An Evaluation of the Shortwave Direct Aerosol Radiative Forcing Using CALIOP and MODIS Observations

Abstract

In this study, all‐sky ShortWave Direct Aerosol Radiative Forcing (SWDARF) at the top of atmosphere is estimated using the method of Oikawa et al. (2013) applied to two generations of Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Level 2 products, i.e., version 2 (V2) and version 3 (V3), and the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud product. The estimated SWDARF in Oikawa et al. (2013) was based on CALIPSO V2 product, which contained significant errors in cloud clearing and low‐altitude aerosols. This error was corrected in V3, resulting in greatly improved and significantly different aerosol and cloud distributions. In clear‐sky conditions, the magnitude of aerosol optical thickness underestimation becomes smaller and SWDARF becomes more negative using the V3 product. In addition, above‐cloud aerosols, which cause positive SWDARF, are less frequently detected and below‐cloud aerosols are more frequently detected in the V3 product than in the V2 product, so that cloudy‐sky SWDARF becomes more negative using the V3 product. From these results, clear‐sky, cloudy‐sky, and all‐sky SWDARFs become more negative using the V3 product than the V2 product. The magnitude of negative SWDARF using the V3 product is more than twice as large as the V2 product under all‐sky conditions due to V3 improvements in the lidar retrieval algorithms. Considering the uncertainties of aerosol and cloud measurements, annual zonal averages of clear‐sky, cloudy‐sky, and all‐sky SWDARFs from 60°S to 60°N are estimated as −4.0 ± 0.2, −1.1 ± 0.3, and −2.1 ± 0.2 Wm−2 from the V3 product.

Aerosol Direct Radiative Effect Sensitivity Analysis

“…Determining these spectral dependencies is often achieved or complemented by the identification of the aerosol type (dust, sea salt, etc.) which then determines the spectral shapes from a predetermined aerosol optical model (e.g., Loeb and Su 2010;Henderson et al 2013;Oikawa et al 2013;Matus et al 2015;Kato et al 2018). Generalizing the uncertainty associated with this approach is difficult since it would involved determining the accuracy of each specific method of aerosol typing and the aerosol optical models.…”

“…This conclusion is even more strongly supported by studies that have made CALIPSO-based DRE estimates. Oikawa et al (2018) showed that the magnitude of the aerosol DRE is about a factor of 7 larger in instances of aerosol below cloud rather than when aerosol is above cloud (their Table 2). The aerosol DRE from Matus et al (2015) was larger by a factor of 12 in magnitude in columns with thin cloud (COD , 1) than those with thick cloud (COD .…”

“…1). While both Oikawa et al (2018) and Matus et al (2015) based their calculations on the standard CALIPSO data products, Kacenelenbogen et al (2019) performed their own, more accurate, CALIPSO retrievals of AOD above cloud. The Kacenelenbogen et al (2019) method gives a larger (more positive) above-cloud aerosol DRE than that inferred from the standard CALIPSO data products.…”

“…Advances have been made in passive retrievals over optically thick cloud (Torres et al 2007;Waquet et al 2009Waquet et al , 2013de Graaf et al 2012;Jethva et al 2013;Meyer et al 2015) and land (Dubovik et al 2011;Lyapustin et al 2011a,b), but, even with these advanced techniques, model information is needed to make truly global all-sky estimates (e.g., Lacagnina et al 2017). Active remote sensing-for example, the lidar on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO; Winker et al 2009Winker et al , 2010-allows for more complete and consistent retrievals in all-sky conditions over both land and ocean (Henderson et al 2013;Oikawa et al 2013;Matus et al 2015;Oikawa et al 2018). However, CALIPSO's detection sensitivity limits the accuracy of the aerosol DRE inferred from its standard data products (Thorsen and Fu 2015;Thorsen et al 2017).…”

“…The shortcomings of all current satellite remote sensing observations are apparent when comparing aerosol DRE estimates over clear-sky ocean, which vary by 61.25 W m 22 (Yu et al 2006;Henderson et al 2013;Oikawa et al 2013;Matus et al 2015). This is despite uncertainties being minimized over clear-sky ocean for most types of remote sensing retrieval techniques.…”