Abstract
Abstract. In situ surface observations show that downward surface solar radiation (SWdn) over the central and southeastern United States (US) has increased by 0.58–1.0 Wm−2 a−1 over the 2000–2014 time frame, simultaneously with reductions in US aerosol optical depth (AOD) of 3.3–5.0 × 10−3 a−1. Establishing a link between these two trends, however, is challenging due to complex interactions between aerosols, clouds, and radiation. Here we investigate the clear-sky aerosol–radiation effects of decreasing US aerosols on SWdn and other surface variables by applying a one-dimensional radiative transfer to 2000–2014 measurements of AOD at two Surface Radiation Budget Network (SURFRAD) sites in the central and southeastern United States. Observations characterized as clear-sky may in fact include the effects of thin cirrus clouds, and we consider these effects by imposing satellite data from the Clouds and Earth's Radiant Energy System (CERES) into the radiative transfer model. The model predicts that 2000–2014 trends in aerosols may have driven clear-sky SWdn trends of +1.35 Wm−2 a−1 at Goodwin Creek, MS, and +0.93 Wm−2 a−1 at Bondville, IL. While these results are consistent in sign with observed trends, a cross-validated multivariate regression analysis shows that AOD reproduces 20–26 % of the seasonal (June–September, JJAS) variability in clear-sky direct and diffuse SWdn at Bondville, IL, but none of the JJAS variability at Goodwin Creek, MS. Using in situ soil and surface flux measurements from the Ameriflux network and Illinois Climate Network (ICN) together with assimilated meteorology from the North American Land Data Assimilation System (NLDAS), we find that sunnier summers tend to coincide with increased surface air temperature and soil moisture deficits in the central US. The 1990–2015 trends in the NLDAS SWdn over the central US are also of a similar magnitude to our modeled 2000–2014 clear-sky trends. Taken together, these results suggest that climate and regional hydrology in the central US are sensitive to the recent reductions in aerosol concentrations. Our work has implications for severely polluted regions outside the US, where improvements in air quality due to reductions in the aerosol burden could inadvertently pose an enhanced climate risk.
Highlights
From 1930 to 2004, the eastern and central United States (US) experienced significant cooling of as much as −0.12 K a−1 (Kumar et al, 2013)
The authors conclude that aerosols reductions enhanced surface temperature through aerosol–radiation interactions, but their study did not consider the covariability between observed clear-sky SWdn and aerosol optical depth (AOD) measurements, which we argue should exist for aerosol–radiation interactions
For AOD to be a controlling factor of clear-sky SWdn, we would expect covariability between variables. To check whether this connection is robust, we develop a statistical model to predict surface monthly mean clear-sky SWdn anomalies based on AOD and cirrus cloud properties using multivariable linear regression (MLR) with no lag
Summary
From 1930 to 2004, the eastern and central US experienced significant cooling of as much as −0.12 K a−1 (Kumar et al, 2013). Previous studies have linked the US warming hole to changing patterns in sea surface temperatures (SSTs) such as the Atlantic Multidecadal Oscillation (AMO; Kumar et al, 2013; Zhang et al, 2013) or to trends in anthropogenic aerosols, which may influence meteorology by interacting with solar radiation or clouds (Leibensperger et al, 2012b; Booth et al, 2012; Yu et al, 2014). Focusing on June–July–August (JJA) during 2000–2011 across the United States, Yu et al (2014) found positive correlations between monthly mean satellite observations of aerosol optical depth (AOD) and cloud optical depth (COD, r = 0.76) and between AOD and shortwave cloud forcing (SWCF, r = 0.84), as well as negative correlations between SWCF and maximum surface air temperatures (r = −0.67) They attribute the 20th century warming hole to aerosol–cloud interactions that lead to surface cooling. Our work has special relevance for developing countries that currently experience heavy aerosol loading but are planning emission reduction strategies (e.g., Lu et al, 2011)
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