SoRPIC (short for Solar Radiation and Phase Discrimination of Arctic Clouds) is a scientific project to investigate the spatial distribution of ice and liquid water in Arctic boundary-layer clouds, and its effects on the solar energy budget (the so-called radiative effect) and on remote sensing of such clouds.
Solar radiation is the main energy input of the Earth-Atmosphere system. Its interaction with the atmosphere, the surface, and clouds drives weather processes and the Earth's climate. Based on the physical understanding of those interactions, radiation emitted or reflected by the atmosphere is used in remote sensing to measure atmospheric properties, e.g., from satellite instruments.
The two different aspects require measurements of two radiation quantities: Irradiance and radiance.   Irradiance is used to quantify the energy budget of solar radiation. It represents the radiant energy flux (Watts, i.e., energy per time) received by a horizontal unit surface. Irradiance therefore bears the unit Watt per square meter. Radiance is used in remote sensing, it is measured by satellite sensors. It is a directional quantity and represents the irradiance per solid angle (field of view of the sensor). Its unit is therefore Watt per square meter and per steradians.

Illustration of irradiance (left) and radiance (right). Irradiance: detection from all directions. Radiance: detection only from a certain solid angle.
The Earth's total energy budget is defined by the incoming and outgoing solar and the outgoing terrestrial (or thermal infrared) radiation. Both are modified by scattering, absorption, and emission by atmospheric particles and the Earth's surface. The energy budget of the Arctic differs significantly from that of the entire Earth's global and annual average. The Arctic areas are a major sink in the Earth's energy budget. This defines the global atmospheric circulation and all related weather processes.
The Arctic regions are highly sensitive to changes in the net solar radiation. Two circumstances increase the impact of atmospheric constituents in the Arctic: the Sun is always low (which increases the optical path of solar radiation, so there is more interaction with the atmosphere), and the surface is very bright (high surface albedo due to snow and ice; the reflected radiation has a second chance to interact with the atmosphere). This results in a high variability of the Arctic climate, as it has been reported in the Arctic Climate Impact Assessment (published by Corell, Cambridge University Press, U.K., 2004). Therefore, it is important to measure the components of the Arctic radiation budget (clouds and aerosols, surface properties, and their interaction with the radiation field), and to validate ground-based and spaceborne remote sensing of Arctic climate parameters, as well as climate models.
In the Arctic, clouds (in particular, boundary-layer clouds) are of special importance in the predictions of Arctic climate warming. On an annual average, the act similar to greenhouse gases: they keep thermal infrared radiation from escaping into space. This effect exceeds the cooling due to the reflection of incoming solar radiation by these clouds. In detail, this radiative effect is highly variable and depends on the surface albedo, aerosol properties, and cloud properties such as water content, cloud-droplet or ice-crystal size, and the thermodynamic phase (ice vs liquid water). Additionally, the long periods of permanent polar day and polar night have a strong impact on the competition between solar and terrestrial radiative effects. For example, the low surface albedo in summer (the dark ocean surface, instead of the bright ice) leads to a seasonal cooling effect of Arctic clouds.
Our knowledge about Arctic boundary-layer clouds largely depends on satellite measurements. Those retrievals are based on radiance at a limited number of wavelengths. However, evaluating the cloud radiative effect and the global energy budget requires knowledge of irradiance over the entire spectral range. The conversion from radiance to irradiance is not straightforward and requires knowledge of the cloud optical properties (which is what is derived from the radiance measurement onboard the satellite). The cloud retrieval itself is accurate for several reasons, such as horizontal cloud inhomogeneities, a preselection of the thermodynamic state, a poor representation of ice-crystal properties, and uncertainties in the vertical structure and the underlying surface albedo. In order to test whether climate models can use the cloud properties retrieved from satellites, we need to check how accurately irradiance can be calculated from these cloud properties. This is only possible with combined airborne measurements of radiation (irradiance and radiance) and of cloud microphysical properties.
Several research groups therefore participate in SoRPIC, and each of them contributes their expertise in measuring different physical properties from aircraft:

• The AMALi is a lidar instrument, i.e., laser light is emitted in pulses, and the photons scattered back by the atmospheric particles are detected in the time between the light pulses. The runtime and intensity of the scattered signal yields information about the vertical structure of the optical properties of cloud and aerosol particles.
• A set of instruments (FSSP, CPI, Nephelometer, Nevzorov probe) provides the microphysical properties of the clouds, such as liquid and ice water content, shape and size distribution, and the scattering coefficient.
• The SMART-Albedometer detects irradiance (upper and lower hemisphere) and radiance (of the nadir point) above and below the cloud.
• The EAGLE is a hyperspectral camera which provides an image of the horizontal, two-dimensional distribution of radiance above a cloud. This is used to extend the precise radiance measurement of the SMART-Albedometer onto a much broader field of view.
  
• The AMSSP-EM measures the complete polarisation information (Stokes vector) of the radiation.
• A sun photometer is used to monitor the optical thickness of the atmosphere along the flight track.

Technical details and the acronyms are explained on the instruments site (left menu). Navigational and meteorological parameters are monitored, as well. All these instruments are mounted on the Polar-5 aircraft of the Alfred Wegener Institute (AWI).