Convective Asymmetries Measured by Dropsondes in Tropical Cyclones

It has been well established that strong updrafts and downdrafts (UDs) in tropical cyclones (TCs) are rare. While most vertical velocities in TCs are relatively weak, there have been documented cases of moderate UDs (magnitude ≥ 5 m s−1), strong UDs (magnitude ≥ 8 m s−1), and extreme UDs (magnitude ≥ 10 m s−1).

The newly developed eXpendable Digital Dropsonde (XDD) allows for higher spatial and temporal resolution of the inner core of TCs. In 2015, a total of 590 XDD sondes were launched into hurricanes Marty (27 – 28 September), Joaquin (2 – 5 October), and Patricia (20 – 23 October). Here, we use these data to calculate vertical velocity within 1500 km of the TC centers for individual storms.

A total of 252, 166, and 124 sondes experienced vertical velocities ≥ +5 m s−1, +8 m s−1, and +10 m s−1 respectively. There was an evident relationship between the number of updraft data points inside the radius of maximum wind (RMW) and the intensity changes of the hurricanes analyzed. UDs tended to occur in the downshear-left quadrant, while there was a clear suppression of convection in the upshear-right quadrant. UDs increased in frequency up to an R* (the RMW normalized radius) of 4 and decreased in frequency after. Individually, there were three favored radii for UDs at 30 km, near 125 km, and near 800 km. We observed that extreme updrafts were located mainly in the upper and lower levels of the TC, while extreme downdrafts occurred almost exclusively in the upper levels of the TC. Abnormally strong UDs were found in all three storms regardless of TC strength.