| 12 Nov 2025
Relationships between Thin clouds, Opaque Clouds, and the Tropical Easterly Jet over the Indian Region observed with Aeolus Spaceborne Doppler Wind Lidar
Abstract. Direct wind observations used to be rare over the Arabian Sea and the Bay of Bengal. Since the launch of ESA's Aeolus Doppler Wind Lidar, profiles of horizontal wind are acquired every day and are perfectly co-located with profiles of thin and opaque clouds. In this study, we show that from June to October 2020, during the South-Asian Summer Monsoon (SASM), high altitude clouds formed over the Bay of Bengal by deep convection in the afternoon are advected westward towards the Arabian Sea in the morning by the fast winds of the Tropical Easterly Jet (TEJ). Consequently, the thin high cloud cover over the Arabian Sea is 14 % at 06 LT on days where westward winds are faster than 23.6 m s-1 between 14 and 17 km of altitude, more than twice as much than on days where westward winds are slower than 23.6 m s-1 (6 %). While the TEJ is primarily driven by the thermal contrast between warm land and cooler Indian Ocean, we observe that the diverging-rotating outflow around deep convective (opaque) clouds can strengthen or weaken the TEJ over the Arabian Sea with respect to the thermal wind, explaining a 3 m s-1 amplitude variation during the SASM. These results suggest that the cirrus cloud cover over the Arabian Sea may decrease in the next decades, as the TEJ and the convection over the Eastern Indian Ocean are expected to decrease in intensity.
General comment:
The manuscript by Titus et al. investigates the relationship between thin clouds, deep convective clouds and upper tropospheric wind structures over the Indian monsoon region using a synergistic cloud and wind profile dataset from Aeolus during the summer monsoon 2020 time period. They conclude that a higher occurrence of thin clouds over the Arabian Sea is associated with stronger westerly winds combined with higher amount of water vapor injected in the upper troposphere by deep convection over India and the Bay of Bengal.
They further show that the strength of the TEJ is mainly consistent with thermal-wind balance, but that the short-term regional variability of the TEJ is modulated by the outflow and divergence, vorticity perturbations caused by deep convection.
While Aeolus’s repeat cycle is usually 7-days, the authors benefit from the orbit configuration to have 12 hours between an ascending and descending orbit with spatial distances of 1150 km, which provides them the opportunity to study advection processes. This is unique for a single observing system.
The dataset is valuable and the topic is very relevant. The study addresses several interesting scientific implications, which are explained in detail. My only concerns are regarding the structural clarify in sections 3-5 plus a few minor comments.
Detailed comments:
The manuscript is clear and well written, the abstract, introduction and conclusion read very well and communicate the motivation and relevance of the study. However, I have several concerns regarding the structure.
While section 2 (Dataset and method) is very descriptive and easy to follow, sections 3, 4 and 5 present a mixture of observational findings (what does the data show us directly without interpretation), interpretations from these findings and hypothesis. Also, the subsection “method” re-appears in section 5 for the individual hypothesis about deep convection and the strength of the TEJ. This causes that the narrative reads as a sequence of results, observations, impacts and hypothesis with no clear logical hierarchy. I recommend separating the observational results, interpretations, discussion and conclusions, either globally, or if you like to keep the hypothesis-driven sectioning, also per section.
Section 5: Deep convection strengthens … This is more like a conclusion or hypothesis. Maybe rephrase title to “Impact of deep convection on TEJ strength over Arabian Sea”?
Section 4: I actually like the idea to already provide the conclusions with the title, but later, with 4.2.1 you refer to the role of temperature and 4.2.2. the role of opaque clouds. Maybe better call section 4: “Drivers affecting the occurrence of thin high clouds over the Arabian Sea” Then you go to 4.1. wind, 4.2. temperature and 4.3. opaque high clouds over India ? This is just a suggestion to make the order clearer.
The abstract is clear and well written, but you have some room for further details. You mention the Aeolus dataset, some results, numbers and implications, which is a nice structure for the abstract. One sentence about the methodology (disentangling the effects) would make it rounder.
The Sub-grid scale variability below the native resolution of Aeolus is not resolved by the merged and re-gridded cloud-wind dataset. You should add some sentence that this high-resolution dataset nicely compares to the resolution of CALIOP, but does not provide higher level of details compared to the native Aeolus resolution, which brings some uncertainties, especially with respect to the cloud top levels.
The manuscript reports convective perturbation wind components (u_conv) on the order of −1.7 to +1.3 m/s at the end of Section 5. However, no uncertainty estimate is provided for these values, nor is their magnitude discussed relative to the wind measurement uncertainty and derived thermal-wind uncertainty described in Titus et al. (2025). Without this context, it is difficult to assess whether the reported perturbations are robust and statistically distinguishable from dataset uncertainty. I recommend providing uncertainty estimates or at least add a reference of typical random errors of Aeolus Mie-cloudy wind and to compare them to the observed amplitude.
Minor technical comments: