Ceilometers as a diagnostic tool for forecasters

In operational meteorology, ceilometers have been widely used for decades. The initial purpose of detecting cloud bases has greatly expanded. As a ground-based LIDAR instrument, the ceilometer provides high temporal resolution vertical profiles of backscatter signals up to 15000 m, depending on the model and configuration. Ceilometers are very important at airports where information on the heights of low clouds is essential for aviation operations, especially in so-called low visibility procedures.

The main advantage of ceilometers is their ability to provide continuous high-frequency measurements every 10-60 seconds. At Croatian airports, Vaisala ceilometers are in use, and together with Lufft instruments operated by the National Meteorological Institute (DHMZ), they form a national ceilometer data network. This spatial representation is important because weather conditions in Croatia can vary significantly between continental, mountain and coastal regions. In Croatia control, we developed visualisation for the forecasters, which includes raw backscatter data profiles (up 15km, PBL zoom up to 1200 - 3000m and up to 150m) together with derived cloudiness, cloud base height over time and direct algorithm cloud data output which is used operationally. From a novel Vaisala CL61 ceilometers at some airports, the linear depolarisation ratio (LDR) is also visualised and used to distinguish lithometeors and hydrometeors. Dozens of examples of typical and particular cases will be shown in the presentation (e.g. convection, growth of PBL, fog, stratus, drizzle, aerosol layers, snow and hail). In order to understand and recognise patterns, a targeted education for forecasters (and observers) is needed, which we started in 2019. 

Using the ceilometer network, which complements the observing system, forecasters can better diagnose weather processes in the atmosphere. This particularly refers to the boundary layer and area around the airports where crucial landing and take-off operations are performed.