The relevance of liquid-dependent freezing processes for the precipitation budget

The formation of precipitation at mid latitudes is well known to occur via ice-phase processes. Only via the Wegener-Bergeron-Findeisen process, which describes the efficient growth of ice crystals at humidities below liquid saturation, hydrometeors can grow to sizes that lead to significant amounts of precipitation, i.e., so-called 'cold rain'. Warm-rain formation at mid-latitudes is limited to drizzle because the cloud thermodynamical processes are too weak to produce large rain droplets solely by coagulation of smaller cloud droplets.

Precipitation formation via the ice phase can be based on different freezing mechanisms. It is known that heterogeneous liquid-dependent freezing processes, such as contact freezing, condensation freezing, and immersion freezing dominate ice formation at temperatures above -25°C. Deposition freezing and homogeneous freezing take place only at lower temperatures. However, especially heterogeneous ice formation processes via the liquid phase are known to depend strongly on the type of ice nuclei. Thus, precipitation that formed solely by immersion, contact, or condensation freezing will be very sensitive to the concentration and type of available ice nuclei.

This study aims on investigating the impact of heterogeneous liquid-dependent freezing processes on the total amount of precipitation observed with the Leipzig Aerosol and Cloud Remote Observations System (LACROS) at Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig, Germany (51.3° N, 12.4° E). We use measurements of the vertically-pointing 36-GHz cloud radar Mira-36 to identify the cloud vertical extent, and a rain gauge and an optical disdrometer to quantify the amount of precipitation reaching the ground. A three-year dataset covering August 2011 to March 2014 was analyzed.

It was found that precipitation formation without deposition freezing and homogeneous freezing can still lead to high rainfall rates of 20 mm/h. In about 40% of the time the precipitation formed at cloud-top temperatures between -25 and 0 °C. In this temperature range about 40% of the total precipitation amount is formed. On average, the precipitation intensity caused by clouds with top temperatures between -25 and 0 °C was 3 mm/h which is also similar to the average total precipitation intensity of all cold-rain processes. Peak precipitation rates of 100 mm/h and more were restricted to events of deep-convection that usually reaches the tropopause level and thus cloud-top temperatures of -50 °C and below.

The derived fraction of precipitation produced at temperatures above -25 °C may be biased towards higher values due to the following reasons: (1) Precipitation may be produced even when the precipitating cloud system already started to dissolve which is usually accompanied by a decrease of cloud-top height. (2) Precipitation with rain rates of more than approximately 30 mm/h cause strong attenuation of the radar return signal which leads to an underestimated cloud top height and corresponding overestimates of the cloud-top temperature. Tracking of the evolution of the precipitation systems that passed the LACROS site as observed with geostationary satellites or with the precipitation radar network will be applied to reduce these uncertainties.

Nevertheless the derived high fraction of precipitation caused solely by liquid-dependent heterogeneous freezing processes implies an important role of the concentration and type of ice nuclei in the formation of precipitation.