the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Sep 2026
High altitude precipitation dynamics using long-term observations in a Himalayan catchment
Abstract. In the Himalayas, the interaction between climate, orography and land cover results in a highly heterogeneous precipitation pattern. Mountain precipitation, being the main driver of the water cycle, ultimately provides water for millions of people downstream. Our current understanding of precipitation patterns within the Himalaya region is mainly based on past modeling efforts and data ensemble products. Yet due to computational limitations these products either cover only a limited time period or are unable to capture fine-scale precipitation patterns due to their resolution. Furthermore, the lack of long-term meteorological observations due to the remoteness and inaccessibility of the terrain results in poor validation of these products, especially above 3000 m a.s.l. We address this knowledge gap by synthesizing thirteen years (2012–2025) of meteorological field data that spans an altitude range between 1406 and 5350 m a.s.l. in the monsoon-dominated Langtang Valley, Nepal. We show that: (i) the diurnal precipitation cycle is bimodal during the monsoon, with an afternoon and a nocturnal peak, but unimodal during the pre-monsoon; (ii) the relative amplitude of the afternoon (nocturnal) peak increases (decreases) linearly with elevation, such that the monsoon precipitation–elevation gradient reverses diurnally from positive in the afternoon to negative at night; and (iii) the nocturnal precipitation, which dominates the monsoon totals, is highly sensitive to valley topography: under the near-saturated conditions prevailing at night, condensation rates scale directly with terrain-forced ascent, concentrating the nocturnal maximum at the steep, confined valley entrance. This elevation gradient of the diurnal cycle offers a simple validation tool for testing whether models capture the topography–circulation interaction, and the observational record provides a unique reference dataset for studying valley-scale precipitation dynamics in this region.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-5005', Anonymous Referee #1, 23 Oct 2026
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RC2: 'Comment on egusphere-2026-5005', Bodo Bookhagen, 02 Nov 2026
Review of "High Altitude precipitation dynamics using long-term observations in a Himalaya catchment"
This manuscript presents a valuable, high-resolution multi-year dataset (2012–2025) synthesized from an extensive network of meteorological stations across an altitudinal gradient (1,406 to 5,350 m asl) in the Langtang Valley, Nepal. The observation and separation of rainfall versus snowfall is an interesting and valuable addition. The authors use their time series data to highlight some core findings. But there are certainly several other (important) analyses that can be done with such data. It is important to make these data available to the community, and I strongly support the authors' effort.
The manuscript contains several good-quality and useful figures. I am pleased to see that the authors will make their data available (it is not yet accessible). Will the published data associated with this manuscript be in hourly time steps or just the seasonal aggregations?
Overall, this is an important contribution to understanding high-elevation climatic settings, but I suggest considering the following comments:
Title:
The title lacks sufficient geographic specificity (the Himalaya span more than 2,000 km in an east–west direction). Because this manuscript presents data from a specific valley in the central Nepalese Himalaya, I suggest specifying this in the title rather than using the generic phrase "in a Himalayan catchment," although it is technically correct.Abstract:
The abstract implicitly assumes that every reader is familiar with the distinction between rainfall (liquid precipitation) and total precipitation (solid and liquid, or rain and snow). Because this distinction is central and represents one of the more unique qualities of this dataset, I suggest clarifying it directly in the abstract. While the authors employ a temperature-based separation of liquid and solid precipitation, this provides crucial additional data. If the authors wish to focus primarily on the rainfall aspect of their data, they should state this explicitly in the abstract rather than broadly referring to "precipitation." Furthermore, the term "precipitation-elevation gradient" is essentially a rainfall-elevation gradient; snowfall scales very differently with elevation than rainfall, and it is worth considering whether the authors want to make that distinction explicit.Figure 5 and Snow vs. Rain Fraction:
Along these lines, the authors present their precipitation data in Figure 5. Is there a measured fraction or breakdown of snow versus rainfall along the elevation gradient? I assume that the station at 1,406 m receives exclusively rainfall, and that the station at 5,090 m also receives some (?) rain, but the current figure does not allow the reader to separate out these processes. Making this distinction clearer would be a compelling addition and highlights a unique aspect of the presented data. For instance, is the largest fraction of precipitation at 5,090 m composed of rain or snow during July and August?While I have no concerns about the analysis separating snow and rain, I am wondering if changing the threshold T for separation would have an impact of the liquid vs. solid precipitation. I see this as a sensitivity analysis. But maybe the authors have done this already and have a solid reason and justification for relying on a single threshold (despite changing rh/wet bulb temperature effects on rain/snowfall).
Data Analysis and Hourly Gradients:
The analysis of the seasonal temperature and rainfall cycle is useful, but it does not reveal entirely novel results; an analysis of ERA5-land T2m data would likely yield broadly similar patterns. What cannot be replicated with other datasets is the analysis of hourly gradients. I am not necessarily suggesting a new analysis, but I encourage the authors to consider more creative and engaging ways to present data from such a unique high-elevation station network. For example, analyzing the rate of warming during morning hours (local time) or the rate of cooling during evening hours (and examining other meteorological parameters similarly) could yield valuable insights. Comparing these dT/dt rates across different elevations would greatly help in understanding the physical processes driving local meteorological conditions.Additionally, are the time series data reported in GMT or Nepali Standard Time? I reread the manuscript and may have missed it, but could not find a clear indication of timing. While timing is less critical for seasonal plots, I assume the diurnal cycle and afternoon peak refer to local time rather than GMT. A more explicit statement regarding the time zone used throughout the manuscript would be very helpful. Similarly, what local time does the timing of the nocturnal peak (0–1 AM) refer to?
Lapse Rate Terminology:
While the station data are extremely valuable and provide support for the study's conclusions, I recommend exercising caution regarding the use of the term "lapse rate." Strictly defined, a lapse rate describes temperature change along the vertical atmospheric gradient rather than across horizontal distances. Although installing stations along a purely vertical profile is often impractical, horizontal temperature gradients derived from stations with spatial separation should be interpreted with care, particularly when comparing them against studies with differing station spacings. I suggest including a brief acknowledgment of this limitation and discussing the horizontal gradient results with appropriate care.Specific Line Comment:
Line 139: It is true that a local, spot measurement is higher than what a ~5 x 5 km grid-averaged rainfall value yields. However, that discrepancy is expected given the difference in spatial scales. Translating this into an absolute water volume makes the comparison much more meaningful; 100 mm/h distributed over a 5 x 5 km2 area generates a massive volume of water that must eventually be routed and discharged through the catchment.Bodo Bookhagen
Citation: https://doi.org/10.5194/egusphere-2026-5005-RC2
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General comments:
This study presents valuable findings from an analysis of ground-based observations collected at multiple sites in the Langtang Valley of the Nepalese Himalayas over a 13-year period. I believe these are important findings based on highly original and valuable data. The seasonal characteristics of precipitation during the post-monsoon, winter, pre-monsoon, and monsoon periods at multiple sites are clearly presented. However, throughout the manuscript, the authors present several physical interpretations as established facts, although the observational results do not directly support them. The authors should clearly distinguish observational findings from possible physical explanations and provide appropriate references where necessary. The authors should also clarify how their findings relate to previous studies and better highlight the novel contributions of this study. The value of this study lies primarily in its long-term, multi-site observations, and the physical interpretations should be presented with appropriate caution. Specific comments are provided below.
Major comments:
1. Title and Line 68: “Precipitation dynamics”
What exactly do the authors mean by “precipitation dynamics”? The term suggests an investigation of the physical processes governing precipitation. However, the present study primarily describes the observed characteristics of precipitation, rather than directly examining the underlying physical mechanisms. Investigating precipitation mechanisms using surface observations alone is challenging. The authors should reconsider whether the term “precipitation dynamics” appropriately reflects the scope of this study.
2. Introduction:
The findings of previous studies should be described in more detail. The fundamental characteristics of the diurnal cycle can be captured even with relatively short observational records. For example, Shea et al. (2015) compared meteorological conditions, including diurnal cycles throughout the year, among different high-altitude glacierized catchments in Nepal using in situ observations. Although their study was based on approximately one year of data, they reported findings similar to those presented in the current manuscript. Ueno et al. (2008) also reported seasonal variations in the diurnal cycle of surface winds in the Nepal Himalaya. The authors should clarify how the 13-year observational record confirms or extends previous findings and what new insights it provides. This distinction should be maintained throughout the manuscript.
Reference:
Ueno, K., K. Toyotsu, L. Bertolani, and G. Tartari, 2008: Stepwise onset of monsoon weather observed in the Nepal Himalaya. Mon. Wea. Rev., 136, 2507–2522.
3. Lines 3–4 and 40–43:
What specific aspects of precipitation patterns remain poorly understood? Do these statements refer to precipitation patterns at high elevations in the Nepalese Himalayas, or to those across the Nepalese Himalayas as a whole? The answer also depends on the spatial and temporal scales considered. Previous studies have investigated spatial precipitation patterns using rain gauge observations from the Department of Hydrology and Meteorology (DHM) of Nepal. Furthermore, spaceborne precipitation radars, such as TRMM PR, have provided detailed information on climatological precipitation patterns with a horizontal resolution of approximately 5 km (Bookhagen and Burbank 2006; Shrestha et al. 2012; Fujinami et al. 2021). The authors should clearly identify the specific gaps in our understanding of precipitation patterns, including the relevant spatial and temporal scales, and explain how the present observations address these gaps. They should also discuss the limitations of existing ground-based observations, numerical models, and satellite measurements to better justify the need for the in situ observations presented in this study.
Reference:
Shrestha, D., P. Singh, and K. Nakamura, 2012: Spatiotemporal variation of rainfall over the central Himalayan region revealed by TRMM Precipitation Radar. J. Geophys. Res., 117, D22106.
4. Line 34 and throughout the manuscript: “Monsoonal jet”
The authors frequently use the term “monsoonal jet.” Does this refer to the low-level winds blowing toward the Himalayas during the monsoon season? Because an easterly jet stream also exists in the upper troposphere during the monsoon season, the authors should explicitly specify the atmospheric level they are referring to. Furthermore, the low-level monsoon winds over land do not necessarily exhibit a jet-like structure, although such a structure may develop when the winds strengthen at night. If the authors are referring to the moist low-level winds associated with the large-scale monsoon circulation, “low-level monsoon flow” would be a more appropriate term.
5. Lines 87–91: Synoptic-scale disturbances during the monsoon and post-monsoon seasons
During the summer monsoon season, monsoon lows and monsoon depressions frequently develop over the Bay of Bengal and surrounding regions. These low-pressure systems (LPSs) can substantially modulate the low-level monsoon flow and moisture transport toward the Himalayas, influencing precipitation over the region (Bohlinger et al. 2017; Hirata et al. 2023). In describing the general atmospheric conditions associated with Himalayan precipitation, the authors should also mention the role of monsoon LPSs, in addition to cyclonic storms during the post-monsoon season (Yanase et al. 2012). The description should be revised based on relevant previous studies to provide a more accurate understanding of the synoptic-scale disturbances affecting the region.
References:
Bohlinger, P., A. Sorteberg, and H. Sodemann, 2017: Synoptic conditions and moisture sources actuating extreme precipitation in Nepal. J. Geophys. Res. Atmos., 122, 12,653–12,671.
Hirata, H., H. Fujinami, H. Kanamori, Y. Sato, M. Kato, R. B. Kayastha, M. L. Shrestha, and K. Fujita, 2023: Multiscale processes leading to heavy precipitation in the eastern Nepal Himalayas. J. Hydrometeor., 24, 641–658.
Yanase, W., M. Satoh, H. Taniguchi, and H. Fujinami, 2012: Seasonal and intraseasonal modulation of tropical cyclogenesis environment over the Bay of Bengal during the extended summer monsoon. J. Climate, 25, 2914–2930.
6. Line 114: hourly totals
Please clarify the accumulation period corresponding to each timestamp of the hourly precipitation data. For example, does the precipitation recorded at 12:00 represent the accumulation during the preceding hour or the following hour?
7. Lines 227 and 231: “Large diurnal temperature ranges” (Figs. 3a and 3d)
Does Fig. 3 provide information to assess the diurnal temperature range? What do the upper and lower boundaries of the shaded areas in Fig. 3 represent?
8. Line 234: “Driven by the monsoonal jet”
This description requires some caution, as it gives the impression that moist low-level monsoon air simply flows from the plains and rises over the Himalayas. Although moisture advection by the low-level monsoon flow is an important factor, the observed water vapor mixing ratio reflects the combined effects of various processes, including local moisture transport associated with daytime anabatic winds, condensation and precipitation processes, and changes in the large-scale circulation associated with monsoon LPSs. The authors should avoid attributing the observed variations solely to the monsoon flow without further evidence.
9. Line 241: “Due to the absence of monsoonal jet”
The term “monsoonal jet” is not appropriate here either, as noted in Comment 4. Although the summer monsoon circulation weakens or retreats during winter, a winter monsoon circulation also exists over South Asia. Therefore, the absence of a “monsoonal jet” does not adequately describe the winter atmospheric conditions. The authors should clarify which aspect of the seasonal change in the low-level circulation they are referring to.
10. Lines 255–256: “The south-westerly directed monsoon flow dominates the prevailing wind direction.”
What exactly does this sentence refer to? Does it describe the prevailing surface wind direction along the valley during July–August, or the large-scale monsoon flow? If the authors are referring to the observed along-valley surface winds, their direction cannot be simply attributed to the large-scale monsoon flow. Thermally induced pressure gradients associated with the valley topography play an important role in driving along-valley winds (Potter et al. 2018). The authors should clarify the distinction between the large-scale monsoon flow and local valley circulations.
11. Section 4.2: Diurnal setting
Given the availability of 13 years of observations, it would be valuable to examine the interannual variability of the diurnal cycles, particularly for precipitation. Are the observed differences in the diurnal precipitation cycle among elevations consistent from year to year? Even a simple presentation of the year-to-year variability would help demonstrate the robustness of the main findings and highlight the value of the long-term observations. Interannual variation in temperature at higher elevations is also important information.
12. Lines 289–290: “Moisture advected into the valley by the monsoonal jet is forced by anabatic winds.”
This statement appears to be based on a conceptual understanding of monsoon and valley circulations rather than on the results presented in this study. Please provide appropriate references to support this interpretation or clarify that it is a possible explanation. In addition, the expression “moisture is forced by anabatic winds” is unclear. If the authors mean that moist air is lifted by anabatic winds, this should be stated more explicitly. As mentioned in Comment 4, “low-level monsoon flow” would be more appropriate than “monsoonal jet.”
13. Lines 291–293: “Convective-type precipitation in the afternoon”
The interpretation that afternoon precipitation is convective is plausible, but the results presented in this study do not directly support this classification. The authors should provide appropriate references supporting this interpretation (e.g., Fujinami et al. 2021) and clarify that the precipitation type has not been directly identified from the present observations.
14. Lines 295–296: “Leading to the nocturnal precipitation”
The results presented in this study do not establish a causal relationship between the observed diurnal variations in meteorological variables and nocturnal precipitation. Therefore, “associated with” would be more appropriate than “leading to.”
15. Lines 296–297: “Katabatic flow”
Isn't katabatic flow less evident during the monsoon season, as shown in Figs. 4a–c? If so, how does this explain the reduced precipitation in the morning?
16. Line 310:
The presented results are insufficient to conclude that daytime precipitation during the pre-monsoon season is convective in nature.
17. Section 5.2: Monsoon nocturnal precipitation
The possible contribution of nocturnal surface radiative cooling to precipitation enhancement is an interesting hypothesis. Although surface cooling may promote saturation and the formation of fog or low-level clouds, it is not clear how this would lead to the observed precipitation, particularly in terms of the amount of rainfall. The authors are encouraged to discuss the limitations of this interpretation and what types of additional observations or analyses would be necessary to evaluate this hypothesis.
18. Conclusions
The Conclusions should be revised in light of the comments above. In particular, the authors should clearly distinguish findings that are consistent with previous studies based on shorter observational records from new findings obtained from their 13-year observations. The unique contributions of the long-term, multi-site observations should be highlighted, while physical interpretations that are not directly supported by the results should be presented with appropriate caution.
Minor comments:
1. Line 46: “(pre-)monsoon”
Does “(pre-)monsoon” refer to both the pre-monsoon and monsoon seasons? Please clarify.
2. Line 63
A meteorological observation network has also been established in the Rolwaling Valley (Hirata et al. 2023).
3. Line 87:
Indian monsoon -> Indian summer monsoon
4. Lines 90–91: “During these events, 3–6 day synoptic scale low pressure systems, developed over and around the Bay of Bengal are diverted towards the Himalayas”.
These sentences describe monsoon low-pressure systems (LPSs) in summer, rather than tropical cyclones in post-monsoon season. Please revise the description accordingly.
5. Line 320: “no statistically significant”
Does this paper include a figure or information showing statistical significance for diurnal cycles in precipitation?