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In this Section, we summarize the most important findings and relevant issues treated in detail in Chapters 2 to 5.
The primary conclusion of this thesis is that it is necessary to take aerosols into account to accurately describe the convective atmospheric boundary-layer (CBL) dynamics and the land-surface processes. We reached this conclusion by systematically studying the land-CBL system and its couplings, and employed a hierarchy of models ranging from an eddy-resolving model (large-eddy simulation; LES) to non-eddy resolving models (mixed-layer model, and single column model). In addition to the numerical component, we used a complete observational data set to help us design and evaluate our numerical framework.
Chapter 2 was devoted to the explanation of the radiative transfer code used in Chapters 4 and 5. We showed that despite the simplified treatment of solar radiation and its interactions with aerosols, our radiative code is in general agreement with a more sophisticated radiative transfer code, even for extreme aerosol loads. Moreover, our results reproduce observations of direct and diffuse radiation at the surface accordingly - as shown in Chapter 4.
Regarding the longwave band, we showed that aerosols are not relevant for the estimation of the incoming longwave radiation at the surface. We concluded that Brunt's formula, depending only on screen level temperature and vapor pressure, is the most adequate to represent the incoming longwave radiation at the surface for the cases relevant for our studies.
In Chapter 3 we investigated the impact of aerosol heat absorption on the dynamics of an idealized CBL with prescribed surface fluxes. We found that the structure and evolution of the CBL were influenced by the vertical distribution of the aerosols. Moreover, we showed that the aerosols influence the exchange of heat between the CBL and the free troposphere by (i) extinction of radiation and consequently reduced surface fluxes, and by (ii) deepening the entrainment zone depth. We highlighted the importance of high-resolution models to properly represent the effects of aerosol absorption of radiation on the dynamics of the CBL, especially in the entrainment zone. We demonstrated that, in addition to the properties of the aerosols, the vertical distribution is an important characteristic to properly describe the CBL height evolution and the dynamics of the upper part of the CBL. To further support the analysis of the LES results, we used a mixed-layer (MXL) model to calculate boundary-layer depth and the potential temperature jump at the inversion layer. In spite of the simplicity of this model, the mixed-layer results obtained for boundary-layer height and the inversion layer jump agreed well with the LES results.
Extending the knowledge acquired with the academical prototypical experiments performed in Chapter 3, in Chapter 4 we quantified the effects of aerosol scattering and absorption of shortwave (SW) radiation both on the surface energy budget and on the CBL dynamics. To this end, we coupled our LES model and the MXL model to (i) a land-surface model and (ii) a broadband SW radiative transfer model, (described in Chapter 2). We successfully validated the results obtained with the LES model and MXL model using measurements of (thermo)dynamic variables and aerosol properties observed in Cabauw (the Netherlands). Our LES results showed that for Cabauw (over well-watered grassland) aerosols significantly alter the magnitude of the available energy at the surface and its partitioning. Under well-watered conditions, the sensible heat flux was more strongly reduced compared to the latent heat flux. Given the satisfactory agreement between the LES results and MXL model results, we further explored the sensitivity of the land-CBL system to a wide range of aerosol optical depths and single scattering albedos using the MXL model. Our results showed that higher loads of aerosols impose an energy restriction at the surface. As a result, we calculated a delay in the morning onset of the CBL and an advance in the CBL afternoon collapse. We also found that entrainment of aerosols from the residual layer plays a significant role in the development of the CBL dynamics during the day. An important aspect of Chapter 4 is the investigation of the different responses of the CBL dynamics depending on aerosol optical properties. Strongly absorbing aerosols deepened and warmed the CBL, while purely scattering aerosols shallowed and cooled the CBL.
We highlighted that the results presented in Chapter 4 can be used as a benchmark to evaluate coupling and performance of the parametrizations for SW radiation, land-surface and boundary-layer schemes, implemented in mesoscale or global chemistry transport models.
In Chapter 5 we increased the complexity of our land-CBL system representation by studying the formation and transport of ammonium nitrate aerosols. In doing so, we coupled in our LES radiation, chemistry, aerosols, CBL dynamics, and surface exchange processes of chemicals, heat and moisture. Our fully coupled LES model was again evaluated against observations of chemistry and aerosol fields and showed a good correspondence. In particular, our results showed a satisfactory agreement between the simulated and observed nitrate partitioning at the surface.
We showed that gas-aerosol conversion of nitrate leads to highly non-linear profiles of nitrate concentrations and turbulent fluxes. Moreover, the shapes of the simulated profiles depended strongly on the time scale of gas-aerosol conversions. Note that the typical timescale of turbulent motions in the CBL is around 10-20 minutes. For shorter time scales of gas-aerosol conversion compared to the CBL dynamics timescale, we found that turbulent fluxes are larger and concentration profiles more tilted within the CBL. These results have a significant impact on the nitrate deposition flux at the surface. Our LES results confirmed that the large deposition velocities for aerosol nitrate close to the surface are actually due to outgassing of aerosol nitrate rather than a real deposition process.
An important aspect discussed in Chapter 5 concerns the inability of non-eddy resolving models to accurately model the turbulent transport of nitrate within the CBL. Based on a detailed analysis of the flux budget equation, we showed that the exchange coefficient of heat used in our 1D model has to be increased to better account for the complex interaction between gas-aerosol conversion of nitrate and 3D turbulence within the CBL. Indeed, the new exchange coefficient also improved the comparison between gas-aerosol partitioning of nitrate calculated with our 1D model and surface observations.
The results discussed in this thesis demonstrate the need for considering the influence of aerosols on the CBL dynamics. Specifically, aerosols influence important phenomena for the CBL evolution namely radiation, surface-atmosphere interactions, chemistry, and (thermo)dynamics. In addition to that, the availability of high-resolution numerical simulations is crucial to validate and evaluate results obtained by numerical models that do not explicitly resolve the turbulent field.
|Qualification||Doctor of Philosophy|
|Award date||20 Mar 2015|
|Place of Publication||Wageningen|
|Publication status||Published - 2015|
- atmospheric boundary-layer
- boundary-layer meteorology
- land surface
- simulation models