The microphysics of clouds over the Antarctic Peninsula - Part 2: modelling aspects within Polar WRF

The first intercomparisons of cloud microphysics schemes implemented in the Weather Research and Forecasting (WRF) mesoscale atmospheric model (version 3.5.1) are performed in the Antarctic Peninsula using the polar version of WRF (Polar WRF) at 5 km resolution, along with comparisons to the British Antarctic Survey's aircraft measurements (presented in Part 1 of this work, Lachlan-Cope et al., 2016). This study follows previous works suggesting the misrepresentation of the cloud thermodynamic phase in order to explain large radiative biases derived at the surface in Polar WRF continent-wide, and in the Polar WRF-based operational forecast model Antarctic Mesoscale Prediction System (AMPS) over the Larsen C Ice shelf. Several cloud microphysics schemes are investigated: the WRF Single-Moment 5-class scheme (WSM5), the WRF Double-Moment 6-class scheme (WDM6), the Morrison double-moment scheme, the Thompson scheme, and the Milbrandt- Yau Double-Moment 7-class scheme. WSM5 used in AMPS struggles the most to capture the observed supercooled liquid phase mainly because of their ice nuclei parameterisation overestimating the number of activated crystals, while other micro- physics schemes (but not WSM5's upgraded version, WDM6) manage much better to do so. The best performing scheme is the Morrison scheme for its better average prediction of occurrences of clouds, and cloud phase, as well as its lowest surface radiative bias over the Larsen C ice shelf in the infrared. This is important for surface energy budget consideration with Polar WRF since the cloud radiative effect is more pronounced in the infrared over icy surfaces. However, our investigation shows that all the schemes fail at simulating the supercooled liquid mass at some temperatures (altitudes) where observations show evidence of its persistence. An ice nuclei parameterisation relying on both temperature and aerosol content like DeMott et al. (2010) (not currently used in WRF cloud schemes) is in best agreement with the observations, at temperatures and aerosol concentration characteristic of the Antarctic Peninsula where the primary ice production occurs (Part 1), compared to parame- terisation only relying on the atmospheric temperature (used by the WRF cloud schemes). Overall, a realistic ice microphysics implementation is paramount to the correct representation of the supercooled liquid phase in Antarctic clouds

The microphysics of clouds over the Antarctic Peninsula – Part 1: Observations

Observations of clouds over the Antarctic Peninsula during summer 2010 and 2011 are presented here. The peninsula is up to 2500 m high and acts as a barrier to weather systems approaching from the Pacific sector of the Southern Ocean. Observations of the number of ice and liquid particles as well as the ice water content and liquid water content in the clouds from both sides of the peninsula and from both years were compared. In 2011 there were significantly more water drops and ice crystals, particularly in the east, where there were approximately twice the number of drops and ice crystals in 2011. Ice crystals observations as compared to ice nuclei parameterizations suggest that secondary ice multiplication at temperatures around −5 °C is important for ice crystal formation on both sides of the peninsula below 2000 m. Also, back trajectories have shown that in 2011 the air masses over the peninsula were more likely to have passed close to the surface over the sea ice in the Weddell Sea. This suggests that the sea-ice-covered Weddell Sea can act as a source of both cloud condensation nuclei and ice-nucleating particles

An assessment of the impact of local processes on dust lifting in martian climate models

Simulation of the lifting of dust from the planetary surface is of substantially greater importance on Mars than on Earth, due to the fundamental role that atmospheric dust plays in the former’s climate, yet the dust emission parameterisations used to date in martian global climate models (MGCMs) lag, understandably, behind their terrestrial counterparts in terms of sophistication. Recent developments in estimating surface roughness length over all martian terrains and in modelling atmospheric circulations at regional to local scales (less than O(100 km)) presents an opportunity to formulate an improved wind stress lifting parameterisation. We have upgraded the conventional scheme by including the spatially varying roughness length in the lifting parameterisation in a fully consistent manner (thereby correcting a possible underestimation of the true threshold level for wind stress lifting), and used a modification to account for deviations from neutral stability in the surface layer. Following these improvements, it is found that wind speeds at typical MGCM resolution never reach the lifting threshold at most gridpoints: winds fall particularly short in the southern midlatitudes, where mean roughness is large. Sub-grid scale variability, manifested in both the near-surface wind field and the surface roughness, is then considered, and is found to be a crucial means of bridging the gap between model winds and thresholds. Both forms of small-scale variability contribute to the formation of dust emission ‘hotspots’: areas within the model gridbox with particularly favourable conditions for lifting, namely a smooth surface combined with strong near-surface gusts. Such small-scale emission could in fact be particularly influential on Mars, due both to the intense positive radiative feedbacks that can drive storm growth and a strong hysteresis effect on saltation. By modelling this variability, dust lifting is predicted at the locations at which dust storms are frequently observed, including the flushing storm sources of Chryse and Utopia, and southern midlatitude areas from which larger storms tend to initiate, such as Hellas and Solis Planum. The seasonal cycle of emission, which includes a double-peaked structure in northern autumn and winter, also appears realistic. Significant increases to lifting rates are produced for any sensible choices of parameters controlling the sub-grid distributions used, but results are sensitive to the smallest scale of variability considered, which high-resolution modelling suggests should be O(1 km) or less. Use of such models in future will permit the use of a diagnosed (rather than prescribed) variable gustiness intensity, which should further enhance dust lifting in the southern hemisphere in particular

Modélisation des nuages de dioxyde de carbone (CO2) sur Mars : application aux nuages mésosphériques.

Carbon dioxide (CO2) ice clouds on Mars stem from the condensation of the main atmospheric component (95% CO2). Numerous theoretical studies suggest that these clouds may have an influence on Mars' current climate, and that they have played a significant role in its past evolution. Only recently, observational constraints on crystal sizes and opacities of these clouds have been obtained after their discovery in the mesosphere. In this context, we have focused on the modeling of these clouds, with the aim of characterizing the microphysics of a near-pure vapor condensing in a rarefied atmosphere. We have developed a growth rate model for crystals, taking into account the potentially high temperature difference between the crystal surface and the environment, which appears to be important for a near-pure vapor. A one dimensional microphysical model has then been developed for these clouds to simulate their formation in the mesosphere. Thanks to this model, we can now explain their short lifetime and their diurnal behavior. We show that it is possible to reproduce the cloud crystal sizes, but not their opacities, as long as clouds are supposed to form onto dust particles (regolith) lifted from the ground by storms. A meteoritic input has been used to simulate denser clouds, in agreement with observations. The new microphysical model is intended to be coupled to Mars' climate and meteorological models.Les nuages de cristaux CO2 sur Mars sont issus de la condensation du constituant majoritaire de l'atmosphère (95% de CO2). De nombreuses études théoriques suggèrent que ces nuages pourraient avoir une influence sur le climat martien actuel et qu'ils ont sans doute joué un rôle significatif au cours de son évolution passée. Seulement récemment, des contraintes précises sur la taille des cristaux qui les composent et leur opacité ont été obtenues après leur découverte dans la mésosphère. C'est dans ce cadre que nous nous sommes intéressés à la modélisation de ces nuages avec pour ambition de caractériser la microphysique de condensation d'un gaz majoritaire dans une atmosphère raréfiée. Nous avons mis au point un modèle de croissance des cristaux tenant compte de la différence de température entre la surface du cristal et son environnement, différence qui s'avère importante dans le cas d'une vapeur majoritaire. Un modèle de microphysique de ces nuages à une dimension a été ensuite développé pour simuler leur formation dans la mésosphère. Grâce à ce modèle, nous sommes maintenant en mesure d'expliquer les faibles durées de vie ces nuages ainsi que leur comportement diurne. Nous montrons qu'il est possible de reproduire la taille de leurs cristaux, mais pas leur opacité tant que ceux-ci sont supposés se former à partir des particules minérales issues du régolite. Des scénarios d'apport exogène de noyaux de condensation ont été étudiés et ont permis de simuler des nuages plus denses conformes aux observables. Ce nouveau modèle de microphysique est appelé à rejoindre des modèles de climat et de météorologie martiens actuellement en développement

Evidence of Meteor Smoke Particles as precursors for formation of mesospheric clouds on Mars

International audienceMesospheric clouds have been systematically observed in the Martian mesosphere for about a decade. Not all of the observations allow for the cloud composition to be defined. However, several observations have revealed clouds formed of CO2 ice crystals, although in some cases a water ice composition has been detected as well. The condensation of the main component of the atmosphere is a fairly unique phenomenon. Although the lower atmosphere of Mars is very dusty and rich in ice nuclei, the mesosphere should be fairly devoid of dust lifted from lower layers (due to weak probability of lifting to high altitudes and low atmospheric densities favouring sedimentation). A very interesting candidate as a source of ice nuclei in the mesosphere comes from a terrestrial analogue. Meteor Smoke Particles have been shown to play a role in the formation of the mesospheric clouds on the Earth, and in a recent modelling study we have been able to show that an exogenous source of ice nuclei is required in the Martian mesosphere to be able to model clouds with observed properties. We will present a short review of observations and a summary of the cloud properties, and then discuss the model results pointing towards Meteor Smoke Particles as a necessary ingredient for the formation of mesospheric clouds on Mars

Formation of mesospheric clouds on Mars: new model results based on updated parameters

International audienceMesospheric clouds have been observed on Mars for about 15 years. Microphysical modeling studies have provided evidence that an exogenous Ice Nucleus (IN) source is needed to form these clouds. These IN are probably Meteor Smoke Particles (MSPs) as in the Earth's mesosphere. Recent studies have provided new information on the properties of the MSPs and of CO2 ice: we are presenting here updated results using these new parameters

Modelling the Martian CO₂ Ice Clouds

Martian CO2 ice cloud formation represents a rare phenomenon in the Solar System: the condensation of the main component of the atmosphere. Moreover, on Mars, condensation occurs in a rarefied atmosphere (large Knudsen numbers, Kn) that limits the growth efficiency. These clouds form in the polar winter troposphere and in the mesosphere near the equator. CO2 ice cloud modeling has turned out to be challenging: recent efforts (e.g. [1]) fail in explaining typical small sizes (80 nm-130 nm) observed for mesospheric clouds [2]. Supercold pockets (T2 clouds could have on the Martian climate, one needs to properly predict the crystal sizes, and so the growth rates involved. We will show that Earth microphysical crystal growth models, which deal with the condensation of trace gases, are misleading when transposed for CO2 cloud formation: they overestimate the growth rates at high saturation ratios. On the other hand, an approach based on the continuum regime (small Kn), corrected to account for the free molecular regime (high Kn) remains efficient. We present our new approach for modelling the growth of Martian CO2 cloud crystals, investigated with a 1D-microphysical model. [1] Colaprete, A., et al., (2008) PSS, 56, 150C [2] Montmessin, F., et al., (2006) Icarus, 183, 403-410 [3] Montmessin at al., (2011) mamo, 404-405 [4] Spiga, A., et al., (2012), GRL, 39, L02201 [5] Wood, S. E., (1999), Ph.D. thesis, UCLA [6] Young, J. B., J. Geophys. Res., 36, 294-2956, 199

Vertical distribution of ozone at the terminator on Mars

International audienceWe will present a global overview of the results on the vertical ozone distribution and de- scribe a new method we have developed for comparing the observations to the model

Modelling the microphysics of Martian CO2 ice clouds

The first unambiguous spectroscopic observation of CO2 ice clouds on Mars came from Montmessin et al. (2007), but observations have been reported for a decade now (Clancy et al. 2003). The CO2 ice clouds are a rare phenomenon in the Solar System, since 95% of the martian atmosphere consist of CO2 gas, and thus on Mars we are dealing with the condensation of the main component of the atmosphere. The condensation is moreover occuring in a rarefied atmosphere (large Knudsen numbers) that can have dramatic consequences on the crystal growth through the limiting effect of the heat transfer. CO2 ice cloud modeling has turned out to be challenging : recent efforts (e.g. Colaprete et al. 2008) fail in explaining typical small sizes (80 nm-130 nm) observed in equatorial mesospheric ice clouds (Montmessin et al. 2006). Recent modelling studies suggest that the effect of the thermal tide in cooling the mesosphere is a prerequisite for the cloud formation (Gonzalez-Galindo et al. 2011), but other perturbations are required to attain the CO2 condensation temperatures. A recent study has shown a strong correlation between mesospheric ice cloud observations and the filtering of gravity waves through the atmosphere (Spiga et al. 2012). Those waves could create cold pockets in wich T ≤ Tcond, and thus provoke a supersaturated environment in which the clouds can form. However, the nature of the key microphysical processes in the formation of CO2 ice clouds remains unclear, especially in the mesosphere. We have adapted a microphysical model previously developed for water ice clouds formation on Mars (Montmessin et al. 2004) for modeling these CO2 ice clouds

Microphysical properties of Martian CO2 ice clouds

We are currently working in adapting a microphysical model previously developed for the water ice clouds formation on Mars ([14], based on that of [17]) with the aim at coupling it to a mesoscale and a general circulation model We will describe the model used and present the modifications we initiated