Spatial Variability of the Atmospheric Boundary Layer Height over an Isolated Mountain: Selected Cases from the MATERHORN-2012 Field Experiment

Weather forecasting in mountainous regions is a great challenge for the atmospheric sciences community. There is evidence that wrong simulation of boundary-, entrainment- and residual layer, including the diurnal/inter-diurnal development of the atmospheric boundary layer (ABL) top height (zi) are crucial for the deficiencies in forecasting mountain weather. Within the first field experiment of Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program conducted at Dugway Proving Ground (DPG, Utah, USA) between 25 September and 24 October 2012, a suite of ground-based and airborne instruments were deployed to collect data at high spatio-temporal resolution in and above the atmospheric boundary layer . In particular, a coherent Doppler lidar onboard a Navy Twin Otter aircraft was deployed to aid in characterizing turbulence features within the ABL as well as the spatial heterogeneity of the zi over an extended area around a steep isolated mountain (Granite peak) of a horizontal and vertical scale of about 8 km and 0.9 km, respectively. Measurements obtained during three selected intensive observation periods (IOPs) that were characterized by quiescent weather conditions provided a data set appropriate for studying spatial variability of zi in a mesoscale domain, in particular, in the vicinity of the Granite peak, located nearly at the center of the experimental region. The zi variability determined from aerosol backscatter profiles across total 1200 km of TODWL flight tracks during three IOPs helped illustrate both orographic forcing and underlying land-surface heterogeneity effect on the boundary layer regimes and associated zi variability. In general, an east-west gradient in zi over the region was found for the well-mixed CBL regimes. For instance, an intercomparison between zi obtained with radiosonde measurements over eastern and western site yielded a difference of around 200 m. The TODWL zi measurements compare well with those from the radiosonde profiles. The near-surface micrometeorological measurements and simultaneous radiosonde profiles of thermodynamic variables over the eastern and the western sites of Granite peak helped illustrate some of the spatial variability that was generated due to the different surface characteristics and relevant thermodynamic regimes present in the area. The tower-based micrometeorological measurements at the eastern and the western sites confirmed two different surface forcing mechanisms that yielded heterogeneity in the near-surface sensible heat flux during entire diurnal cycle, thus on spatial variability in zi. Additionally, a precipitation event that occurred between two IOPs helped investigate the impact of two different moisture regimes on the thermodynamic features, thus, on the spatial zi variability.