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"Near winter solstice, Mars' distance from the Sun approaches its
minimum, and the atmosphere becomes dusty (optical depth at visible
wavelengths >> 1). Absorption of solar radiation by dust increases the
daily mean temperature and diurnal temperature range above the
surface, but the surface diurnal temperature range decreases and the
daytime convective layer disappears. Daily average temperature near the
surface is not affected by dust loading."

Reference:
Leovy, Conway, Weather and climate on Mars, Nature 412, 245-249 (2001) .
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Thanks for the ref. I'll take a look at that.

Bob Clark