Overview

Scientific Motivation

Clouds represent the primary atmospheric influence on the surface radiation budget, which in turn provides the energy for boundary layer processes. The response of clouds to changes in climate, whether anthropogenically forced or resulting from natural variability, is the largest uncertainty in short- and long-term predictability. Comprehensive observations that provide information for understanding changes in clouds through time as well as understanding of fundamental cloud physical processes are required in a range of climatic regimes. Determining an instrument suite that is robust, relatively operationally efficient, while providing the most comprehensive and accurate information on cloud physical properties is a driver for this study.

Experimental Design

All participating instruments will be co-located at the Table Mountain Facility within a 150 m² area, and the RadSys systems within a 2 km² surrounding area. Since each instrument has a different field-of-view, siting is done in consideration of this fact, ensuring that hemispheric FOV instrument are unobstructed and narrow FOV instruments are representative of the wider FOV. All instruments will sample at their native or typically operating temporal resolutions, and comparisons will be made at the largest sampling resolution or as specific comparisons dictate.

Participants and Products

Data Availability and Access:

Data will be shared and made available with the participants of the CPEX campaign. Use of the data for reports and publications requires permission and consultation with the other participants, especially given some of the products are preliminary and in development.

Background on Cloud Physical and Radiative Properties

The surface cloud radiative effect (CRE) is a measure of the impact of clouds on the surface radiation budget. For a cloud field, CRE is a function of cloud cover, cloud optical depth which is dictated by cloud microphysical properties, and cloud field geometrical configuration or morphology which are cloud macrophysical properties.

Cloud microphysical properties are internal to the cloud and include the cloud drop size distribution, commonly represented by an effective radius r_e, the cloud drop number concentration N_d, and the liquid water path LWP. These properties are all physically related to each other and the cloud optical depth by τ_c ∝ LWP/ r_e. For a given LWP, τ_c ∝ N_d^1/3. Further, droplet number concentrations N_d are related to aerosol number concentrations N_a in a way that is dictated by aerosol size and composition and the available water and cloud dynamics/thermodynamics.

Cloud cover (cloud fraction, f_c), cloud base height (CBH), cloud geometric thickness (CTH), and cloud layering are all macrophysical characteristics of a cloud field. Cloud base height (CBH) is a primary determinant of longwave irradiance, but also impacts the shortwave as well, especially through interactions with the surface. Thus cloud height is a major indicator of the cloud radiative forcing. In the case of multi-layer clouds, the relative heights, optical depths, and overlap characteristics can greatly impact radiative transfer by creating a highly multiple-scattering environment.

In addition, the dynamics of the boundary layer play an important role in the vertical transport of aerosols and moisture which affect cloud formation and dissipation. Simultaneous measurements of boundary layer dynamics such as the wind, turbulence kinetic energy (TKE) and boundary layer depth along with the radiation variables allow a more thorough investigation of the boundary layer processes occurring in cloud topped boundary layers. It is also important to measure and understand the vertical structure of the aerosol as this perturbs not only the radiation reaching the surface, but also the cloud formation.

Measurements of Cloud Properties

The surface radiation budget is typically observed by hemispheric field-of-view broadband (passive) radiometric measurements in the shortwave and longwave and these measurements can be used to retrieve some cloud properties. These instruments integrate the total irradiance from the cloud field in that ~180° sky view. In overcast conditions this sky sample may be fairly uniform and representative of a single cloud. In broken or partly cloudy conditions this includes diffuse irradiance from the cloud scattering and direct irradiance from the clear areas between clouds. Thus, some of the critical properties discussed above can only be accurately observed with narrow field-of-view (active or passive) radiometric instruments. Ultimately, the instrument field-of-view must be filled with the cloud being measured, else radiation from clear-sky portions of the scene will confound the retrieval of cloud properties. The exception is inferring cloud fraction from hemispheric FOV measurements. A goal for a comprehensive, cloud-sensing instrument suite is to combine measurement techniques that provide each of the critical properties for determination of CRE with acceptable uncertainty.