The Depth of the Daytime Convective Boundary Layer on Mars : A Case of Extremes

Introduction: It has long been realized that the daytime convective boundary layer on Mars is generally much deeper than that on Earth [1]. This is fundamentally the consequence of the low density of the Martian atmosphere, coupled with the fact that the solar thermal forcing of the surface is not that much weaker than on the Earth. Simple consideration of these factors leads to a basic recognition that the depth of the convective boundary layer on Mars will tend to be deeper – other factors being similar – in regions of high topography. The Mars topography is enormous in size, and thus very large variations in the convective boundary layer depths can be expected on this basis alone. Mars also is characterized by large variations in the thermal properties of its surface, the albedo and the thermal inertia. These produce very substantial variations in the thermal forcing (via both infrared radiation and surface sensible heat fluxes) of the daytime boundary layer. There has been a great lack of direct observations of the depth of the daytime convective boundary layer on Mars. Observations of dust devils (as well as some of convective clouds) can provide estimates of minimum boundary layer depths, where heights of the dust devils can be determined from shadows in the imagery [2,3]. In the case of dust devils it is important to note that regions with plentiful and more easily lifted dust may tend to exhibit the most extensive activity, regardless of boundary layer depth. The Mini-TES instruments on the Spirit and Opportunity rovers have allowed the first sampling at relatively high spatial and temporal resolutions of the lower portion of the boundary layer [4]. However, these temperature profiles only extend to about 2 km above the surface, well below the typical top of the convective boundary layer. Recently, a set of temperature profiles (~ 50) obtained from radio occultation experiments conducted by Mars Express have been analyzed [5]. These profiles lie within the ~ 15 S – 50 N region, and were obtained at local times close to 1700; the season was northern midspring (Ls ~ 35-70). Most of these temperature profiles are characterized by a lower region of relatively constant potential temperature overlain by a region in which potential temperature increases rapidly with height, consistent with a well-defined convective boundary layer. The determined depths of the boundary layers in the profiles range between ~ 3-10 km, with the larger values tending to occur over higher topography. There is also some correlation of the depths with surface temperature as observed by the MGS TES. Unfortunately, except for a small number of profiles obtained in the pre-mapping phases of the MGS mission, the MGS radio occultation temperature profiles are all at local times of ~ 0300-0600, making them unsuitable for investigations of the daytime boundary layer depths. The Phoenix lidar experiment is now making observations that should be able to directly determine the depth of the daytime boundary layer at the high northern latitude landing site in summer. Mesoscale as well as LES modeling of the Martian atmosphere has certainly shown that very large variations in the depth of the convective boundary layer are generally present. Extensive mesoscale modeling carried out in connection with the MSL EDL engineering efforts has shown this result even more explicitly than previous modeling, as it has been employed to produce global maps of the maximum convective boundary layer depth – this being a specific concern of the EDL design. The focus of this paper is on the results of modeling of this type, in order to examine the daytime convective boundary layer depth and its seasonal variations and compare it with available observational data.