| 12 Aug 2025
The potential of GNSS radio occultation data for the analysis of the tropical width: a comparison with reanalyses
Abstract. The tropics are expanding poleward as a result of anthropogenic climate change. This in turn has great implications on the temperature and precipitation patterns in the subtropical regions. Previous studies have found varying widening trends, most of which have been derived using reanalysis and climate model data. These trend discrepancies underline the need for studies using alternative datasets. Here, we explore the potential of GNSS radio occultation (RO) data for analyzing the tropical width as an independent observational source of information with key characteristics: high accuracy, global availability, and long-term consistency. We evaluate the skill of RO temperature and newly established RO wind records to accurately capture tropical width features, using tropopause break and jet stream metrics. The results are compared to three state-of-the-art reanalysis datasets (i.e., ERA5, MERRA-2, and JRA-3Q). Zonal-mean patterns and the regional structure of tropical width features are investigated to test the utility of RO in respect to its spatial robustness. Furthermore, we provide a perspective on the necessary record length for reliable trend estimation of the tropical width. Comparisons of RO to reanalyses show overall high agreement of the zonal-mean values. As for the zonally resolved metrics, results from reanalyses and RO align well with exceptions over the northern hemisphere. While the RO record length is still a bit too short for detecting tropical width trends, the results are encouraging and confirm that RO is a valuable alternative observation-based dataset, with increasing relevance towards the future.
The authors leverage 15 years of GNSS radio occultation (RO) temperature profile data to examine the width of the tropics and its change over time. They compare the resulting diagnoses to longer records from several state-of-the-art reanalyses. Ultimately, it is demonstrated that RO data provide useful characterizations that broadly agree with the reanalysis diagnoses, especially upper troposphere lower stratosphere metrics where RO data are complete and most reliable. While I find the study to be mostly well-constructed and detailed, there are a few aspects that require a bit more clarification which I outline below.
General Comments
There are many instances of “on the NH” or “on the SH” that should all be revised to “in the NH” or “in the SH”.
Specific Comments
Line 7: the opening sentence of the abstract is a too strong of a statement. As the authors acknowledge later, this result is contingent upon the metric used. The language should be softened here.
Line 29: delete unnecessary period after “include”
Line 32: “systems” should be “system”
Lines 71-75: these sentences are entirely unnecessary
Line 85: a brief discussion of the wind retrieval is warranted here. It is later stated that the wind isn’t a simple geostrophic retrieval, so what is it?
Lines 92-93: is a ±2 day Gaussian time-weighting approach appropriate? This could be better justified/explained.
Line 250: “extend” should be “extent”
Line 257: “then” should be “than”
Line 311-312: there are a few studies that have revealed this narrowing via tropopause break metrics – Martin et al. 2020, https://doi.org/10.1175/JCLI-D-19-0629.1; Zou et al. 2023, https://doi.org/10.3389/feart.2023.1177502; Turhal et al. 2024, https://doi.org/10.5194/acp-24-13653-2024
Line 330: “this a globally” should be “this globally”
Line 335: delete unnecessary comma after “(Nimac et al., 2025a)”