Modeling of albedo and thermal inertia induced mesoscale circulations in the midlatitude summertime Martian atmosphere

Abstract

Mesoscale circulations in the Martian atmosphere induced by variations in surface albedo (a) and soil thermal inertia (I) or both have been studied numerically with a two‐dimensional primitive equation model. The latitudinal and seasonal parameters correspond to northern midlatitudes and midsummer. The variations in the surface properties induce surface temperature variations, which, in turn, drive horizontal winds and vertical motions. A circulation cell not unlike a terrestrial seabreeze cell is formed at 1300–1400 local time (LT), the vertical motion peaks just prior to sunset and persists until 2100–2300 LT. The horizontal winds accelerate throughout the day and peak around 2100 LT. The acceleration picks up at 1600–1800 LT owing to the collapse of vertical momentum mixing. In the thermal inertia case the surface temperature gradient reverses around 1800 LT, leading to reversed forcing and nocturnal return flow. Owing to the Coriolis effect, the perpendicular wind component (ν) dominates the magnitude of the horizontal flow. The extrema of horizontal winds and vertical motions occurring for variations of a = 0.20–0.30 and I = 250–350 SI units and optical thickness τ = 0.4 can be up to 7–8 m/s and approximately 3–4 cm/s, respectively. The strength of the circulation is sensitive to the amount of suspended dust; the maximum wind speed is reduced by a factor of more than 2 when τ is changed from 0.1 to 1.0. The effects of superimposed thermal inertia and albedo variations depend on their respective magnitudes and signs: if a and I increase in the same direction, the circulations are amplified, and the phases are close to phases of circulations induced by thermal inertia variations alone. If the variations have opposite signs, the circulations are attenuated, and the time of largest forcing can shift from daytime to the night; circulation and surface stress patterns are also shifted in phase by up to several hours. Surface stress τ0 induced by the circulations is discernible as of a few hours after sunrise and peaks in the region of a and I variations. If the forcing is due to a or I variation alone or due to the amplifying combination of the two, τ0 peaks in the early afternoon (typically at 1200–1400 LT) and collapses 2–3 hours prior to sunset, primarily owing to the collapse of vertical momentum mixing. In case of opposing surface property variations, τ0 maximum is shifted in phase and can occur as early as 0800 LT owing to predominantly nocturnal forcing. The circulations studied here appear not to play a significant role in dust raising, as the magnitude of the stress generated is at least an order of magnitude below the estimated dust raising threshold.