SPECS - SPEctral Cloud microphysicS model

SPECS is a spectral cloud microphysics model. The aerosol, droplet, and ice particle size or mass spectra are divided into fixed size classes (Fig. 1). The Linear Discrete Method (LDM, Simmel et al., 2002) is used to sort the microphysical sizes into the fixed size bins. The various microphysical processes are calculated as explicitly as possible. Predicted variables are the number, liquid water, ice and aerosol mass mixing ratios.

Fig. 1: Schematic representation of the spectral description of hydrometeors and the microphysical processes that shift number and mass within the spectrum, between the spectra as well as between condensate and vapor phase.

SPECS has been continuously developed and refined. The formation of liquid droplets, their interaction with water vapor (condensation, evaporation) and their collision processes within a cloud were described in a discrete spectrum  (Simmel et al., 2002; Simmel and Wurzler et al., 2006). This stage of development of the model system has been successfully applied to the simulation of low orographic clouds (Simmel et al., 2005). With the description of immersion and contact freezing (Diehl et al., 20062007), important processes for the formation of cloud ice were introduced. For this purpose, a second discrete spectrum for ice particles was implemented. The developed description of the freezing and melting processes is intermediate between the spectrum for liquid droplets and that for ice particles. In recent years, the description of ice microphysics has been further refined (Simmel et al., 2015). 


SPECS is thus a comprehensive model of cloud microphysics, which in particular allows investigations of the importance of aerosol properties for the formation of cloud droplets, cloud ice and ultimately precipitation. Special attention is paid to the description of aerosol-cloud interactions, e.g. the formation of cloud droplets at so-called cloud condensation nuclei (CCN) and the freezing of cloud droplets triggered by certain aerosol particles, so-called ice nucleating particles (see also cloud microphysics). For example, Figs. 3 and 4 show a more pronounced mixed phase with COSMO-SPECS, in which cloud droplets and ice particles coexist, and less formation of precipitation.

SPECS can be embedded in different dynamic forcings. Originally developed as a pure box model, SPECS can also be applied within air parcel models for idealized cases (constant updraft velocity, convective forcing, trajectory from orography or 3D models, observed updraft velocity profiles, ...). In addition, SPECS was coupled to the meteorological forecast model of the German Weather Service COSMO (formerly LM, now ICON) (Grützun et al., 2008). With COSMO-SPECS, the feedbacks on the dynamics of the more complex description of the cloud microphysics by SPECS compared to COSMO’s original 1- or 2-moment cloud microphysics can be investigated. Furthermore, the model system allows the simulation of realistic scenarios, which can be used to accompany field campaigns (Figs. 3 & 4).

Fig. 2: Time evolution of the spectrum of the combined droplet and ice crystal mass mixing ratio for a model run for the CyCare campaign. While the cloud droplets and ice crystals (1-100 µm) are located at about 3-7 km (middle part of the spectrum), large droplets and ice crystals (> 100 µm) can be seen at the right edge of the spectrum, which have sufficient falling velocities to reach the ground as precipitation.

Fig. 3: Time-height section of hydrometeor classification comparing COSMO (2-moment scheme) and COSMO-SPECS for a simulation over Cyprus on 24.01.2017 (CyCare -campaign).

Fig. 4: Time-height cross section of ice water content comparing COSMO (2-moment scheme) and COSMO-SPECS for a simulation over Cyprus on 24.01.2017 (CyCare -campaign).