The Role of Clouds in Modulating Global Aerosol Direct Radiative Effects in Spaceborne Active Observations and the Community Earth System Model

Abstract

Observational benchmarks of global and regional aerosol direct radiative effects, over all surfaces and all sky conditions, are generated using CloudSat's new multisensor radiative fluxes and heating rates product. Improving upon previous techniques, the approach leverages the capability of CloudSat and CALIPSO to retrieve vertically resolved estimates of cloud and aerosol properties required for complete and accurate assessment of aerosol direct effects under all conditions. The global annually averaged aerosol direct radiative effect is estimated to be 21.9 W m 22 with an uncertainty range of 60.6 W m 22 , which is in better agreement with previously published estimates from global models than previous satellite-based estimates. Detailed comparisons against a fully coupled simulation of the Community Earth System Model, however, reveal that this agreement on the global annual mean masks large regional discrepancies between modeled and observed estimates of aerosol direct effects. A series of regional analyses demonstrate that, in addition to previously documented biases in simulated aerosol distributions, the magnitude and sign of these discrepancies are often related to model biases in the geographic and seasonal distribution of clouds. A low bias in stratocumulus cloud cover over the southeastern Pacific, for example, leads to an overestimate of the radiative effects of marine aerosols in the region. Likewise, errors in the seasonal cycle of low clouds in the southeastern Atlantic distort the radiative effects of biomass burning aerosols from southern Africa. These findings indicate that accurate assessment of aerosol direct effects requires models to correctly represent not only the source, strength, and optical properties of aerosols, but their relative proximity to clouds as well.

Quantifying the low bias of CALIPSO's column aerosol optical depth due to undetected aerosol layers

“…CALIPSO was launched in 2006 and has produced vertical profiles of aerosols and cloud optical properties over the globe for more than 10 years [Winker et al, 2010]. CALIOP aerosol products are widely used for aerosol studies such as aerosol long-range transport [e.g., Huang et al, 2008;Liu et al, 2008;Uno et al, 2009;Yumimoto et al, 2009], validation of and assimilation in aerosol models [e.g., Koffi et al, 2012;Kim et al, 2014;Pan et al, 2015;Sheng et al, 2015;Sekiyama et al, 2010], estimation of aerosol radiative effect [e.g., Huang et al, 2009;Oikawa et al, 2013;Andersson et al, 2015;Matus et al, 2015], and aerosol above cloud (AAC) effects [e.g., Chand et al, 2009;Kacenelenbogen et al, 2014;Liu et al, 2015].…”

The impact of lidar detection sensitivity on assessing aerosol direct radiative effects

“…CALIPSO provides vertically resolved aerosol properties over all surface types during both day and night and can more accurately separate cloud from aerosol in the same profile [ Liu et al , ]. Recognizing these advantages, recent studies have used CALIPSO to provide new all‐sky estimates of the global top of atmosphere (TOA) SW aerosol DRE [ Oikawa et al , ; Matus et al , ]. Notably, the clear‐sky ocean aerosol DRE from these CALIPSO‐based estimates [ Oikawa et al , ; Matus et al , ] are significantly smaller in magnitude than the passive sensor‐based ones [ Yu et al , ].…”

Shortwave and longwave radiative forcings of aerosols depending on the vertical stratification of aerosols and clouds

“…LWDARF sometime is not taken into account in aerosol radiative forcing studies, while LWDARF is comparable to SWDARF in AAC case. Shortwave plus longwave DARFs are calculated by using CALIPSO/CloudSat data in this study and Matus et al [9]; however, the global mean values of all-sky DARF are 1.4 Wm 2 in this study and 1.9 Wm 2 in Matus et al [9]. It is possible that the negative SWDARF of cloudy-sky condition is underestimated in this study, because COT of this study is larger than MODIS measurements and aerosol absorption in cloudy-sky condition is overestimated.…”

“…We also perform the model simulation of the global aerosol model, MIROC5-SPRINTARS [7,8], in the same time period. The calculated DARF using observation data are compared with the model simulation results and the previous study [9]. Figure 1 shows the annual mean distribution of SW and LW DARF at the top of atmosphere (TOA) in clear-sky, AAC, ABC, and all-sky cases.…”

“…Global annual-mean shortwave plus longwave DARF at TOA under clear-sky (left), cloudy-sky (middle), and allsky (right) conditions. TS: CALIPSO/CloudSat observation-based estimation of this study; M15: CALIPSO/CloudSat observation-based estimation of Matus et al [9]; MIR5: MIROC5-SPRINTARS model of this study; CAM5: CESM1-CAM5 model [9].…”