Estimation of daytime planetary boundary layer height (PBLH) over the tropics and subtropics using COSMIC-2/FORMOSAT-7 GNSS–RO measurements

Abstract

The advantage provided by the unprecedented data density and deeper penetration of Global Navigation Satellite System (GNSS) Radio Occultation (RO) based refractivity profiles from COSMIC-2/FORMOSAT-7 mission over the tropical and subtropical regions is leveraged to get a seasonal picture of daytime planetary boundary layer height (PBLH) distribution from 2 years of data (from October 2019 to September 2021) using the traditional method and a new approach proposed in this study. Traditional PBLH retrieval relies on the minimum refractivity gradient (MRG). In the new method, preference is given to the lowest significant peak in refractivity gradient (if present) rather than the MRG alone for determining PBLH. Comparison with the MRG method shows the highest decrease in seasonal mean PBLH over the oceanic ITCZ regions (by 500–700 m) which is in line with studies reporting low PBLH from in situ observations. The lowest decrease was observed over subtropical oceanic subsidence regions with marine stratocumulus clouds (MSC region; ~100–150 m) where the MRG method was thoroughly validated. The seasonal scale PBLH uncertainty (standard deviation) decreased by ~300 m over the ITCZ regions in the proposed method; it partially increased over the offshore MSC regions by ~100 m indicating detection of infrequent cases of marine boundary layer decoupled from stratocumulus clouds aloft which is not possible in the traditional method. The overestimation and higher uncertainty in PBLH derived from the traditional method over deep convective regions are attributed to the erroneous detection of seasonally varying cloud tops, subsidence base, and elevated water vapour layers as PBL top. An eastward shift by more than 5o longitude is observed in the location of PBLH maxima (seasonal mean) over the southeastern Pacific Ocean in the proposed method (with respect to the traditional method) for all seasons. The proposed method shows a better comparison with daytime PBLH from ERA5 reanalyses over the oceans but there is a partial overestimation of PBLH by ERA5 over continental regions in the summer hemisphere.