All‐Sky Aerosol Direct Radiative Effects at the ARM SGP Site

Abstract

All‐sky aerosol direct radiative effect (DRE) was estimated for the first time at the Atmospheric Radiation Measurement Southern Great Plains site using multiyear ground‐based observations. The NASA Langley Fu‐Liou radiation model was employed. Observed inputs for the radiation model include aerosol and cloud vertical extinction profile from Raman lidar; spectral aerosol optical depth, single‐scattering albedo, and asymmetry factor from Aerosol Robotic Network; cloud water content profiles from radars; temperature and water vapor profiles from radiosondes; and surface shortwave spectral albedo from radiometers. A cloudy‐sky radiative closure experiment was performed. The relative mean differences between modeled and observed surface downwelling shortwave total fluxes were 6% (7%) for transparent (opaque) cloudy‐skies. The estimated annual mean all‐sky aerosol DRE is −2.13±0.54 W m−2 at the top of atmosphere (TOA) and −5.95±0.87 W m−2 at the surface, compared to −3.00±0.58 W m−2 and −6.85±1.00 W m−2, respectively, under clear‐sky conditions. The seasonal cycle of all‐sky aerosol DRE is similar to that of the clear‐sky, except with secondary influences of the clouds: The cloud radiative effect is strongest (most negative) in the spring, which reduces the all‐sky aerosol DRE. The relative uncertainties in all‐sky aerosol DRE due to measurement errors are generally comparable to those in clear‐sky conditions except for the aerosol single‐scattering albedo. The TOA all‐sky aerosol DRE relative uncertainty due to aerosol single‐scattering albedo uncertainty is larger than that in clear‐sky, leading to a larger total relative uncertainty. The measurement errors in cloud properties have small effects on the all‐sky aerosol DRE.

An Investigation of the Ice Cloud Detection Sensitivity of Cloud Radars Using the Raman Lidar at the ARM SGP Site

“…There are 315,224 total RL profiles from August 2008 to December 2014. The clear-sky and opaque cloudy-sky occurrence fractions are 0.39 and 0.31, which is in good agreement with Balmes and Fu [43] (see their Figure 2). Note that the small differences in occurrence fractions in this study compared with Balmes and Fu [43] are due to difference in the time period considered.…”

“…We follow Wu et al [42] and Balmes and Fu [43] to set up the RT model inputs. The temperature and water vapor profiles are from radiosondes, interpolated to the RL-FEX vertical resolution [35,36].…”

“…The MODIS MCD43C1 Version 6 Bidirectional Reflectance Distribution Function and Albedo Model Parameters data set [48,49] are used for surface albedo. We linearly interpolate the monthly mean surface albedo from seven spectral bands to the model wavelengths; the values of the closest observational wavelengths are used for the model wavelengths outside the MODIS spectral bands [42,43]. The total sky albedo combines the black-sky and white-sky albedo weighted by the diffuse ratio.…”

“…The ice cloud effective diameter is derived as a function of temperature following Heymsfield et al [50]. For SW calculations, we run the RT model with the diurnal cycle of solar zenith angle from sunrise to sunset at an interval of 30 min, following Wu et al [42] and Balmes and Fu [43]. For LW calculations, only one RT simulation is considered.…”

An Overview of Aerosol Properties in Clear and Cloudy Sky Based on CALIPSO Observations

“…Due to the fact that a cirrus layer is able to modify aerosol direct radiative effect at the TOA and heating rates in the atmosphere, it is important to quantify the degree to which of cirrus clouds change the underlying aerosol direct effect in order to obtain an accurate estimate of the role of aerosols in Earth's radiation budget. The ground‐based observations such as from the ARM program provide important measurements of clouds, aerosols and radiation that will serve for simulating and constraining regional aerosol direct radiative effect at the TOA in both clear and cloudy skies as demonstrated in two recent studies (Balmes & Fu, 2021; Wu et al., 2021).…”