A numerical study on the interaction of nonclassical mesoscale circulations and baroclinic systems

Nonclassical mesoscale circulations (NCMCs) are thermally-induced circulations similar to sea-breezes, except that they are established when horizontal gradients in soil moisture, soil type, vegetation, snow cover, or cloud cover exist. Numerical studies of these phenomena have focused on the effect of simple discontinuities in soil type, soil moisture, or vegetation, while neglecting synoptic forcing and three-dimensional effects; therefore, these studies may tend to over-predict the impact of NCMCs on the structure of the boundary layer. Synoptic forcing and three-dimensional effects may be strong enough to mask or suppress NCMCs;In the present study, a hydrostatic, three-dimensional, mesoscale model has been developed to evaluate the effects of horizontally heterogeneous soil moisture and soil type on the passage of a summer cold front in the central United States. The atmospheric portion of the model is coupled to the earth by incorporating forecasts of both moisture and heat fluxes within the soil;Numerical simulations demonstrated that evaporation of soil moisture significantly affected the boundary-layer structure embedded in the baroclinic circulation. Evaporation cooled the boundary layer near the surface and induced a mesohigh over the regions of moist soil. The reduced heating at the surface suppressed the development of the mixed layer and reduced the boundary-layer height considerably. Although the position of the front was not altered, the thermal and momentum fields were affected enough to weaken the front near the surface. Evaporated soil moisture was transported into the free atmosphere and advected ahead of the cold front, far from its source region. Moisture convergence was significantly enhanced in several locations, indicating that soil moisture may play an important role in modifying the spatial distribution and intensity of precipitation;Simulations with no imposed synoptic flow and similar inhomogeneous surface characteristics produced NCMCs that were weaker than those embedded in the frontal zone. The particular synoptic field chosen for the baroclinic circulations interacted nonlinearly with the NCMCs to magnify the effect of forcing at the surface. This illustrates that the impact of surface inhomogeneities in soil moisture and soil type on the atmosphere is expected to be highly dependent on the particular synoptic conditions;Realistically coupling the earth and atmosphere in numerical models is of prime importance because the parameterization of horizontally inhomogeneous surface characteristics in operational models may influence short-range forecasts. NCMCs also may play an important role in patterns related local meteorology and climatology, cumulus convection, and air quality

Overview of the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES)

Substantial uncertainties still exist in the scientific understanding of the possible interactions between urban and natural (biogenic) emissions in the production and transformation of atmospheric aerosol and the resulting impact on climate change. The US Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program’s Carbonaceous Aerosol and Radiative Effects Study (CARES) carried out in June 2010 in Central Valley, California, was a comprehensive effort designed to improve this understanding. The primary objective of the field study was to investigate the evolution of secondary organic and black carbon aerosols and their climate-related properties in the Sacramento urban plume as it was routinely transported into the forested Sierra Nevada foothills area. Urban aerosols and trace gases experienced significant physical and chemical transformations as they mixed with the reactive biogenic hydrocarbons emitted from the forest. Two heavily-instrumented ground sites – one within the Sacramento urban area and another about 40 km to the northeast in the foothills area – were set up to characterize the evolution of meteorological variables, trace gases, aerosol precursors, aerosol size, composition, and climate related properties in freshly polluted and “aged” urban air. On selected days, the DOE G-1 aircraft was deployed to make similar measurements upwind and across the evolving Sacramento plume in the morning and again in the afternoon. The NASA B-200 aircraft, carrying remote sensing instruments, was also deployed to characterize the vertical and horizontal distribution of aerosols and aerosol optical properties within and around the plume. This overview provides: (a) the scientific background and motivation for the study, (b) the operational and logistical information pertinent to the execution of the study, (c) an overview of key observations and initial findings from the aircraft and ground-based sampling platforms, and (d) a roadmap of planned data analyses and focused modeling efforts that will facilitate the integration of new knowledge into improved representations of key aerosol processes and properties in climate models