Solar radiation measurements compared to simulations at the BSRN Izaña station. Mineral dust radiative forcing and efficiency study

Abstract

[1] This paper presents a comparative study of shortwave downward radiation (SDR) measurements and simulations, obtained with the radiative transfer model LibRadtran, at the Baseline Surface Radiation Network (BSRN) site of Izaña Atmospheric Observatory (IZA, Spain). The analysis is based on cloud-free days between March 2009 and August 2012 (386 days), including aerosol-free and Saharan mostly pure mineral dust conditions and comparing the day-to-day, annual, and interannual variability. The observed agreement between simulations and measurements is excellent: the variance of daily measurements overall agrees within 99% with the variance of daily simulations, and the mean bias (simulations-measurements) is -0.30˙0.24 MJm -2 (-1.1˙0.9%) for global, -0.16˙0.34 MJm -2 (-0.4˙0.9%) for direct, and +0.02˙0.25 MJm -2 (+0.9˙9.2%) for diffuse SDR. Furthermore, the diurnally averaged aerosol radiative forcing (DF) and radiative forcing efficiency (DF eff ) due to Saharan mostly pure mineral dust events has been computed at Izaña Observatory. The mean DF values are -7˙1, -96˙5, and 44˙2 Wm -2 for global, direct, and diffuse BSRN SDR, respectively (mean aerosol optical depth, AOD, at 500 nm of 0.18˙0.01), whereas the mean DF eff values are -59˙6, -495˙11, and 230˙8 Wm -2 per unit of AOD at 500 nm for global, direct, and diffuse BSRN SDR, respectively. These values highlight the importance of scattering processes for mineral dust aerosols: the ratio between DF and the corresponding SDR without aerosols is 2.5% for diffuse SDR versus 0.2% for direct SDR. This illustrates the significant potential of mineral dust particles to cool the Earth-atmosphere system.

Compatibility of different measurement techniques of global solar radiation and application for long-term observations at Izaña Observatory

“…Similar values were found by García et al (2014c), they present a comparative study of GSR H measurements and simulations at BSRN Izaña station. The analysis was based on cloud-free days between March 2009 and August 2012 (386 days), including both aerosol-free and Saharan almost-pure mineral dust conditions.…”

“…The BSRN establishes strict quality controls for shortwave surface radiation and other measurements (Long and Dutton, 2002;Long and Shi, 2006). Applying the BSRN quality controls aforementioned to the Izaña GSR H measurements for solar zenith angles (SZA)< 90 • , García et al (2014c) shows that the measurements largely satisfy the quality control recommended by the BSRN obtaining less than 0.1 % of the measurements out of the physically possi-ble and extremely rare limits (see Table 1 of García et al, 2014c). Also, other validation of the GSR H measurements is routinely made by comparison with the LibRadtran model (http://www.libradtran.org; Mayer and Kylling, 2005) reporting a high consistency between simulations and measurements of GSR H (see Table 5 in García et al, 2014c).…”

“…To simulate the GSR H values, we used daily aerosol optical depth (AOD) data from AERONET (Holben et al, 1998) and precipitable water vapour (PWV) observations from Vaisala meteorological radiosondes (Miloshevich et al, 2009), launched at Güi-mar (WMO station #60018) at 11:15 UTC (Romero Campos et al, 2011). Further details about the LibRadtran simulations can be found in García et al (2014c).…”

“…where Y is obtained in the image processing method already discussed and C are coefficients computed with the LibRadtran model which has demonstrated to be a very useful measurement quality control tool (García et al, 2014c). This study is performed for a sample of cloud-free days in May and June (2015), a month in which there is a maximum SZA range.…”

“…The Izaña Atmospheric Observatory (IZO) is an optimal station to carry out this quality assessment study, since solar radiation observations have been continuously performed since the early 1920s. At IZO, the SD observations started in 1917 with a CampbellStokes sunshine recorder (henceforth, CS recorder), which was recently replaced by a Kipp & Zonen sunshine duration sensor (henceforth, CSD recorder García et al, 2012García et al, , 2014c IZO is a high-mountain observatory above a strong subtropical temperature inversion layer, which acts as a natural barrier for local pollution, and it provides atmospheric measurements representative of free troposphere conditions of the North Atlantic region due to the quasi-permanent subsidence regime typical of the subtropical region (Cuevas et al, 2013;Gómez-Pelaez et al, 2013). It is optimal for solar radiation measurements given that it has about 3436 h of sunshine per year and a mean annual number of cloudfree days of 221 (61 %) between 1916 and 2015, as well as for calibration and validation activities due to a high atmospheric stability, stable ozone total column amounts, very low column water content and low aerosol concentrations.…”