Daytime Cirrus Cloud Top-of-the-Atmosphere Radiative Forcing Properties at a Midlatitude Site and Their Global Consequences

Abstract

One year of continuous ground-based lidar observations (2012) is analyzed for single-layer cirrus clouds at the NASA Micro Pulse Lidar Network site at the Goddard Space Flight Center to investigate top-of-the-atmosphere (TOA) annual net daytime radiative forcing properties. A slight positive net daytime forcing is estimated (i.e., warming): 0.07–0.67 W m−2 in sample-relative terms, which reduces to 0.03–0.27 W m−2 in absolute terms after normalizing to unity based on a 40% midlatitude occurrence frequency rate estimated from satellite data. Results are based on bookend solutions for lidar extinction-to-backscatter (20 and 30 sr) and corresponding retrievals of the 532-nm cloud extinction coefficient. Uncertainties due to cloud undersampling, attenuation effects, sample selection, and lidar multiple scattering are described. A net daytime cooling effect is found from the very thinnest clouds (cloud optical depth ≤ 0.01), which is attributed to relatively high solar zenith angles. A relationship involving positive/negative daytime cloud forcing is demonstrated as a function of solar zenith angle and cloud-top temperature. These properties, combined with the influence of varying surface albedos, are used to conceptualize how daytime cloud forcing likely varies with latitude and season, with cirrus clouds exerting less positive forcing and potentially net TOA cooling approaching the summer poles (not ice and snow covered) versus greater warming at the equator. The existence of such a gradient would lead cirrus to induce varying daytime TOA forcing annually and seasonally, making it a far greater challenge than presently believed to constrain the daytime and diurnal cirrus contributions to global radiation budgets.

Technical note: Fu–Liou–Gu and Corti–Peter model performance evaluation for radiative retrievals from cirrus clouds

“…As described in both Campbell et al (2016) and Lolli et al (2017), daytime is specifically defined in these experiments as those hours where incoming net solar energy exceeds that outgoing. Only under such circumstances can the net TOA CRE term become negative.…”

“…for both FLG and CP. The method to estimate MPLNET cirrus cloud optical properties is described in Lewis et al (2016) and Campbell et al (2016), for both 20 and 30 sr solutions from the unconstrained singlewavelength elastic lidar equation at 532 nm . The latter constraint provides "bookend" estimates for TOA CRE designed to approximate system variance.…”

“…A summary of this dataset and net daytime TOA CRE results can be found in Campbell et al (2016). As this site is land-locked, only the single albedo was, again, used, though varied monthly based on climatological passive satellite estimates.…”

“…In particular, TOA CREs are evaluated by means of radiative transfer modeling, designed with different degrees of complexity. What is not yet known is how the relative simplicity of some models translates to a relative retrieval uncertainty, given that the CRE effect of cirrus clouds, at both the ground and TOA, is typically on the order of 1 W m −2 (e.g., Campbell et al, 2016;Lolli et al, 2017). Whereas some studies show the relative uncertainty of such models as static percentages (Corti and Peter, 2009), the absolute magnitude of uncertainty with respect to cirrus CRE is necessary to understand whether or not they fit within acceptable tolerance thresholds sufficient for quantitative use.…”

Vertical Profiling of Aerosol Types Observed across Monsoon Seasons with a Raman Lidar in Penang Island, Malaysia

“…7). An accurate description of the model can be found in Lolli et al (2016) and in Campbell et al (2016). The FLG model evaluated the radiative effect (calculated as the difference between aerosol and pristine condition cases) at surface (SFC) and top of the atmosphere (TOA) for three different measured aerosol profiles:…”