Interpretation of the extremely long and narrow mid-latitude orographic clouds on Mars

Following our report of new cases of extremely long and narrow mid-latitude clouds at Thaumasia Highlands (267°E, 39°S), Alba Patera (250°E, 40°N) and Lyot crater (29.3°E, 50.4°N) (Larsen et al., 2026), we analyze the Planetary Climate Model (PCM) (Forget et al., 1999)  predictions in order to understand the conditions under which they form. For this study, we take into account the predicted water vapor concentrations and temperatures, as well as the wind regime (velocity and direction).

We see that elongated clouds form when wind speeds close to the surface are higher (20 m/s at the origin point) compared to those during spring and summer. The same happens at higher heights, where horizontal wind speeds are far greater during fall and winter. In regards to wind direction and cloud orientation, cloud orientations at Thaumasia and Lyot agree with the predicted wind direction at 10 – 22 km and 5 – 16 km respectively, while at Alba Patera, there is a discrepancy. At the volcano, winds matching the initial cloud orientation are located higher up (∼40-50 km).

Orographic clouds, both on Earth and Mars, often reveal the presence of mountain waves. Since there are many types of mountain waves, and each produces a different type of cloud, identifying a particular type of wave can help understand and deduce certain atmospheric parameters, such as, wind speed and stability. Therefore, in order to see if the formation of these clouds is consistent with the model’s atmosphere, we have used a simple, single layered, two-dimensional model (Houze 2014) to understand the flow over arbitrary “bell shaped ridges” roughly the same shape of the aforementioned obstacles. In order to examine the fundamental properties of the mountain waves, we consider a steady-state, two-dimensional airflow, so that linear theory can be used. Linearazing and combining the equations for an inviscid Boussinesq fluid one can obtain a single equation for the vertical wind speed,  w_zz + w_xx + l²w = 0  (Durran 1986), where l is the Scorer parameter (Scorer 1949).  With the basic state wind profile and static stability, considered to be those upstream, and the aid of the continuity equation (u_x +w_z = 0),  we obtain the streamlines in the steady airflow over the ridges.

Figure 1. (a) Elongated cloud that forms at Warrego Rise (Thaumasia Highlands) on January 5^th 2025 (MY38, Ls=26°). The dot marks the head or beginning of the cloud, while the arrows point at the tail. (b) Streamlines in the steady airflow over an isolated bell-shape similar in size to Warrego Rise for a Scorer value of l=0.00018 m^-1 . The red solid line is the zonal topographic profile of Warrego Rise (x=0 at 267.3°E, 40.4°S).

The streamlines that describe the steady airflow are very dependant on the Scorer parameter and the size of the obstacle. Therefore, vertical displacement is seen to depend very much on the details of the airstream, such as, atmospheric stability and wind speed. Obtaining the vertical stability and wind speeds from the PCM we are able to calculate the vertical displacements and, with the vertical temperature and water abundance profiles, see if clouds similar to the ones observed are obtained.

 

References

Durran, D.R. (1986) Mountain Waves. In: Ray, P.S. (eds) Mesoscale Meteorology and Forecasting. American Meteorological Society, Boston, MA. https://doi.org/10.1007/978-1-935704-20-1_20

Forget, F. et al. (1999) Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res. Planets 104, 24155–24175

Houze, R. A., Jr. (2014) Clouds and Precipitation Associated with Hills and Mountains. In International Geophysics (pp. 369–402). Elsevier. https://doi.org/10.1016/B978-0-12-374266-7.00012-3

Larsen, E. et al. (2026) Orographic elongated clouds in mid-temperate and subpolar latitudes of Mars. I-Observations. Icarus, 445, 116864. https://doi.org/10.1016/j.icarus.2025.116864

Scorer, R. S. (1949) Theory of waves in the lee of mountains. Quarterly Journal of the Royal Meteorological Society, 75(323), 41–56. https://doi.org/10.1002/qj.49707532308