Accurate short-term forecasts of low ceiling and visibility are vital to air traffic operation, in order to maximize the use of an airport. The research presented here uses specific local observations and a detailed numerical 1D model in an integrated approach. The goal of the proposed methodology is to improve the local prediction of poor visibility and low clouds at Paris’s Charles de Gaulle International Airport. In addition to the development of an integrated observations and model-based forecasting system, this study will try to assess whether or not the increased local observing network yields improvements in short-term forecasts of low ceiling and poor visibility. Tests have been performed in a systematic manner during 5 months (the 2002/03 winter season). Encouraging results show that the inclusion of dedicated observations into the local 1D forecast system provides significant improvement to the forecast. Inspection of events indicates that the improvement in very short-term forecasts is a consequence of the ability of the forecast system to more accurately characterize the boundary layer processes, especially during night. Accurate forecast of low cloud seems more difficult since it strongly depends on the 3D mesoscale flow. This study also demonstrates that the use of a 1D model to forecast fogs and low clouds could only be beneficial if it is associated with local measurements and with a local assimilation scheme. The assimilation procedure used in this study is based on different steps: in the first step the atmospheric profiles are estimated in a one-dimensional variational data assimilation (1DVAR) framework, in the second step these atmospheric profiles are modified when fog and/or low clouds are detected, and in the third step the soil profiles are estimated in order to keep the consistency between the soil state and atmospheric measurements.

Large‐eddy simulations (LES) of radiation fog were performed over an airport area to study the effect of urban canopy on fog. These LES were performed with the Meso‐NH research model at very high resolution: 1.5 m in the horizontal and 1 m in the vertical and over a domain 4.5 km × 1.5 km.The blocking effect of the airport buildings led to strong wind shear and consequently to the production of turbulent kinetic energy (TKE). The airport buildings also had a strong effect on vertical velocity, with a subsidence region behind the buildings. The increase of both turbulence and vertical velocity strongly modified the fog formation. The fog layer took more time to form in the airport area, but the increase in turbulence facilitated the vertical development of the fog layer.The fog took 1.5 h to form over the whole simulated airport area. The fog height was heterogeneous during the formation phase, with the formation of very low clouds locally. The effect of airport buildings on vertical velocity could explain these heterogeneities of the fog height. During the mature phase of the fog, the buildings had little impact on the fog layer characteristics. The fog dynamics were mainly controlled by processes at its top.These results suggest that the inclusion of high levels of detail in the building representation remains important for the local forecasting of fog formation. Particularly, small‐scale heterogeneities can explain the spatial variability of fog formation. It seems necessary to take small‐scale variability of the urban canopy into account for local and accurate forecasts of fog formation over airport areas.

A 100 m resolution simulation of radiation fog observed during the Local And Non‐local Fog EXperiment (LANFEX) was performed over the Shropshire hills (UK) in order to understand the impact of local circulation on valley fog formation. The model correctly resolves all valleys and their different geometries, their associated dynamical features, and the different fog conditions between the measurement sites. Passage of stratocumulus during the night led to fog dissipation and gave the opportunity to study two fog formation stages. In the narrow valleys, fog formed at the valley floor and non‐local drainage‐flow processes acted to dissipate it. This equilibrium determined the fog optical thickness, which varied within and between valleys. Wider basins were more subject to dense fog conditions (due to local formation) than narrower valleys, where advecting fog events are locally observed through basins overflowing. The largest and most open valley of Jay Barns was impacted by numerous circulations from narrower tributary valleys, and their complex interactions affected fog formation differently between events. The impact of cloud microphysics on the simulated fog is studied by comparing simulations with one‐moment and two‐moment schemes. The use of a two‐moment scheme brings improvements when the prognostic number concentration is used to compute cloud optical properties, meaning that the radiative impact of droplet concentration is greater than its gravitational settling effect.

The feasibility of the recently proposed concept of adaptive observations is tested on a typical case of poorly forecast North Atlantic cyclogenesis. Only numerical tools are employed, no special observations. Although based on simulated data, this study addresses both theoretical and practical problems of adaptive observations. In the first stage of this study, the role of the data assimilation processes is neutralized; the correction is done by forcing correct continuous fields within the target area. These experiments prove that it is necessary to correct the projection of the initial errors on the first unstable plane (the first two leading singular vectors) in order to significantly improve the forecast. These results also clearly demonstrate that the quality of the initial conditions on a limited, but quite large, area could be a major factor influencing the forecast quality. In a second stage, the focus is on operational aspects. The correction is done through the assimilation of a discret... Abstract The feasibility of the recently proposed concept of adaptive observations is tested on a typical case of poorly forecast North Atlantic cyclogenesis. Only numerical tools are employed, no special observations. Although based on simulated data, this study addresses both theoretical and practical problems of adaptive observations. In the first stage of this study, the role of the data assimilation processes is neutralized; the correction is done by forcing correct continuous fields within the target area. These experiments prove that it is necessary to correct the projection of the initial errors on the first unstable plane (the first two leading singular vectors) in order to significantly improve the forecast. These results also clearly demonstrate that the quality of the initial conditions on a limited, but quite large, area could be a major factor influencing the forecast quality. In a second stage, the focus is on operational aspects. The correction is done through the assimilation of a discret...

Large‐eddy simulations (LES) of radiation fog were performed to study the dissipation phase of fog in detail. These LES were carried out with the Meso‐NH research model at a resolution of 4.5 m in the horizontal and 1 m in the vertical, over a domain of dimensions 4.5 × 1.5 km².The dissipation phase is characterized by a large scatter of the liquid water path and there is evidence that dry downdraughts at the top of the fog layer have a large impact on the liquid water content (LWC) variability inside the layer. These downdraughts promote an increase in solar radiation reaching the ground over small areas, leading to an increase of turbulence near the ground, through an increase of wind variance and an increase of the convective structure of the fog layer. However, downdraughts can also move strong LWC located at the top of the fog layer, transporting it to the ground. These coupled processes between the ground and the top of the fog layer can explain the spatial variability of fog during dissipation of the layer.Sensitivity studies proved that small‐scale surface heterogeneities have no impact on the range of dissipation times, but do have an impact on the horizontal structure of fog at small scales. The wind intensity modifies the balance between warming of the surface and wind shear at the fog top, thus impacting the dissipation time of the fog, but it has little impact on the fog heterogeneity during the dissipation phase.