To improve the understanding of the impact of boundary layer clouds on the earth's climate system, helicopter measuring flights over the Azores (39°N, 28°W) are planned for June and July 2017. Over this area extended fields of stratocumulus clouds frequently occur, especially in June/July and October/November. The homogeneous water surface allows studying the processes in stratocumulus clouds without the impact of an inhomogeneous surface.

Figure 1: Schematically overview of the conditions and processes at the top of a stratocumulus layer. (adapted from Wood, 2012)

Fig. 1 gives a schematically overview of the conditions and processes at the top of a stratocumulus layer. The cloud layer (grey) is shown in the lower part of the figure, whereas the free troposphere is located near the upper edge of it. Between both the EIL (Entrainment Interfacial Layer) is located, representing the transition between the cloud layer and the free troposphere. The Entrainment ist represented by the red arrows. Absorption and emission of terrestrial infrared radiation as well as absorption of solar radiation within the cloud and the atmosphere generates heating and cooling rates, normally resulting in a weak warming near cloud base and a stronger cooling near the top of the cloud. The vertical profile of the radiative cooling due to emission of terrestrial infrared radiation near the cloud top is shown in the left part of the figure, showing the strongest cooling right at the top of the cloud layer. Due to entrainment and thus a stronger evaporation of the cloud droplets evaporative cooling is also enhanced near cloud top. Both radiative cooling/warming and evaporative cooling lead to an increasingly unstable atmospheric layering (convective instability), as colder air parcels with higher density are located over warmer air parcels with lower density. This leads to mixing and the generation of turbulence within the cloud. The right part of the figure contains a typical vertical profile of the temperature (T) in the environment of stratocumulus clouds. Over the cloud top often exists a distinct temperature inversion with a strength of 5 to 10 K, which weakens the rising motions and thus prevent deep convection. In the region of the Azores the temperature inversion develops due to subsidence within the subtropical belt of high pressure. During the subsidence the air warms up and creates a persistent subsidence inversion.

Figure 2: Planned measurement setup with the two helicopter-borne measuring platforms SMART-HELIOS 2.0 (1) und ACTOS (2).

The objectives of the campaign are (a) to improve the understanding of the fine-scale structure of the EIL, (b) to quantify the influence of the EIL on the entrainment in stratocumulus clouds, and (c) to quantify the role of radiative heating and cooling rates in cloud entrainment and convection processes. Due to the existence of so called feedback mechanisms between the processes in a stratocumulus cloud numerous meteorological parameters need to be measured to quantify the processes and the correlation between them. Fig. 2 shows the planned measurement setup. The helicopter (Bo-105CB4) carries two measuring platforms on an overall 170 m long cable. SMART-HELIOS 2.0 (1) is located 20 m below the helicopter and ACTOS (2) is fixed at the end of the cable. SMART-HELIOS 2.0 (HELIcopter-borne Observations of Spectral Radiation) flies over the cloud and measures radiative quantities. It is maintained by LIM. ACTOS (Airborne Cloud Turbulence Observation System) is located within the cloud layer and thus measuring cloud microphysical, dynamic, thermodynamic and radiative quantities within the cloud. This platform is maintained by TROPOS. The helicopter enables measurements up to 3000 m height and flight durations up to 2 hours. Due to the relative low horizontal airspeed of about 20 m s-1 also small-scale variations can be measured and vertical profiles of the lower troposphere can be derived with a smaller horizontal drift.

SMART-HELIOS 2.0 contains up- and downward looking pyrgeometers and pyranometers to measure the broadband terrestrial (TIR) and solar (SW) radiation, respectively. Together with the radiation sensors on ACTOS this setup allows the derivation of heating and cooling rates. A downward looking radiance inlet combined with two grid spectrometers is needed to measure the spectral distribution of the solar and near infrared radiation. Thus the optical depth of the clouds as well as the effective radius of the cloud droplets can be derived. A downward looking infrared camera will be used to measure the fine-scale variability of the brightness temperature (Tb). With the camera variations of the cloud top temperature down to 50 mK can be measured with a horizontal resolution of less than 10 cm. To measure the distance (d) between the platform and the cloud top, the usage of a laser altimeter is planned. This allows on the one hand side the measurement of variations in cloud top height and on the other hand side the identification of the position of ACTOS relative to the top of the cloud. A GPS receiver and a position sensor complete the platform.

Last modification on 2016/8/17 by Felix Lauermann