A Comparison of Methods for Estimating Vertical Wind from Dropsondes

Measuring vertical wind using dropsondes is useful for studying dynamic and thermodynamic processes in the atmosphere, such as those in tropical cyclones. Vertical wind can be estimated from dropsondes by finding the difference between the dropsonde's measured fall rate and terminal velocity. Determining an accurate dropsonde terminal velocity is challenging since it requires either laboratory experimentation or analyses of dropsonde observations within potentially strong convective motions. Two methods for finding the dropsonde terminal velocity and resulting vertical wind were examined. The first method determines a theoretical terminal velocity based on the dropsonde design, parachute size, and a drag coefficient. The second method estimates the terminal velocity by finding the median fall rate of a large sample of dropsondes with similar designs. The fall rates and vertical winds were calculated from dropsondes released by NOAA and NASA aircraft from 2010 to 2024 using four different dropsonde models. For three of the dropsonde models, the median fall rates were faster than the corresponding theoretical terminal velocity. This difference resulted in the vertical wind estimated using the theoretical terminal velocity having a larger percentage of downward motion than upward motion. In contrast, the vertical wind estimated using the median fall rates had a more symmetric distribution of upward and downward motions. Overall, estimating vertical wind from dropsondes using the median fall rate for the corresponding drop-sonde model is recommended versus using the theoretical terminal velocity. This study focuses on observations in tropical cyclones and their surrounding environments; however, the results can be applied to observations of other atmospheric phenomena.