Analysis of the peripheral characteristics of Tropical Storm Sinlaku(2020) based on dropsonde data

Global Positioning System (GPS) radio occultation (RO) from low Earth‐orbiting satellites has increased the quantity of high‐vertical resolution atmospheric profiles, especially over oceans, and has significantly improved global weather forecasting. A new system, the Global Navigation Satellite Systems Instrument System for Multistatic and Occultation Sensing (GISMOS), has been developed for RO sounding from aircraft. GISMOS also provides high‐vertical resolution profiles that are insensitive to clouds and precipitation, and in addition, provides greater control on the sampling location, useful for targeted regional studies. The feasibility of the system is demonstrated with a flight carried out during development of an Atlantic tropical storm. The data have been evaluated through a comparison with dropsonde data. The new airborne RO system will effectively increase by more than 50% the number of profiles available for studying the evolution of tropical storms during this campaign and could potentially be deployed on commercial aircraft in the future.

The thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TCs) were investigated using dropsonde observations collected from 1996 to 2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 h following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.

Abstract. The formation of west Pacific tropical cyclone Nuri (2008) was observed over four days from easterly wave to typhoon stage by aircraft using scanning Doppler radar and dropsonde data. This disturbance developed rapidly in a significantly sheared environment. In spite of the shear, overlapping closed circulations existed in the frame of reference of the storm in the planetary boundary layer and at 5 km elevation, providing a deep region protected from environmental influences. The rapid spinup of Nuri can be attributed to the strong increase with height at low levels of the vertical mass flux during and after the tropical depression stage, and the correspondingly strong vorticity convergence in the planetary boundary layer. As Nuri developed, convective regions of boundary layer vortex stretching became fewer but more intense, culminating in a single nascent eyewall at the tropical storm stage. A non-developing tropical wave case was also analyzed. This system started with much weaker circulations in the boundary layer and aloft, leaving it unprotected against environmental intrusion. This may explain its failure to develop.

The development of a compact warm core extending from the mid-upper levels to the lower troposphere and related surface pressure falls leading to tropical cyclogenesis (TC genesis) is not well understood. This study documents the evolution of the three-dimensional thermal structure during the early developing stages of Typhoons Fanapi and Megi using aircraft dropsonde observations from the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign in 2010. Prior to TC genesis, the precursor disturbances were characterized by warm (cool) anomalies above (below) the melting level (~550 hPa) with small surface pressure perturbations. Onion-shaped skew T–logp profiles, which are a known signature of mesoscale subsidence warming induced by organized mesoscale convective systems (MCSs), are ubiquitous throughout the ITOP aircraft missions from the precursor disturbance to the tropical storm stages. The warming partially erodes the lower-troposphere (850–600 hPa) cool anomalies. This warming results in increased surface pressure falls when superposed with the upper-troposphere warm anomalies associated with the long-lasting MCSs/cloud clusters. Hydrostatic pressure analysis suggests the upper-level warming alone would not result in the initial sea level pressure drop associated with the transformation from a disturbance to a TC. As Fanapi and Megi intensify into strong tropical storms, aircraft flight-level (700 hPa) and dropsonde data reveal that the warm core extends down to 850–600 hPa and has some characteristics of subsidence warming similar to the eyes of mature TCs.

This paper examines the moist entropy and moisture budgets in tropical cyclones, as well as their relation to tropical cyclone's development. This analysis focuses on the dropsonde data collected during Hurricane and Severe Storm Sentinel project and the accompanying satellite data. Two tropical cyclones of interest are Tropical Storm Gabrielle (2013) and Hurricane Edouard (2014). There were three research flights into Gabrielle (2013), during its nondeveloping and decaying stages. Edouard (2014) was visited four times in different stages of its life cycle, twice during the intensification and twice during the decay. Also, we extended our analysis on the larger data set, consisting of 11 nonintensifying and 12 intensifying systems. Our study shows that the moist entropy tends to increase during intensification and decrease during nonintensifying stages. On the other hand, the moisture budget relates better to the tropical cyclone's current intensity than its development. The sign of the moist entropy tendency depends on the ability of surface fluxes and irreversible moist entropy generation to overcome lateral export of moist entropy and loss due to radiative cooling. Edouard's decay during the last research flight was likely the result of increasing wind shear and low sea surface temperatures. During its decay, Gabrielle had strong column‐integrated lateral export of moist entropy and drying between 1 and 4 km height. This is probably the consequence of a dry environment at multiple levels, amplified by a warm and dry anomaly left behind by previous convective activity.

On the distribution of helicity in the tropical cyclone boundary layer from dropsonde composites

The invaluable meteorological observations for a late autumn tropical cyclone that came very close to Hong Kong, namely, Nalgae in November 2022, are documented in this article. In particular, the dropsonde data for two consecutive days for this late season storm close to Hong Kong are presented. Meteorological data revealed that while Nalgae appeared to be rather weak from the meteorological satellite image under the cool sea surface water and the cool and dry northeast monsoon, it still maintained considerable intensity near the lower-boundary layer and managed to bring gale to storm force 10 min mean winds over many places in Hong Kong, necessitating the issuance of a gale or storm wind signal in November since 1972. The consideration in the issuance of the warning signal in Hong Kong and the difficulty in numerical weather prediction (NWP) model in forecasting Nalgae are also discussed in this article.