Conventional observations of meteorological variables are restricted to a limited number of heights near the surface, with the lowest observation often made above 1 m. This can result in missed observations of shallow fog as well as the initial growth stage of thicker fog layers. At the same time, numerical experiments have demonstrated the need for high vertical grid resolution in the surface layer to accurately simulate the onset of fog; this requires correspondingly high-resolution observational data for validation.
This page presents a field experiment I worked on in collaboration with Bart Schilperoort of TU Delft's Water Management Group. We set out to try and capture the growth of fog from the ground up using new techniques for obtaining high resolution observations in the near-surface layer.
An article describing the work is currently under revision at the journal Boundary-Layer Meteorology.
High Resolution Observations
In order to observe the lowest few metres of the atmosphere at very high resolution, we needed to use alternatives to conventional sensors. For temperature, we employed a technique known as "Distributed Temperature Sensing" (DTS; see the Silixa Ultima page). While this technique has been used for some time in other fields, only recently has it been possible to use it in the atmospheric sciences. DTS works by sending a laser pulse along a fibre optic cable and measuring the return signal of the pulse which is related to the temperature along the fibre. Using this technique, we were able to sample temperature every 12.5 cm along the fibre.
This page presents a field experiment I worked on in collaboration with Bart Schilperoort of TU Delft's Water Management Group. We set out to try and capture the growth of fog from the ground up using new techniques for obtaining high resolution observations in the near-surface layer.
An article describing the work is currently under revision at the journal Boundary-Layer Meteorology.
High Resolution Observations
In order to observe the lowest few metres of the atmosphere at very high resolution, we needed to use alternatives to conventional sensors. For temperature, we employed a technique known as "Distributed Temperature Sensing" (DTS; see the Silixa Ultima page). While this technique has been used for some time in other fields, only recently has it been possible to use it in the atmospheric sciences. DTS works by sending a laser pulse along a fibre optic cable and measuring the return signal of the pulse which is related to the temperature along the fibre. Using this technique, we were able to sample temperature every 12.5 cm along the fibre.
High-resolution observations of temperature alone aren't enough to observe fog; visibility estimates are also needed. To do this, we used a simple camera-LED setup. Previous methods have been developed to obtain visibility estimates from camera images, but they all rely on ambient daylight. As a result, we had to develop our own method for obtaining estimates of visibility, which we achieved through taking time lapse photos of an LED light strip. The intensity of the pixels within the camera images allowed us to estimate visibility: in the most simple sense, the pixels will be darker when fog is present and the light is scattered and absorbed by the water particles. Through comparing the pixel intensity at 1.5 m height with conventional observations at the same height, we came up with a relationship to convert pixel intensity at any height to an estimate of visibility.
The Experimental Set-Up
The experiment was conducted at the Cabauw Experimental Site for Atmospheric Research (CESAR) located in the centre of the Netherlands (see Fog in the Netherlands for more). Our setup was located in the "Energy Balance Field" 200 m to the north of the tower.
The DTS fibre ran from the tower building to the measurement site along a small waterway. It was suspended from a 7-m tall pneumatic tower to obtain a profile of temperature in the lowest 7 m, as well as wrapped in a coil in the lowest metre in order to get even higher vertical resolution. At the base of the pneumatic tower, we had the LED strip (from the ground to 2.5 m height), with a GoPro Hero4 Session camera mounted on the tower building to obtain our camera images for estimating the visibility.
Performance of the High-Resolution Techniques
The high-resolution techniques agreed well with their conventional counterparts. In particular, the DTS temperatures 10 cm and 1.5 m off the ground were within a few tenths of a degree when compared with the KNMI observations at the site. Only nights with calm, cold conditions showed a significant deviation from the conventional sensor at 10 cm. This is likely because of the difference in where the measurements were made, with the local surface properties becoming significant for regulating surface temperature.
The Experimental Set-Up
The experiment was conducted at the Cabauw Experimental Site for Atmospheric Research (CESAR) located in the centre of the Netherlands (see Fog in the Netherlands for more). Our setup was located in the "Energy Balance Field" 200 m to the north of the tower.
The DTS fibre ran from the tower building to the measurement site along a small waterway. It was suspended from a 7-m tall pneumatic tower to obtain a profile of temperature in the lowest 7 m, as well as wrapped in a coil in the lowest metre in order to get even higher vertical resolution. At the base of the pneumatic tower, we had the LED strip (from the ground to 2.5 m height), with a GoPro Hero4 Session camera mounted on the tower building to obtain our camera images for estimating the visibility.
Performance of the High-Resolution Techniques
The high-resolution techniques agreed well with their conventional counterparts. In particular, the DTS temperatures 10 cm and 1.5 m off the ground were within a few tenths of a degree when compared with the KNMI observations at the site. Only nights with calm, cold conditions showed a significant deviation from the conventional sensor at 10 cm. This is likely because of the difference in where the measurements were made, with the local surface properties becoming significant for regulating surface temperature.
More than providing accurate measurements, the unprecedented resolution provided by DTS enables resolving strong gradients near the surface. On the night of 6-7 November, for example, the temperature gradient was as high as 5 degrees in the lowest metre alone!
The visibility estimates also agree surprisingly well with the conventional observations at 1.5 m. At large visibilities (clear conditions), the camera pixels were saturated meaning the camera was unable to give accurate visibility information. When the visibility was reduced, however, the visibility estimates from the camera were within a few tens of metres and provided confidence in the estimates they provide, and certainly the ability to distinguish between "clear" and "foggy" conditions at different heights.
The visibility estimates also agree surprisingly well with the conventional observations at 1.5 m. At large visibilities (clear conditions), the camera pixels were saturated meaning the camera was unable to give accurate visibility information. When the visibility was reduced, however, the visibility estimates from the camera were within a few tens of metres and provided confidence in the estimates they provide, and certainly the ability to distinguish between "clear" and "foggy" conditions at different heights.
Shallow Fog
Based on the conventional observations at 1.5 m height, fog was observed twice during our 2.5-week experiment. From our camera estimates in the lowest 0-0.5 m, however, shallow fog was observed on a further two nights.
Based on the conventional observations at 1.5 m height, fog was observed twice during our 2.5-week experiment. From our camera estimates in the lowest 0-0.5 m, however, shallow fog was observed on a further two nights.
The first night of fog (6-7 November) was a radiative fog event. Strong cooling of the surface resulted in a temperature gradient of up to 5 degrees within just the first metre. This extremely strong gradient corresponed to a shallow region of saturated air which formed into the fog layer which grew steadily throughout the night. Interestingly, the fog layer was seen to be present in the lowest 50 cm up to two hours before fog was observed at 1.5 m. Two hours is a long time when it is put in the context of forecasting and event warning.
Further Reading
Further Reading
- Izett, J. G., B. Schilperoort, M. Coenders-Gerrits, P. Baas, F. C. Bosveld, B. J. H. van de Wiel (2019). Missed Fog? On the potential of obtaining observations at increased resolution during shallow fog events. Boundary-Layer Meteorol. DOI: 10.1007/s10546-019-00462-3
- Schilperoort, B, M. Coenders-Gerrits, W. Luxemburg, C. Jiménez Rodríguez, C. Cisernos Vaca, and H. Savenije (2018). Technical note: using Distributed Temperature Sensing for Bowen ratio evaporation measurements. Hydrology and Earth System Sciences 22, 819-830. https://doi.org/10.5194/hess-22-819-2018