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The Effect of Land Surface Heterogeneity on the Atmospheric Boundary-Layer Structure and Measurements

Fachliche Zuordnung Physik und Chemie der Atmosphäre
Förderung Förderung von 2009 bis 2014
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 117404101
 
Erstellungsjahr 2014

Zusammenfassung der Projektergebnisse

During the LITFASS-2003 field campaign, a comprehensive experimental data set on the structure of the atmospheric boundary layer (ABL) over a heterogeneous land surface has been collected. Measurements included the operation of 14 surface flux stations, two eddy-covariance systems at a 99 m tower, three large-aperture scintillometers (LAS) and 35 low-level flights using the helicopterborne turbulence probe Helipod. The project analysed this data base and added large-eddy simulations regarding the turbulent ABL structure and processes over a heterogeneous land surface. Large-eddy simulations (LES) for selected days of the LITFASS experiment showed that surface heterogeneity-induced heat-flux patterns, as well as secondary circulations (SC), extend throughout the entire depth of the boundary layer. In order to separate the heterogeneity signals from the randomly-distributed turbulence, virtual measurements within the LES showed that extensive averaging is required, making it difficult to capture heterogeneity signals within the boundary layer by e.g. aircraft observations. Further-more, LES runs showed that SCs take over a significant part of the vertical transport in the boundary layer, however, they do not modify the total vertical transport, except for the entrainment layer. Depending on the amplitude and scale of the surface heterogeneity, LES runs over idealized surface heterogeneity showed that entrainment can be either decreased or increased by weak or strong SCs, respectively. Moreover, for the first time, footprints over complex heterogeneous terrain were evaluated by means of LES, and compared against footprints derived from a Lagrangian stochastic model, in order to estimate the uncertainty of the footprint prediction for operational turbulence measurements. The detailed analysis of airborne high-resolution surface temperature measurements revealed considerable local heterogeneity of the surface temperature distribution even over areas of “homogeneous land use”. These local temperature differences were analyzed quantitatively over sectors of 30 deg width covering the footprint area around several of the LITFASS-2003 surface flux stations. It could be shown that enhanced values of the surface temperature variance (increased local surface temperature heterogeneity) correspond with increased values of the local energy budget closure deficit and vice versa. It is therefore recommended to perform a careful mapping of the local surface conditions at a given patch of land use based on airborne photography before installing a flux station. 34 cases studies using Helipod data measured below 100 m above ground (performed in a variety of wind and buoyancy conditions) show that the spatial variability of mean thermodynamic quantities is not significant, while second order moments related to potential temperature exhibit a clear decrease over the vicinity of the lake Scharmützel. The latter was chosen for being the largest surface discontinuity in the experimental site and for statistical reason: The observed variability for turbulent variables due to transient motions and large flux sampling errors over land patches are reduced significantly over the lake. Most likely, the lack of thermals over a coldest surface favours such drop for the turbulent fluxes and its random variability. Several vertical length scales for the surface heterogeneity influence were calculated. It was found that only those scales that consider the variance of vertical velocity are compatible with the observations. Since the variance of vertical velocity describes vertical mixing originated by both wind shear and thermal heating, this stability parameter seems more suitable to account for a dataset with a large range of buoyant conditions. Additionally, the application of a convective scale indicates that the lake may influence the entire ABL for scale wind speeds below 4 m/s. A statistical analysis on the local energy budget closure for a long-term data set collected at the boundary layer field site GM Falkenberg of the MOL-RAO during the seven-year period 2003-2009 showed a mean energy budget closure deficit of about 20 % of the net radiation. Enhanced values of the closure deficit (22 – 23 %) occur for winds from the South-West while lower values (17 – 19 %) were found for wind directions around East and from North-West. A reduction of the data set towards daytime conditions and to situations around noon resulted in a decrease of the closure deficit by about 2 – 4 %. Similarly, the residual is smaller for weak wind conditions by about 2 – 3 % when compared with moderate to strong winds. The results from both time and spatial-averaged analysis of ground, tower and airborne data suggested that SC (secondary circulations) transport significant energy, especially as sensible heat. However, their contributions are not measurable by EC (eddy-covariance) measurement within 30 minutes, and thereby cause the imbalance in the energy budget. Therefore, the appropriate energy balance correction is necessary. All the findings show that an appropriate energy balance correction shall use the buoyancy flux ratio. However, a better spatial-averaged measurement that could overcome all limitations discovered would improve any energy-balance correction.

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