Daytime Top-of-the-Atmosphere Cirrus Cloud Radiative Forcing Properties at Singapore

Abstract

Daytime top-of-the-atmosphere (TOA) cirrus cloud radiative forcing (CRF) is estimated for cirrus clouds observed in ground-based lidar observations at Singapore in 2010 and 2011. Estimates are derived both over land and water to simulate conditions over the broader Maritime Continent archipelago of Southeast Asia. Based on bookend constraints of the lidar extinction-to-backscatter ratio (20 and 30 sr), used to solve extinction and initialize corresponding radiative transfer model simulations, relative daytime TOA CRF is estimated at 2.858–3.370 W m−2 in 2010 (both 20 and 30 sr, respectively) and 3.078–3.329 W m−2 in 2011 and over water between −0.094 and 0.541 W m−2 in 2010 and −0.598 and 0.433 W m−2 in 2011 (both 30 and 20 sr, respectively). After normalizing these estimates for an approximately 80% local satellite-estimated cirrus cloud occurrence rate, they reduce in absolute daytime terms to 2.198–2.592 W m−2 in 2010 and 2.368–2.561 W m−2 in 2011 over land and −0.072–0.416 W m−2 in 2010 and −0.460–0.333 W m−2 in 2011 over water. These annual estimates are mostly consistent despite a tendency toward lower relative cloud-top heights in 2011. Uncertainties are described. Estimates support the open hypothesis of a meridional hemispheric gradient in cirrus cloud daytime TOA CRF globally, varying from positive near the equator to presumably negative approaching the non-ice-covered poles. They help expand upon the paradigm, however, by conceptualizing differences zonally between overland and overwater forcing that differ significantly. More global oceans are likely subject to negative daytime TOA CRF than previously implied.

Fu-Liou Gu radiative transfer model used as proxy to evaluate the impact of data processing and different lidar measurement techniques in view of next and current lidar space missions

“…The one-dimensional FLG radiative transfer model, developed in the early 1990's, has recently been adapted to retrieve the cloud and aerosol radiative forcing using the aerosol and cloud vertical profile of lidar extinction as input (Heymsfield et al, 2014;Campbell et al, 2016, Lolli et al, 2017. FLG calculates the direct effect of aerosol/cloud forcing at each altitude level as a function of optical depth, single-scattering albedo, phase function and asymmetry factor of the layer, and for the column as the partial contribution to the total aerosol optical depth (AOD) for each aerosol species or more simply the total 20 cloud optical depth (COD).…”

“…FLG distinguishes eighteen different types of aerosols, with single scattering aerosol properties parameterized through the OPAC (Optical Properties of Aerosol and Clouds) database (d'Almeida et al, 1991;Tegen and Lacis, 1996;Hess et al, 1998 Heymsfield et al (2014) where D e is retrieved through the atmospheric temperature (Campbell et al, 2016, Lolli et al, 2017.…”

“…The emerging prominence of ground-based lidar, however, strengthens the necessity for further studies as claimed by several papers in literature for aerosol (Pappalardo et al, 2004, Mona et al, 2006, Wang et al, 2012, Pani et al, 2016 and cloud (Lolli et al, 2013, Campbell et al, 2016, Lolli et al, 2017 optical and geometrical properties resolved from multi-spectral lidar techniques. Unlike elastic-scattering instruments, multi-spectral and Raman lidars can retrieve aerosol and cloud 20 properties with much better accuracy without strong assumptions, (e.g., Grund and Eloranta, 1991;Ansmann et al, 1992;Goldsmith et al 1998, Mona et al, 2012, Pappalardo et al, 2014, thought with greater operational expenses.…”

“…This is depending if the so-called albedo effect (or the capability of aerosols of reflecting the incoming solar light) is outweighing the greenhouse effect (or the capability of trapping/absorbing outgoing longwave radiation) 2 Clouds, especially thin cirrus clouds, represent another fundamental contributor to the earth radiative budget. Clouds are similarly subject to current uncertainty surrounding their net radiative contribution globally (e.g., Campbell et al, 2016, Lolli et al, 2017.…”

“…Since lidarderived optical properties obtained from different instrument techniques are being more frequently used to assess the radiative effects of clouds and aerosols (e.g., Campbell et al, 2016, Lolli et al, 2017, corresponding uncertainties in determining net radiative forcing, which may help reconcile inconsistencies in studies carried out at the global scale based on different lidar techniques, are compulsory, especially now that several new space missions with lidar on board have been 5 launched (Cloud-Aerosol Transport System; CATS, McGill et al, 2015) or are scheduled very soon (ESA Earth Care; Illingworth et al, 2015).…”