MODELING AND OBSERVING KEY PROCESSES DRIVING THE FOG LIFE CYCLE

S. Stolaki (3), J.-C. Dupont (2), M. Haeffelin (2), C. Lac(1), T. Bergot(1), F. Burnet (1)

(1) CNRM/Météo-France, France, (2) IPSL, France, (3) LMD, Ecole Polytechnique, France E-mail: stolaki.stavroula@lmd.polytechnique.fr

ABSTRACT

The study focuses on a radiation fog event that occurred on the 15 November 2011 and lasted around 7.5 hours at the SIRTA Observatory (Instrumented Site for Atmospheric Remote Sensing Research), near Paris, in France. At SIRTA, the radiative, microphysical and dynamical processes driving the fog life cycle are fully monitored near the surface and throughout the boundary layer. An analysis combining these measurements and numerical simulations with the single-column mode of the Meso-NH non-hydrostatic, pseudo-compressible model has been performed. Sensitivity tests have been performed with the aim of defining an appropriate configuration for a realistic simulation of the fog event. Moreover, the influence of the CCN activation parameterizations available in the model on fog characteristics, as well as the role of the aerosol distribution and the type of the aerosols (maritime or continental) on fog life cycle and the interaction of these processes with turbulence kinetic energy in the vertical are examined. 1. INTRODUCTION

The application of single-column models on fog studies, allows for efficient experimentation with the different physical processes (dynamical, radiative and microphysical) that influence the fog life cycle. Moreover single-column models provide a powerful tool that can add to the knowledge of quantifying the interaction among the various processes and mechanisms, that contribute to the appearance or no of the phenomenon. All in all, model sensitivity tests, in combination with measurements, lay the basis for a better understanding, in a computationally cheap and quick way, of the complex nature of the various fog events and their life cycle. The ultimate objective of the present study is to contribute to the better understanding of the interactions between turbulence and microphysics on the radiation fog life cycle, as well as to identify the role of aerosols (type, distribution etc.) on fog formation and development. To reach this goal though, a detailed examination has been necessary in order to define the appropriate configuration for a realistic simulation of the fog life cycle at SIRTA with the numerical tool applied here.

2. DATA AND METHODS

The SIRTA observatory (48.7oN, 2.2oE), at 156 m a.m.s.l., is a French National Observatory located in a semi-urban area, on the Saclay plateau 25 km south of Paris, where the radiative, microphysical and dynamical processes driving the fog life cycle are fully monitored near the surface by in-situ sensors and throughout the boundary layer by remote sensing backscatter Lidar (aerosol profiles), Doppler Lidar (wind and turbulence profiles), Doppler radar (liquid/ice water profiles), and microwave radiometer (temperature and humidity profiles) [6]. An analysis combining measurements and numerical simulations with the single-column mode of the Meso-NH model has been performed. 2.1 Numerical model

Meso-NH is a 3D non-hydrostatic, pseudo-compressible research model jointly developed by CNRM/GAME (Météo-France) and the Laboratoire d’ Aérologie (CNRS) [8], intended for the study of meteorological mesoscale and microscale phenomena. Meso-NH, either in its 3D mode [4] or in 1D mode [1] has been used in several research efforts in order to study fog. In the current study the 1D mode is applied with the ultimate goal of focusing on the examination of the individual and combined role that processes related to turbulence and microphysics play during the fog life cycle. It uses a second order microphysics scheme [5] and the [3] turbulence scheme used with the [2] mixing length. Radiation is parameterized with the ECMWF radiation scheme code [10], while the three-layered soil-vegetation scheme ISBA [9] is used. The model is initialized and forced with radiosonde data derived from radiosondes launched by Météo-France in TRAPPES (48.7oN, 2oE) and 30 m meteorological mast vertical profiles of temperature, humidity and wind with hourly frequency. The numerical simulations are validated against real measurements of parameters such as: temperature and relative humidity at different levels from 1 m up to 30 m, horizontal visibility (at 4 m and 19 m), wind speed (at 10 m and 30 m), liquid water content (at 2.5 m), shortwave and longwave radiation (at 17 m and 30 m), droplet number concentration. Initializing and forcing a model with data that represent the local meteorological and surface conditions of a

certain place is an important asset that provides the basis for realistic simulations of fog. Having this in mind, an extensive investigation on the appropriate configuration of the 1D model concerning the forcings and the surface parameters has been performed in order to achieve realistic simulations. Moreover the model’s ability to represent the fog characteristics such as the onset and dissipation times, the fog development and height are investigated. The model has been initialized at 1200 UTC of the 14 November 2011 and the simulation lasted 36 h. 3. FOG EVENT OF 15 NOVEMBER 2011

Based on SIRTA’s observations (1-30 m mast thermohygrometric measurements, 10 m and 30 m sonic anemometer data, visibilimeter, sky imager etc.), the near surface meteorological conditions prevailing during the evening of 14 November 2011 were typical for the formation of a radiation fog event. The fog appeared at 0200 UTC of 15 November 2011 and dissipated after 7.5 h, at 0930 UTC. The gradual cooling (~0.7 oCh-1) of the airmass close to the surface (in the column from 1 m up to 30 m a.g.l.), as well as the high relative humidity values (94.6% to 98.3%) a few hours before the fog onset, resulted in saturation shortly before the fog formation. Radiative cooling was further enhanced by the absence of clouds and light 10 m winds were as weak as 3 ms-1. The thermal inversion that formed at 1400 UTC between 1m and 30 m height and lasted until the fog onset, provided more evidence about the radiative cooling taking place, favouring the fog appearance. 4. RESULTS

Defining the most suitable configuration for fog simulations with Meso-NH’s 1D mode is not an easy task, as applies for any 3D model. Several tests have been performed in order to achieve this by examining surface parameters and different types of temperature, humidity and wind initialization data, as well as forcings. Fig. 1 demonstrates the observed and simulated temperature and relative humidity time series for the 36 h simulation. It is evident that there is generally a good agreement between the simulated and the observed values of the two principal, for fog formation, meteorological variables. A few hours before fog formation, the temperature inversion simulated by the model was less strong than the observed one. What is more, the relative humidity was very close to observations until 1700 UTC of 14/11/2011 after which and in the course of the rest simulation, it was overestimated by the model (maximum 5%). Such an overestimation is partly responsible for a slightly earlier fog formation by the model and a delay in fog dissipation. According to the temporal change of the cloud mixing

ratio (Fig. 2), the fog layer has a height of almost 400 m above ground level (maximum thickness ~250 m) but the duration of it is longer than in reality. According to the same parameter, the fog onset is simulated around half an hour earlier, while the dissipation of it takes place with almost a 3 h delay, in regard with the actual dissipation time. Moreover, at the end of the day (2000 UTC), fog is formed anew and lasts until the end of the simulation. This is evidently connected to the overestimation by MesoNH of relative humidity near the surface (that reaches 100%), after 1630 UTC of 15/11/2011 and until the end of the simulation. It should be noted though that a second fog event occurred at 0000 UTC of 16/11/2011 that lasted 24 h. Concerning the parameter of visibility, the model seems to underestimate it, which translates to the simulation of a denser fog, since the minimum simulated visibility reaches 50 m (Fig. 3). It should be noted that the visibility formula [6] applied in the model takes into consideration only the liquid water content and not the droplet size distribution. Since the liquid water content (not shown) is overestimated by the model, as a result, the visibility is underestimated.

Figure 1. Observed and simulated time series of

temperature (top) and relative humidity (bottom) at 2 m, 10 m and 30 m a.g.l. height. Simulation initialized at

1200 UTC, 14/11/2011. Observed fog formation: 0200 UTC; observed fog dissipation: 0930 UTC.

Figure 2. Cloud mixing ratio forecast by MesoNH.

Initialization at 1200 UTC, 14/11/2011. The time axis refers to the elapsed time since 1200 UTC.

Figure 3. Observed and simulated values of visibility.

As depicted in Fig. 4, MesoNH fails to correctly simulate the 10 m wind speed pattern during the simulation, but although reality is overestimated, the discrepancy is not too high. The same applies for the 10 m turbulent kinetic energy (not shown). As far as longwave downward and upward radiation during the simulated fog event is concerned, MesoNH performs generally quite well (Fig. 5). It seems that the simulated and observed values are quite close for the mentioned period with a difference that reaches 5 Wm-2 for both parameters. The variability of the longwave radiation is closely related to the presence or not of clouds and fog, which is also evident in the simulations. The upward infrared radiation is related to the surface temperature and the surface emissivity. Since the temperature is well simulated by the model, this has a positive impact on the simulation of the upward longwave radiation, which is also satisfactory. The downward (at 17 m) and upward (at 30 m) shortwave radiation is depicted in Fig. 6. In the

beginning of the simulation obviously the model performs quite well for both variables, with a slight overestimation of reality (error < 50 Wm-2). For the downward shortwave radiation, in a first phase (0700-1100 UTC), the model overestimates it, whereas in a second phase (1100-1500 UTC), it underestimates it. This might be linked to the fact that the simulated fog layer has higher liquid water content and is thicker than in reality. Note also that during 1000-1200 UTC low clouds are observed (355nm Backscattering LIDAR) which might explain the lower values in observations. On the other hand, MesoNH overestimates upward shortwave radiation between 0700 UTC and 1200 UTC, and underestimates it after that hour.

Figure 4. Observed and simulated values of wind speed

at 10m.

Figure 5. Observed and simulated downward (at 17 m, top) and upward (at 30m, bottom) longwave radiation.

Last, as a preliminary step, the droplet number concentration (FM-100 fog monitor) has been plotted against the activated CCN concentration simulated by MesoNH (Fig. 7), although no direct comparison can be made. In the tests presented herein, the runs have been performed with the default values of the activation scheme parameters that are related to the source type of aerosols and their characteristics (concentration, size distribution etc.). This default configuration considers a

standard, not too polluted and more pristine air mass (activated CCN ~300 cm-3). Since this is an ongoing work, such a result underlines the need to perform sensitivity tests in order to ameliorate the current configuration according to the atmospheric environment and the aerosol load of SIRTA, and, as a second goal, to examine whether the radiation fog characteristics are affected by changes in the aerosol activation process and in general by other microphysics features.

Figure 6. Observed and simulated downward (at 17 m, top) and upward (at 30 m, bottom) shortwave radiation.

Figure 7. Observed and simulated droplet number

concentration at 4 m. (SIRTA, FM-100 fog monitor) 5. CONCLUSIONS

This study demonstrates an attempt that has been made in order to simulate a radiation fog event that occurred at SIRTA Observatory in November 2011 with the application of the 1D mode of the MesoNH model. The present work has shown that applying the 1D mode of the model for the simulation of fog at a specific place is not a straightforward process and it requires a more detailed examination of the initial surface and upper level conditions and the forcings provided to it. The big range of sophisticated measurements available at SIRTA (derived from: Lidar, Doppler Lidar, Doppler

radar and microwave radiometer) provide the source of information necessary for conducting further tests and comparisons between simulated and observed data aiming at validating the numerical tool at a first step. At a next step it remains to use these measurements in order to deepen the knowledge concerning the microphysics and turbulence role on the fog life cycle. 6. REFERENCES

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10. http://www.ecmwf.int/research/ifsdocs/CY23r4/