Parametrizations of subgrid-scale mountains are commonly used in numerical weather prediction and climate models. They try to represent quite separate processes; namely, the enhancement of the turbulent drag by orography, gravity-wave drag, and the effects of low-level flow blocking. Among the gravity wave schemes, some of them distinguish between upward-propagating waves and trapped lee waves. This article makes use of a recent theoretical methodology to propose a formalism that includes all these effects. This theory handles enhanced turbulent drag in the neutral case, gravity waves in the stratified case, and trapped lee waves in the transition. Mountain drag associated with all these processes is estimated analytically, as well as the fraction of the drag that stays within the boundary layer instead of being radiated in the far field. Although the theory used is adapted to gentle hills with small slope, we also try to evaluate the blocked layer depth by combining the sheltering effects that dominate when stratification is small and the blocking effects when stratification is strong.
This is the second part of an investigation of water vapor in the Martian atmosphere using solar occultation observations by the Nadir and Occultation for MArs Discovery (NOMAD) spectrometer on board the ExoMars Trace Gas Orbiter. Following the analysis of six Earth years of NOMAD observations performed in the first part, a cross-validation between NOMAD and ACS results is presented, showing global as well as profile-by-profile comparisons. The results reveal an overall good agreement between different teams and instruments, taking into account the different retrieval methodologies. In order to compare with model predictions, we perform an exhaustive analysis of the water vapor simulated by Mars Planetary Climate Model (MPCM). It shows that the MPCM reproduces most of the water vapor climatological features observed in the atmosphere. However, several discrepancies between model and observations are noticed. Some of these are possibly related to the vertical distribution of dust and its effect on the global circulation and on the water vapor vertical transport. Other data-model differences found at 60 km seem to be related to discrepancies on the water ice cloud formation in the MPCM. We include a cluster analysis of Martian water vapor vertical profiles for the first time. This technique applied to MPCM and NOMAD water vapor retrievals reveal distinct groups of profiles being representative of specific seasons and latitudinal regions, similarly distributed in both model and observations. Moreover, it allows us to provide a simplified water vapor climatology, useful to detect out-of-season events and biases in the retrieval processes.
We present vertical profiles of water vapor obtained during six continuous years of solar occultation observations in the infrared by the Nadir and Occultation for MArs Discovery (NOMAD) instrument on board Trace Gas Orbiter. The retrievals have been performed with an inversion code previously applied to smaller samples of this data set, but improved to combine pairs of diffraction orders allowing for sounding water vapor up to about 120 km altitude. As a first part of a set of two papers, this study presents the most extended data set of water vapor measurements from the NOMAD instrument to date, covering three full and consecutive Martian Years. Building upon previous researches primarily focused on the perihelion season, this analysis now includes the aphelion season, offering a comprehensive view of Mars' water cycle. Observations from April 2018 to December 2023 were analyzed, covering perihelion of Mars Year (MY) 34 to aphelion of MY 37 and presenting water vapor vertical profiles from approximately 5-10 km to 110-120 km in altitude. This study reveals consistent seasonal and latitudinal water vapor patterns, showing water vapor systematically more vertically extended during the perihelion season than during the aphelion. We present an extensive analysis of the water vapor local time variability, confirming overall larger abundances during the evenings than during mornings. These data provide new insights into the vertical distribution of atmospheric water vapor on Mars, aiding future comparisons and global climate model validation.
High-level ice clouds exert a net warming on the climate system because their greenhouse effect outweighs their albedo effect. Focusing on in-situ cirrus, their formation involves the conversion of upper-tropospheric water vapor into ice crystals. Anthropogenic perturbations to the upper-tropospheric water budget-such as aviation-induced cloudiness or the proposed cirrus cloud thinning concept-also trigger atmospheric adjustments that contribute to the total effective radiative forcing. This study presents an idealized climate-model pulse experiment in which water exceeding saturation within cloud-free, ice-supersaturated regions is instantaneously condensed to form cirrus clouds. In contrast to sustained perturbations, this transient modification of cirrus cloud cover enables the direct isolation and examination of atmospheric adjustments. We further introduce a novel ensemble-based framework that suppresses the statistical impact of atmospheric variability, allowing for a robust assessment of the comparatively weak atmospheric response. Cirrus cloud formation initially increases high-level cirrus cloud cover, followed by a decay as ice crystals sediment and sublimate. This process induces an immediate reduction in upper-atmospheric specific and relative humidity, with recovery occurring on substantially longer timescales. The combined cloud and humidity responses generate an initially positive radiative perturbation that transitions to a negative signal after a couple of hours. Notably, relative humidity requires up to four days to return to its equilibrium value. Adjustments in high-level cloud fraction and top-of-atmosphere longwave radiation exhibit non-linear behavior. Together, these findings elucidate how the radiative effects of cirrus clouds-whether natural or anthropogenic-are partially counteracted by atmospheric adjustments, with important implications for the climate efficacy of cirrus-related forcings.
We present simulations of lunar exospheric neon (20Ne) that include diurnal variations of the surface temperature measured by the Diviner radiometer onboard the Lunar Reconnaissance Orbiter (LRO). The Diviner-based model predicts a different exospheric density than the models based on analytical and empirical expressions of the surface temperature. The discrepancy is most evident in craters with a high abundance of rocks, which at night radiate more heat compared to the surrounding areas. We focus on Tycho, Tsiolkovskiy, and Giordano Bruno. For Tycho, the most prominent of these, the expected local depletion in exospheric density can be as great as similar to 18%, up to an altitude of similar to 20 km. This deviation could be detectable by a mass spectrometer orbiting the Moon at a few tens of kilometers of altitude. We compare the modeled exospheric densities with data from the Neutral Mass Spectrometer (NMS) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE). The Diviner-based model and the model based on the empirical formula of the surface temperature demonstrate superior agreement with the data, compared to the model based on analytical expression of the surface temperature. The radial profiles of exospheric 20Ne density at several lunar phases, latitudes, and local time show a common altitude (90-100 km) above which the exospheric density is greater on the dayside compared to the nightside, opposite to what is observed at lower altitudes. We find that charge exchange with solar wind protons is a negligible loss process for lunar exospheric 20Ne.
