Fog, As the Factor, Which Worsens/Impairs Visibility Will Bring Large

Automatic Meteosystem for Fog Parameters Monitoring

Mikhail Khaikine, ArkadyKoldaev, Eugene Miller and Alexandre Mironov

Central Aerological Observatory, Roshidromet,

3 – Pervomayskaya Str., Dolgoprudny, Moskow Reg. 141700, Russia

Tel/Fax: (+7)(095) 408-7758

e-mail:

1. Introduction

Fog, as the factor, which impairs visibility, creates large harm on many fields of national economy. In spite of development and improvement of technical equipment for "blind navigation", the urgency of the problem of artificial fog dissipation does not decrease as in field of aviation as in marine transportation. An essential effect fog have also on car and truck traffic, increasing the danger of collision and considerably reducing the velocity of motion. The problem of fog is especially urgent for the large highways, where along with an increase in traffic volume there is the grows of loses induced by fog.

The development of methods and technical equipment for artificial fog dispersion is the perspective direction of works to the decrease of the losses, connected with the fog. At the same time, as it is shown by practical experience, super-short term forecast (nowcasting) of the fog formation/dissipation makes it possible to decrease considerably the losses. Under the word nowcasting in application to the fog, we mean reliable (more then 80% probability) local (valid for the area about 10-20 square kilometers) forecast within the time scale 1-2 hours prior the real event.

It is well known that not only saturation of air is required for the fog formation, but air mass should be supersaturated, so that the part of the water vapor would be condensed and would create the water content of fog. The process of air saturating in the atmospheric boundary layer depends by two reasons: either by a temperature decrease or by an increase of the moisture supplies. Important role in the fog formation and scattering plays the vertical distribution of meteorological elements in the boundary layerof the atmosphere. Among them are: temperature profile, humidity profile, wind and so forth. Stable atmosphere stratification is common for all forms of fog. It is characterized by the weak temperature decrease in the layer of fog and by the inversion above it, therefore turbulent moisture and heat exchange above it is very weak. It is increased with a change of the synoptic situation, when wind speed is increased and the inversion is destroyed. In this case the fog is scattered both due to the mixing of dry air with the fog and due to the redistribution of the temperature and humidity in its layer. While the turbulence during stable stratification is increased, the temperature rise in the lower layer, and fog is scattered or transformed into the low level cloud.

Radiation fluxes play large role in the process of the formation of fog. As is known, water vapor is the key atmosphere component which absorb IR radiation. Therefore the changes in saturation of boundary layer has the influence on the effective emission of the earth's surface. These changes can be detected in a good time prior the fog formation by carrying out of the corresponding measurements. Thus, for the fog "nowcasting" it is necessary to obtain in operative mode following data about the state of the boundary layer of the atmosphere:

- near ground temperature, humidity, pressure, velocity and direction of wind;

- vertical pulsation of wind at the near ground level;

- the temperature profile in the atmosphere boundary layer;

- radiation balance at the near ground level;

- visibility

2. Meteorological system description

The meteorological system composed with a set of remote and "in situ" sensors was developed in Central Aerological Observatory (CAO) for monitoring of the atmosphere boundary layer conditions associated with fogy conditions. The system is consist with:

• -Meteorological Temperature Profiler MTP-5 (temperature profiles up to 600m);
• Net Radiometer (up and down radiation fluxes of solar and IR radiation as well as radiation balance);
• IR Visibility Meter (measuring of the atmosphere transparency);
• Ultrasonic meteostation (3 component of wind as well as pulsation, relative humidity, temperature and pressure);
• Humidity and temperature probes HMP-45A.

The main technical parameters of the meteorological system are given in the Table 1.

Table 1.

MTP-5 specification

3. Field measurements and the results.

During the time of measurements were registered two days with the intensive fog (10 and on 11 March), when visibility dropped below 100 m. The moderate fog, when visibility did not decrease below 200 m, was observed on 2 March. Weak fogy were noted on 4, 5 and on 12 March.

Visibility during these days was more than 500 m.The analysis of the temperature profiles, obtained by the profiler MTP -5, indicates that in entire time of observations the temperature inversions were not observed only on 3 March and on 6 March. Unstable profile on March 3 is associated with the rain and the same for March 6 is explained by the fresh wind (> 40km/h). During the remaining days the inversion were formed, as a rule, in night hours. While the inversion was formed in the evening hours, its destruction occurred next day and, as a rule, after sunrise.

The comparative analysis of the parameters of atmosphere boundary layer allowed us to define a number of the necessary environment conditions for fog formation .at the location of measurements. Namely the following events were observed:

- temperature drop with the rate not less than 0.5 degree/hour;

- increase in the relative humidity to the values higher than 90%;

- temperature inversion in the layer 0-200 m;

- wind speed not more than 3.0 m/h.

However, the presence of these conditions insufficiently for the beginning of fog formation. Time series of the temperature, measured at heights 2.0m, 6.5m, 10.5m and 50 m in the morning hours for clear (March 7, 2001) and fogy of days (March 10, 2001) are presented on the Figure 2.

For these days was observed a 3 degree C temperature decrease within 6 hours , and an increase in the relative humidity to 4-6%. A change in the visibility for the case of fog on 10 March is shown in figure 3.

As can be seen well on figure 2, in time the temperature decrease is observed in both fogy/non-fogy conditions the variations in the temperature, which exceed one degree, was observed at the height 2m. With an increase in the height the amplitude of these oscillations decreased. For the clear conditions was observed stable stratification practically from the earth's surface. For the fogy of condition prior to the beginning of the formation of steady fog (3:05) in the layer to 50 m were observed strong variations in the temperature profile. After 3:30, when visibility was reduced to 150 m, the elevated inversion was formed. In figure 3 it is evident that several cases of the short-term (20-40 minutes) decrease of visibility were observed from 0:00 to 3:00 hours. These changes in the visibility were accompanied by temperature drops at the level 2 m to 2.0-2.5 degrees and, with a certain delay, by the drop in relative humidity on 1.5-2.0%

As has already been spoken above, changes in saturation of boundary layer have an effect on the effective emission of the earth's surface. K. S. Shifrin proposed the equation, which expressed the effective emission of the earth's surface through the terms of the average water content in the sub inversion layer. With the aid of this equation was calculated the gradient of average water content in the sub inversion layer for the fogy days and for the clear ones. For the calculations were chosen just those clear days when the parameters of the lowest layer of the atmosphere were close to those observed in the case of the formation of fog. Calculations showed that if the conditions for the formation of fog are absent, then the gradient of average water content is close to zero. In the case of the formation of fog 1,5-2,5 hours prior to the formation of steady fog the gradient of average water content initiated to fall sharply. Then was observed its growth, and after the formation of fog, the gradient of average water content varied around zero. Typical time series of the gradient of the average water content for clear and fog conditions are shown on the Figure 4.

4. Conclusion

The measurements made by means of the described meteorological system have shown that :

- Measurements of the radiation balance, and especially in IR range, of spectra are very useful addition for the understanding of fog formation processes.

-The developed inversion in the boundary layer temperature profile along with the high relative humidity are not "enough" predictors for fog formation

-The number of meteorological parameters, measured by the designed system, is "necessary" and "enough" in mathematical meaning for the proposed methodic of fog formation. "nowcasting"

- The case study of the number of fogy events in the North Italy indicates that the synergy of different sensors is a very perspective direction for fog formation "nowcasting".

- The meteorological system described in the presented report is the bright example of the system solution of the particular "Customer" tasks using the standard set of meteorological devices.

Fig.2. Temperature and relative humidity change for clear and fog condition.

Fig.3. Visibility change during fog at March 10, 2001

Fig. 4 The change of the average water content in inversion layer for clear (March 7, 2001) and fog conditions (March 10, 2001).