Tropical Cyclone Planetary Boundary Layer Heights Derived from GPS Radio Occultation over the Western Pacific Ocean

According to GPS radio occultation data from previous studies, the height of the planetary boundary layer (PBLH) is defined as the altitude at which the vertical gradient of refractivity N is at its local minimum, called the gradient approach. As with its density, the atmosphere's refractivity falls broadly exponentially with height. The spherically symmetric refractivity N-ss(r) was established to account for the standard deviation of atmospheric refractivity with altitude. N-i is the residual from the fundamental vertical variations of refractivity, defined as N-i(r) = N(r) - N-ss(r). In this study, the vertical gradient of N is replaced by the vertical gradient of N-i to optimize the gradient approach, called the local gradient approach. Using the US radiosonde and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultations (ROs) data from 2007-2011, these two PBLH-determining approaches are evaluated. The PBLHs estimated by the gradient approach and the local gradient approach have RMSE values of 0.73 km and 0.65 km, respectively. The PBLH obtained by the local gradient approach is closer to the radiosonde-derived value. In this paper, the COSMIC-2 ROs data and the western Pacific typhoon best track data are collocated in time and space during 2020-2021, and the axisymmetric composite structural characteristics of the tropical cyclone (TC) PBLs are analyzed. The lowest vertical gradients of N and N-i of TCs correspond closely with the average PBLHs. We find that the mean PBLHs of tropical depressions (TD), tropical storms (TS), and typhoons (TY) all have their local maxima at a radial distance of 125 km with heights of 1.03 km, 1.12 km, and 1.36 km, respectively. After 375 km, 575 km, and 935 km of TD, TS, and TY radial distances, the mean PBLHs become stable and cease to vary. The mean PBLH undulations increase significantly with the increase in tropical cyclone intensity. N-iwet is the residual from the fundamental vertical variations of wet refractivity, defined as N-iwet(r) = N-wet(r) - N-sswet(r). Local minima of N-iwet and N-i vertical gradients of TD, TS, and TY have comparable distributions and are concentrated between 0.5 km and 1 km.

Automated summary

According to GPS radio occultation data, two approaches for determining the height of the planetary boundary layer (PBLH) were evaluated in this study. The results show that the PBLH estimated using the local gradient approach is closer to the actual value. Additionally, the analysis of tropical cyclones reveals that the mean PBLH increases with the increase in cyclone intensity.