Tropical Dust Cyclones on Mars

Introduction: Atmospheric dust plays an important role in regulating the climate of Mars and strongly influences the deposition of incoming solar energy and outgoing infrared radiation, thereby affecting atmospheric dynamics through heating. The lifting of dust from the surface is fundamentally a result of atmospheric circulations, and this provides an opportuntity for feedback between atmospheric dynamics, dust lifting processes, and the radiative forcing perturbations that ensue. Dynamical atmospheric instabilities resulting from radiatively active atmospheric dust have been previously recognized [1,2,3]. None of these studies included the process of dust lifting, and there was no direct investigation of whether the disturbances were capable of maintaining or enhancing the dynamically unstable atmospheric dust distribution through the replenishment of dust from the surface. Furthermore, none of these studies investigated the instabilities or feedbacks that might result from horizontally finite dust disturbances from which any effectively horizontally infinite or global dust distribution must originate. More recent general circulation modeling studies have included dust-lifting parameterizations tied directly to the atmospheric wind stress and atmospheric lapse rate [4,5,6]. However, these studies did not investigate the possible interplay between atmospheric dust, local atmospheric dynamics, and dust lifting. Does the lifted dust lead to amplification of the local circulation producing the initial dust lifting (as opposed to an amplification of the large-scale mean circulation)? Or, is the lifted atmospheric dust primarily passive, revealing only the presence of an atmospheric circulation while contributing little to its dynamical forcing? The proposed positive feedback mechanism explored in this study mechanism works as follows: 1) wind lifts dust from the surface into the atmosphere; 2) the increased atmospheric dust load results in increased radiative heating of the atmosphere during the day, or less radiative cooling during the night, thereby producing a relatively warm region on the scale of the lifted dust; 3) Surface pressure is hydrostatically lowered in the warm region, which leads to an amplification of the low-level pressure gradient force; 4) The increased pressure gradient results in stronger winds, which lift more dust, and thus completes the positive feedback loop. This mechanism is not unlike the Wind-Induced Sensible Heat Exchange (WISHE) mechanism proposed for the development and maintencance of tropical cyclones on Earth [7].