Adverse impact of terrain steepness on thermally-driven initiation of orographic convection
Abstract. Diurnal mountain winds precondition the environment for deep moist convection through horizontal and vertical transport of heat and moisture. They also play a key role in convection initiation, especially in strongly inhibited environments, by lifting air parcels above the level of free convection. Despite its relevance, the impact of these thermally-driven circulations on convection initiation has yet to be examined systematically. Using idealized large-eddy simulations with the WRF model, we study the effect of cross-valley circulations on convection initiation under synoptically undisturbed and convectively inhibited conditions, considering quasi-2D mountain ranges of different heights and widths. In particular, we contrast convection initiation over relatively steep mountains (20 % average slope) and moderately steep ones (10 %). One distinctive finding is that, under identical environmental conditions, relatively steep mountain ranges lead to a delayed onset and lower intensity of deep moist convection, although they cause stronger thermal updrafts at ridge tops. This finding cannot be explained considering the temporal evolution of convective indices, such as convective inhibition and convective available potential energy. Analysis of the ridgetop moisture budget reveals the competing effects of moisture advection by the mean thermally-driven circulation and turbulent moisture transport. In general, at mountaintops, the divergence of the turbulent moisture flux offsets the convergence of the advective moisture flux almost entirely. The weaker total moistening found over steep mountains can be explained by considering that buoyant updrafts over their ridgetops are on average relatively narrow. Thus, they are more strongly affected by the turbulent entrainment of environmental air, which depletes their moisture and cloud water content and makes them less effective at initiating deep convection. In our simulations, convective updrafts over moderately steep mountains, on the other hand, gain more moisture from the vapor flux at cloud base and lose less moisture due to horizontal vapor fluxes over the course of the day, leading to significantly higher moisture accumulation. The precipitation efficiency, a measure of how much of the condensed water eventually precipitates, is also considerably larger over the moderately steep mountains. The weaker convection over steep mountains is a robust finding, valid over a range of background environmental stratifications and mountain sizes.
Matthias Göbel et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-648', Anonymous Referee #1, 05 May 2023
- RC2: 'Comment on egusphere-2023-648', Anonymous Referee #2, 15 May 2023
Matthias Göbel et al.
Data and code for "Adverse impact of terrain steepness on thermally-driven initiation of orographic convection" https://doi.org/10.5281/zenodo.7794166
Matthias Göbel et al.
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This manuscript uses idealized simulations to describe and quantify the sensitivity of orographic moist convection to terrain steepness. Experiments are performed with two different mountain heights and two different terrain slopes, allowing the two effects to be roughly separated. The simulations suggest that wider mountains generate thermal circulations that are more successful at accumulating moisture in the subcloud dry updraft, and that the moist convection over wider mountains more efficiently converts condensate into precipitation.
I found this manuscript to be well written and interesting, with precise analyses that help to gain insight into the problem. Therefore, I think it will provide a useful addition to the literature. However, I have some concerns about some of the analyses and associated interpretation, particularly regarding the cloud-layer budget analysis, the distinctions between subcloud and cloud-layer convection, and the resolvability of the orographic clouds. Therefore, I have comments that would be classified as “major”, which I hope the authors will address in their revision.
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