KNMI purchased 50, CHM15k "NIMBUS" units
Figure 1: Types of clouds to be observed by ceilometers
Long before automated weather stations were available, weather and clouds were already observed visually by officially trained and certified meteorological observers. Today, there are still human observers active in many weather services around the globe. Observers look at the sky and estimate cloud fraction in octa and the height of the cloud base. This provides essential information for (aviation) meteorology and, on longer time scales, for climate research and monitoring.
In the nineties, the Royal Netherlands Meteorological Institute( KNMI) started automating the visual observations of present weather, visibility and clouds using automatic weather stations with dedicated optical sensors. For cloud observations, ceilometers were purchased and deployed in the measurement network on a 24/7 operational basis.
After 15 years of use, the old ceilometer had reached the end of their service lives and technical support is no longer available, as the product was phased out by the manufacturer. The time had come to look for a new cloud height sensor replacing the old ones. The requirements included, that the new ones need to be more sensitive for high cirrus clouds and should have limited capabilities for aerosol layer detection. Moreover, the sensors should be accessible remotely.
To find the suitable technology, KNMI published a call for tender in May 2014 to replace the outdated systems with about 50 new ones and integrate them into the networks of the weather service, that serves different end users:
- One part of the network is dedicated for general meteorology purposes: ground-based observations of clouds are considered primary meteorological data from the meteorological observation network. This information is provided by KNMI to the general public and to professional users on a national as well as an international level;
- A second (and larger) part of the ceilometers is deployed to serve aviation customers. Accurate and real-time cloud information is an essential part of the meteorological information required for aeronautical applications. In its role as Air Navigation Service Provider for Meteorology (MET ANSP), KNMI continuously measures and distributes cloud base height and cloud cover data for all Dutch (civil and military) airports and helidecks, in accordance with International Civil Aviation Organization (ICAO), World Meteorological Organization (WMO) and European Aviation Safety Agency (EASA) regulations.
During the bidding and testing phase, KNMI compared the performance of the Lufft CHM 15k with their old operational ceilometer, reference LIDARs and visibility measurements in a special 3-month lasting acceptance test. This evaluation took place at the Cabauw Experimental Site for Atmospheric Research of KNMI (CESAR), a unique experimental facility used by the Dutch atmospheric research community. The primary goal of this test period was to verify whether the CHM15 k sensor was able to operate in accordance to the specifications provided by Lufft. One special challenge was the evaluation of low cloud detection, because there was no suitable reference instrument available for this so far. Thus, the meteorologists at KNMI needed to become creative and decided to use seven visibility sensors on a 213 m high tower for this.
In 2015, after four months of testing and comparing, the decision fell on the Lufft CHM 15k cloud sensors. It stood out due to its good price-performance-ratio, precise measurements of aerosol layers up to 15 km, the differentiation of multiple cloud layers, easy network integration as well as remote access. By implementing the sensor in its network, KNMI was confident that it would be able to sustain its duties regarding delivery of cloud information to its users for a period of, at least, 10 years. Third line maintenance activities were also part of the contract.
Another challenge was the transport of the around 70 kg heavy Lufft CHM 15k ceilometers. To ease this task, the project members developed a special caterpillar vehicle.
In close collaboration with KNMI, the Lufft developers managed to enhance the quality of the system in total, e.g. through a firmware update to stabilize it during a 24/7 operation.
The installation of the 50 ceilometers took place from 2016 to 2019 and so far involved 41 measurement sites: 7 automated weather stations (AWS), 12 offshore platforms in the North Sea, and a total of 22 ceilometers are installed at or around Defense and civil airports, including a dense network of fog stations around Amsterdam Schiphol Airport and one at Bonaire International Airport, located near Kralendijk on the island of Bonaire in the Caribbean Netherlands. In the coming years, KNMI expects expansion of the network at the offshore substations deployed within the upcoming wind farms in the Dutch North Sea.
In total, the contract between KNMI and Lufft comprised project costs of more than one million Euros.
Figure 2: Photograph of the acceptance test setup with two CHM 15k sensors, close to the tall tower of CESAR Observatory. This observatory is situated on the KNMI meteorological research site near Cabauw, located in the western part of the Netherlands (51.971° N, 4.927° E) in a polder 0.7 m below mean sea level. At the site a large set of instruments is operated to study the atmosphere and its interaction with the land surface.
Figure 3: 24 hours data sample of the CHM15k ceilometer at Cabauw on 31 October 2014, during the acceptance test. Cloud layer (in white) is detected up to 13 km height. The blue and green colors are dust layers (aerosols) reaching up to 3 km and probably contain smoke from wildfires in Canada which were transported to the Netherlands (©KNMI)
Figure 4: Transport and installation of the sensor is facilitated by a special, self-developed vehicle. ©KNMI
Figure 5: Map of the Dutch meteorological measurement network. Sites where a Lufft ceilometer is deployed (blue) or is scheduled (red), Status in November 2019. The one installed on Bonaire is not displayed on the map.