The martian daytime convective boundary layer: Results from radio occultation measurements and a mesoscale model

Abstract

We investigate the behavior of the Martian daytime convective boundary layer (CBL) through a combination of data analysis and modeling. This study relies on two subsets of Mars Express radio occultation (RO) measurements that sounded the atmosphere in northern spring of successive Mars years. Only the first year of observations has been examined previously (Hinson et al., 2008); the second year provides complementary spatial coverage and greatly increases the total number of observations. Analysis of the RO profiles yields basic characteristics of the CBL, such as its depth D and the average potential temperature of the mixed layer θm. We also combine RO retrievals of surface pressure with surface temperatures from infrared sounding to characterize the surface forcing, expressing the result as a potential temperature θs. These observations are at local times in early afternoon for θs and late afternoon for θm and D, when each parameter is near its diurnal maximum. We use measurements at mid-to-low latitudes, which sample a wide range of θs (227–294 K), to determine the response of the lower atmosphere to spatial variations in surface forcing. The depth of the CBL ranges from less than 3 km in the midlatitude topographic basins to more than 9 km above elevated terrain in the tropics. The dependence of θm on θs is linear, with a characteristic slope of about 0.7 in both years. We gain further insight by performing a simulation with the Oregon State University Mars Mesoscale Model in a region centered on Isidis Planitia, which includes two potential landing sites for the Mars 2020 Rover. As expected from previous modeling of much smaller craters, the arc of steep topography along the western and southern margins of Isidis produces a distinctive, diurnally varying, mesoscale circulation. The simulation captures key features of the observations, such as the wide variations in θm and D — by 34 K and 9 km, respectively — that occur within this region. The model also accounts for peculiar features of RO profiles on the rim of Isidis, where the wind field strongly influences the depth and diurnal evolution of the CBL. Detailed comparisons with the observations validate the general performance of the model and confirm several aspects of the simulated wind field.