Essential Observations

The Oklahoma Atmospheric Radiation Measurement (ARM) site, which has been operating for 15 years, provides an example of detailed ground-based cloud process observations over long time series. Although these measurements contribute greatly to the improvement of process understanding and the development of process representations in models, studies that provide reliable information about cloud-climate feedbacks derived directly from these data are still lacking. One reason might be that the available data relate only to a specific, individual location. An ensemble of dedicated ground-based measurement sites positioned over a large spatial scale and over a long time period could resolve this issue. Such sites might be less well equipped than ARM or Cloudnet sites. However, as a minimum requirement, they would need to observe some main cloud properties (e.g., cloud-base and cloud-top heights, integrated liquid water path (LWP), radiative cooling and fluxes, precipitation) together with thermodynamic properties derived from radiosondes. Maintaining such an ensemble for a few years (to encompass, e.g., El Niño and La Niña events) could provide the opportunity to explore relationships between cloud properties, large-scale atmospheric circulation, and thermodynamic stratification of the atmosphere (e.g., low-level tropospheric static stability, low-level humidity gradients).

For cloud-climate feedback studies, a sustained long-term monitoring of the Earth's radiation budget (ERB) is essential. Loeb et al. (2007) suggest that with the current measurement stability of broadband radiometers, such as CERES, a record of about ten years of consistent observations would allow for the detection of some cloud-climate feedbacks. Since a substantial part of cloud feedbacks act on longer timescales, sustained ERB observation is necessary. The commitments to operate broadband radiometers onboard the EarthCARE and NPOESS NPP platforms are important, as are the contributions of instruments monitoring ERB at high temporal resolution. However, for the latter, spatial coverage is limited (e.g., GERB on MSG and ScaRaB on Mega-Tropique). In addition, data are not able to be intercompared as a result of possible instrument failures, suggesting the need for an absolutely calibrated

ERB instrument. Retrievals with such an instrument at any two times could then be compared to estimate the temporal change in the ERB.

A dataset of reanalyses, including the assimilation of satellite observations of cloudy atmospheres, may prove particularly useful for the investigation of cloud-climate feedbacks.

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