Estimation of the Uncertainty in Daytime Cirrus Cloud Radiative Forcing and Heating RatesDue to Ice Crystal Optics

The uncertainties in absolute daytime top-of-the-atmosphere (TOA) net cirrus cloud radiative forcing (CRF) and radiative heating rates are estimated at five Micro-Pulse Lidar Network (MPLNET) sites spanning the tropics to high-latitudes. One month of semi-transparent cirrus cloud (optical depth < 3.0 and cloud top temperature < –37 °C) measurements taken at 532 nm are subject to spectrally consistent optical properties of nine different ice crystal shapes, thus providing a range of possible forcing values. The daytime absolute TOA net CRF is on average positive at Barbados, Goddard Space Flight Center (GSFC), and Singapore (+0.008, +0.677, and +0.958, W∙m^–2, respectively) and negative at Kanpur and Fairbanks (–0.026 and –1.203 W∙m^–2, respectively). At Singapore, GSFC, and Fairbanks, the CRF depends on ice crystal shape; in particular, plates produce a relatively strong absolute value and decreases for bullet rosettes and columns. Uncertainty in the monthly absolute daytime TOA net CRF of cirrus is estimated based on its spread considering all particle habits at each respective site. The standard deviation ranges from 0.004 W·m^–2 at Barbados to 1.508 W·m^–2 at Singapore, or 50 – 150%. In-cloud daytime net radiative heating rates are positive at all five sites (ranging from 0.52 to 4.82 K/day) and have an estimated average uncertainty of less than 20%. The uncertainties in cirrus radiative forcing and heating characterized by assumptions regarding the ice crystal optics must be considered for other applications that rely on resolving ice cloud single-scattering properties, including satellite retrievals and numerical weather prediction.