Atmospheric boundary layer, ABL

As Brenguier (2003) noted, a contributing factor to the uncertainty is drizzle in clouds that form in the atmospheric boundary layer (ABL). In particular, this circumstance illustrates the importance of the adequate retrieval of cloud cover dynamics in the ABL. Another problem is connected with consideration (parameterization) of small-scale processes in the ABL and their non-linearity. For instance, aerosols acting as cloud concentration nuclei (CCN) can be determined from upward motions at the cloud bottom which should be reproduced at a spatial resolution (in the horizontal) of the order of 100 m. The present parameterization schemes still do not meet these requirements. 

To estimate isotopic discrimination at the ecosystem, local or regional scales, an estimate of the exchange flux, or the 6 C value of NEE, at the relevant scale is required. Such estimates are obtained by sampling air in and above plant canopies, or in the and above the atmospheric boundary layer (ABL). A simple and powerful approach employs a two-member mixing model as first proposed by Keeling (1958, 1961). The equation used in the Keeling approach is derived from the basic assumption that the atmospheric concentration of a substance in an ecosystem reflects the combination of some background amount of the substance that is already present in the atmosphere and some amount of substance that is added or removed by a source or sink in the ecosystem ... 

Whole ecosystem and regional-scale discrimination is also assessed by evaluating the isotopic composition of CO2 in the atmospheric boundary layer (ABL also called planetary boundary layer (PBL), as well as convective boundary layer (CBL), when considered during times of convective enhancement by surface heating during the day). 

Figure 8 A representative atmospheric boundary layer (ABL) profile of CO2 (obtained by airplane) during a summer mid-afternoon at a forest study site in Wisconsin (Helliker and Berry, unpublished). Representative 6 C values for the mixed ABL and the free troposphere are indicated. The distinct ABL and its homogeneity provide a basis to estimate changes in CO2 storage and its isotopic composition in the entire ABL column above large surface areas. Alternatively, the large concentration and isotopic gradients at the top of the ABL provide a robust basis for estimating the flux across the top of the ABL, which is assumed to be similar to that at the bottom of the ABL (i.e., the...

The atmospheric boundary layer (ABL) is that portion of the atmosphere where surface drag due to the motion of the air relative to the ground modifies synoptic-scale motions caused by horizontal pressure gradients, Coriolis forces, and buoyancy. The depth of the ABL is highly variable (50 to 2000 m), but it generally increases with proximity to the equator, with wind speed, and as the earth surface roughness, but it decreases... 

Atmospheric boundary layer (ABL) Lower part of the atmosphere that is directly influenced by the presence of the Earth s surface and its typical characteristics. 

The parameterisation has been tested on the city of Basel (Switzerland), Mexico City (Mexico), Copenhagen (Denmark), and verified versus the BUBBLE experiment (Basel Urban Boundary Layer Experiment Rotach et al., 2005 [549]). The verification results (Figure 9.11) show that the urban parameterization scheme is able to catch most of the typical processes induced by an urban surface Inside the canopy layer, the wind speed, the friction velocity and the atmospheric stability are reduced. In the other hand, even if the main effects of the urban canopy are reproduced, the comparison with the measurement seems indicates that some physical processes are still missing in the parameterization. In most of the cases, the model still overestimates the wind speed inside the canopy layer and it can have difficulties to simulate the maximum of the friction velocity which appears above the building roofs. 

The few observations of nucleation in the free troposphere are consistent with binary sulfuric acid-water nucleation. In the boundary layer a third nucleating component or a totally different nucleation mechanism is clearly needed. Gaydos et al. (2005) showed that ternary sulfuric acid-ammonia-water nucleation can explain the new particle formation events in the northeastern United States through the year. These authors were able to reproduce the presence or lack of nucleation in practically all the days both during summer and winter that they examined (Figure 11.16). Ion-induced nucleation is expected to make a small contribution to the major nucleation events in the boundary layer because it is probably limited by the availability of ions (Laakso et al. 2002). Homogeneous nucleation of iodine oxide is the most likely explanation for the rapid formation of particles in coastal areas (Hoffmann et al. 2001). It appears that different nucleation processes are responsible for new particle formation in different parts of the atmosphere. Sulfuric acid is a major component of the nucleation growth process in most cases. 

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