KITcube receives new Lidar systems for water vapor profiles

In addition to its property as a greenhouse gas, water vapor is the transport medium for energy in the atmosphere, because the heat used in the evaporation of water is released again during the condensation of water vapor, e.g. during cloud formation. It is therefore referred to in meteorology as latent heat which can be transported over thousands of kilometres. This can be associated with extreme events such as the major floods on the Ahr and Elbe, when large amounts of water vapor from the Mediterranean are transported to our area by stationary low-pressure systems, rise in the mid-mountain regions, condense and fall as precipitation. Water vapor is also the main energy source for severe thunderstorms with precipitation amounts of sometimes more than 200 mm (liter per square meter) per hour and hail. Water vapor in the atmosphere is subject to great variability both horizontally and vertically. The maximum content of water vapor in the atmosphere depends primarily on the temperature and increases by 7% with each degree Celsius. This means that significantly more water vapor can be present at ground level and in warmer regions (tropics) than at higher altitudes and in polar regions. Due to climate change with higher temperatures, extreme events can therefore increase in intensity. How much more water vapor the atmosphere can absorb is expressed by the relative humidity.

Precise knowledge about the distribution and transport of water vapor in the atmosphere is therefore an essential basis for weather forecasting and climate research. While the measurement at ground level with in situ methods e.g. with hygrometers or psychrometers is easy to carry out and part of every household weather station, as well as the measuring networks of the weather services worldwide, the measurement at greater height above the ground is difficult. For this purpose, radiosondes are used, small measuring devices that rise to heights of over 25 km attached to balloons and measure humidity, temperature and wind on their way and transmit the data to the ground. The KITcube of the IMKTRO includes such an automated radiosonde system together with stations for manual ascends. However, measurements with radiosondes are complex, associated with high costs and are not carried out continuously.

In early 2024, KITcube measurement capabilities were extended by five innovative Differential Absorption Lidar (DIAL) water vapor profilers, which measure the vertical distribution of water vapor in the lower four kilometers of the atmosphere every 2 minutes using a laser method. The DIAL measurement method uses two slightly different wavelengths which are sent vertically into the atmosphere, from which only one is absorbed by the water molecules. The amount of water vapor is determined from the signal difference of the backscattered laser light received by the same device. The associated measurement height results from the runtime of the laser light.

Currently, the new devices of the type Vaisala DA10 are being tested on our Testsite at Campus Nord and then integrated into the KITcube data processing and instrument control system.  Fig. 1 shows the experimental setup including the DIAL water vapor profilers, the automated radiosonde system and the IMKTRO precipitation radar. Fig. 2 shows the water vapor measurements from May 14th. The great variability of the water vapor concentration can be clearly seen both with the height and with the time. From around 08:30 UTC, the water evaporated from the ground, mainly by plants, is transported higher and higher by convection and reaches a height of 2000 m at 12:00 UTC. Smaller clouds can be seen at 14:30 UTC at a height of approx. 2200 m due to high water vapor concentrations. This moist layer reaching up to there is the atmospheric boundary layer, which is in direct exchange with the ground surface. The black line in the figure represents the maximum height up to which reliable measurements are possible. A weakness that currently still exists is shown in the transition area of two measurement zones of the device at a height of approx. 500 m, which will soon be remedied by firmware upgrades.

Over the summer, the new devices will be tested in Karlsruhe, Jülich and Innsbruck before they are then used operationally in 2025 at the TEAMx measurement campaigns in South Tyrol and NAWDIC in Brittany.