CIRES (Cooperative Institute for Research in Environmental Sciences)

Cloud feedbacks remain the largest source of uncertainty in future climate predictions. Simulations robustly project an increase in cloud height, which is supported by some observational evidence. However, how much of this increasing trend is due to climate warming and how much is due to multiyear natural variability still remains unclear because of the brevity of existing observational records. Here we estimate when the signal will become detectable at 95% confidence by existing radar technology. We use output from a Representative Concentration Pathway 8.5 Community Earth System Model version 1 simulation in a Monte Carlo analysis to determine (1) what is the first year at which changes in the altitude of high cloud can be confidently estimated if we continue to fly W-band cloud radar, (2) what radar sensitivity is required to detect those changes, and (3) at what latitude will we first detect these changes? In Community Earth System Model version 1 a cloud radar record would be able to confidently detect upward shifts in cloud height over 20-60 degrees N before 2030 for a radar with a sensitivity of -15dBZ and stable calibration errors of 0.25dBZ. Furthermore, vertical resolution could be degraded to 1.6km with little effect on detection year. Results are more sensitive to the magnitude of calibration errors than to the minimum detectable echo. Our earlier midlatitude detection contrasts with a previous lidar-based analysis, which may be due to radar detecting different parts of the clouds and our use of simulations that account for changing geographical patterns of forced warming through time.

https://doi.org/10.1029/2018JD030242

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