed low equivalent potential temperature (u e ) air originating from one of the TUTT cells to the north (upshear) to impact the thermodynamic environment of TCS025. This in turn weakened subsequent convection that might otherwise have improved alignment and contributed to the transition of TCS025 to a tropical cyclone.

[1] Saturn's atmosphere contains numerous vortices that reside predominantly within specific localized latitude bands. Two-dimensional turbulence theory predicts that vortices which do form are readily destroyed as they interact with dispersive Rossby waves in a process called the ''Rhines effect,'' which acts to organize turbulent energy into alternating zonal flows through the interaction of Rossby waves and turbulence of similar scales. Observations show that at some latitudes, vortices are more prevalent, suggesting that Rossby waves are suppressed in these regions. Following the method applied to Jupiter by Theiss (2006), we generalize the 2-D Rhines scale to include depth-dependent flow with a finite deformation radius; this allows for a simple estimate of the conditions under which Rossby waves are suppressed in the cloud layer. We then compare the latitudes of known vortices to the predicted latitudes where Rossby waves may be suppressed on Saturn. We find a good correlation, suggesting that, as on Jupiter, Rossby wave suppression helps explain the prevalence of vortices at specific latitudes on Saturn.

An analysis of in situ observations from the nondeveloping tropical disturbance named TCS025 revealed that a combination of unfavorable system-scale and environmental factors limited further development. In this study, a multiphysics ensemble of high-resolution simulations of TCS025 are analyzed and compared. A simulation that overdeveloped the TCS025 disturbance is compared with one that correctly simulated nondevelopment and reveals that convection was stronger and diabatic heating rates were larger in the developing simulation. This led to continued spinup of the low-level circulation primarily through vorticity stretching. In contrast, convection was much weaker in the nondeveloping simulation, and after an initial period of deep convection, average vorticity tendencies from stretching became weakly negative, which allowed for the frictional spindown of the low-level circulation. Convective-scale differences identified early in the simulations appear to have resulted from the explicit representation of graupel in the developing simulation. The net impacts resulting from these differences in convection are manifest in the average diabatic heating profiles that are important for determining the developmental outcome. Additional simulations are conducted whereby the diabatic heating rates are artificially adjusted. Relatively small changes in the diabatic heating rate led to significantly different outcomes with respect to storm development, and the degree of overdevelopment is largely dictated by the diabatic heating rate. These findings suggest the correct representation of convective processes and associated diabatic heating are necessary to adequately forecast tropical cyclogenesis, especially for systems near a threshold of development like TCS025.