Measuring cloud supercooled liquid water
On 21 February it was raining and we did radiosonde launches together with the cloud sensor to measure the amount of the supercooled liquid water content in the clouds. At temperatures between 0ºC and -15ºC most clouds are composed of supercooled water droplets. Between -15ºC and -40ºC clouds typically made of a mixture of ice crystals and supercooled water droplets. The presence of supercooled liquid water in clouds occurs because of the relatively low amounts of cloud ice nuclei. At temperatures below -40ºC ice will form spontaneously (without ice nuclei).
In Antarctica, satellite observations showed an important presence of mixed-phase clouds containing supercooled liquid water. The amount of the supercooled water in cloud is important for the formation of precipitation and also cloud radiative forcing at the surface (how much clouds cools and/or warm the surface by reflecting shortwave radiation and emitting longwave radiation).
Supercooled liquid water in clouds pose icing problems for the planes and in fact this principle of icing of metallic objects forms the basis of our measurements. The sensor we use is called Universal Water Content (UWC) sensor and is manufactured by the company Anasphere. It has two vibrating wires, one for the total water content (warm and supercooled) and one only for the supercooled water content. When passing through a cloud with supercooled water - icing occurs on the wire reducing its frequency of vibration. The second wire is coated with hydrophilic gel, absorbing water droplets at warm temperatures. From this we can calculate cloud water content both for above and below 0ºC temperatures.
Photo: Testing UWC on the ground before the launch: this allows to see the wires behaviour during the rain and at different wind speeds before the launch.
In meteorology we use the so-called skew-T diagram to have the first-order look into the structure of the troposphere and lower stratosphere using radiosondes. The temperature lines are skewed at 45º angle (see the thin black lines go from the bottom left to the upper right for temperature and magenta lines for the dew-point temperature). In the skew-T diagram on 21 February we see the entire troposphere being saturated (cloudy) and relatively warm below 600 hPa (the temperatures below 600 hPa decreasing much slower with height). At 300 hpa temperature stops decreasing and then starts to increase - this is the tropopause boundary where troposphere ends and lower stratosphere starts.
Figure: Skew-T profile on 21 February 2023, 21 UTC.
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