Tropical Depression

A tropical cyclone that forms over tropical waters and is in the stage with maximum sustained winds below 17 m/s; it is the stage preceding typhoon development.

Tropical Depression

Overview

Tropical Depression (열대 저압부, Tropical Depression, abbreviated TD) is a tropical cyclone in the stage of a cyclonic circulation system that forms over tropical waters with maximum sustained winds below about 17 m/s (34 knots). It corresponds to the early stage before a strong tropical cyclone—called a typhoon, hurricane, or cyclone—matures, and is classified according to the World Meteorological Organization (WMO) scale and the criteria of each Regional Specialized Meteorological Center. Convective clouds become organized and the central pressure falls, but because it is not yet accompanied by storm-force winds, it is often tracked by a number or temporary designation rather than a formal name.

Main Content

Definition and Classification System

Tropical cyclones are divided into several stages according to maximum sustained wind speed. In general, intensity is classified in the order ① tropical depression (TD, below 17 m/s), ② tropical storm (TS, 17–24 m/s), ③ severe tropical storm (STS, 25–32 m/s), and ④ typhoon (TY, 33 m/s or higher). Names and criteria differ slightly by region: in the Northwest Pacific, the Japan Meteorological Agency (JMA) is responsible; in the North Atlantic and eastern Pacific, the U.S. National Hurricane Center (NHC); and in the Indian Ocean and South Pacific, each Regional Specialized Meteorological Center (RSMC) handles monitoring and classification. At the tropical depression stage, an official name is not yet assigned, and it is usually designated as "Tropical Depression No. ○" or by a numerical code.

Formation Conditions and Mechanism

A tropical depression usually forms when conditions such as the following are met. First, sea surface temperatures must remain at about 26.5°C or higher to supply sufficient water vapor and latent heat energy. Second, the latitude must be sufficiently far from the equator (approximately 5–20 degrees) for the Coriolis force to act so that cyclonic rotation can be maintained. Third, vertical shear, the difference in wind speed between the upper and lower levels, must be weak so that convective clouds can develop vertically. Fourth, humidity in the middle atmosphere must be high and low-level convergence must be smooth.

When these conditions are met, a tropical wave, monsoon depression, or sometimes a cyclonic vortex shed from a front gathers convective clouds, and as latent heat is released the central pressure falls, starting a self-reinforcing feedback. This stage is the tropical depression; if conditions continue to be favorable afterward, it develops into a tropical storm and receives a name.

Structure and Characteristics

In a tropical depression, strong convective cells gather around the center, but an eye or a distinct eyewall structure has often not yet formed. In satellite imagery, it appears as a bright, thick cloud mass (central dense overcast, CDO) only partially organized, and the center position is blurred and difficult to track. The central pressure often remains roughly in the 1000–1008 hPa range, but varies greatly depending on the rate of development.

Observation and Forecasting

Because a tropical depression is relatively weak compared with a strong typhoon, precise observation is difficult. Infrared and water vapor imagery from geostationary and polar-orbiting satellites, microwave sensors, and scatterometer observations are key means, while aircraft reconnaissance (such as the Hurricane Hunters), buoys, and ship observations at sea are used as supplementary data. In forecasting, ensemble results from various numerical models are compared to judge the possibility of development and the track, and uncertainty is greater in the earlier stages.

Impacts and Damage

Although the tropical depression itself has weak strong winds, because it moves while carrying large amounts of water vapor, it can bring torrential rain, squalls, and high waves at sea. Indeed, there have been many cases in which a tropical depression that failed to develop into a typhoon made landfall and caused record heavy rain, landslides, and urban flooding. In addition, whether a tropical depression will develop into a typhoon is the starting point for disaster response, so meteorological agencies in each country strengthen monitoring and information issuance from this stage.

Recent Trends

In 2024–2025, satellite observations and AI-based forecasting models are being actively introduced for tropical depression monitoring. As AI models from the European Centre for Medium-Range Weather Forecasts (ECMWF) and graph-based neural network models from Google DeepMind show track forecasting performance similar to or better than existing numerical models, they are contributing to improving the accuracy of distinguishing early development stages.

In addition, analysis continues that as sea surface temperatures rise due to climate change, cases of tropical depressions rapidly intensifying into strong typhoons are increasing. Accordingly, monitoring of "Rapid Intensification" has been strengthened, and some agencies are expanding systems that issue early warnings from the tropical depression stage. Around the Korean Peninsula as well, as cases of tropical depressions moving north and causing localized heavy rain are reported, the trend is to treat them not merely as a "weak stage" but as an independent hazard.