Downwelling surface solar irradiance in the tropical Atlantic Ocean: a comparison of re-analyses and satellite-derived data sets to PIRATA measurements

Abstract

This paper assesses the merits and drawbacks of several data sets of solar downwelling radiation received at the horizontal surface of the tropical Atlantic Ocean where the magnitude of this radiation and its spatial and temporal variability are not well known. The data sets are compared to quality-controlled measurements of hourly means of irradiance made at five buoys of the PIRATA network for the period 2012-2013. The data sets comprise the re-analyses MERRA-2 and ERA5, and three satellite-derived data sets: HelioClim-3v5, SARAH-2 and CAMS Radiation Service v2. It was found that the re-analyses MERRA-2 and ERA5 often report cloud-free conditions while the actual conditions are cloudy, yielding an overestimation of the irradiance in such cases; and reciprocally, they report actual cloud-free conditions as cloudy, yielding an underestimation. The re-analyses exhibit more bias in irradiance in cases of medium and highlevel clouds than for low-level clouds. They correlate well with the hourly means of irradiance (as a whole, correlation coefficients greater than 0.85 for MERRA-2 and 0.89 for ERA5); they correlate very poorly with daily means of irradiance (coefficients of less than 0.48 and 0.59 for MERRA-2 and ERA5, respectively) and with the hourly and daily clearness indices (coefficients of less than 0.53 and 0.46 for MERRA-2 and less than 0.63 and 0.59 for ERA5). The irradiance pattern at both hourly and daily timescales is spatially distorted by re-analyses, especially for MERRA-2. The three satellite-derived data sets exhibit similar performances between them. The correlation coefficients are greater than 0.95 and 0.78 for irradiance and clearness index, respectively, in most cases for hourly values and 0.90 and 0.88, respectively, for daily values. The relative standard deviation of errors is of the order of 15 % for hourly values and 8 % for daily values. It is concluded that these data sets reproduce well the dynamics of the irradiance and clearness index at both hourly and daily timescales. They exhibit overestimation, with the lowest biases reached by the CAMS Radiation Service v2 and ranging between 11 and 37 W m −2 depending on the buoy. It is suggested that HelioClim-3v5 and CAMS Radiation Service v2 are suited for reproducing the spatial gradients of the irradiance and reflecting the spatial variability in the irradiance.

The Tropical Atlantic Observing System

“…All Q net products have problems achieving a balanced energy budget at the ocean surface, with an overestimation of the downward heat input to the ocean ranging from 5 to 20 W m −2 (Yu, 2019). For reanalysis fluxes, there are major uncertainties in tropical shortwave and longwave radiation associated with the long-standing problems of parameterizing tropical convective clouds and low-altitude stratocumulus clouds in reanalysis models (Trolliet et al, 2018). For satellite fluxes, major uncertainties are related to turbulent bulk flux parameterization schemes and also satellite retrieval algorithms.…”

“…Gliders have already proven valuable for measuring upper-ocean turbulence and the factors that control it (St. Laurent and Merrifield, 2017). Satellite-based retrievals of surface radiative and turbulent fluxes show considerable skill (Yu and Weller, 2007;Trolliet et al, 2018) and are likely to improve further if satellite and in situ measurements are maintained.…”

Verifying the spatial consistency of the CAMS Radiation Service and HelioClim-3 satellite-derived databases of solar radiation using a dense network of measuring stations: the case of The Netherlands

“…Several meteorological reanalyses provide estimates of E for the entire world and span several decades back in time, such as ERA-Interim, ERA5, MERRA, and MERRA-2. Published validations of these reanalyses against measurements of E performed by instruments based inland or offshore found that the bias is not constant in space and reported that these reanalysis exhibits large standard deviation of errors (Bengulescu et al, 2017;Boilley and Wald, 2015;Jones et al, 2017;Trolliet et al, 2018). It is also reported that reanalyses often exhibit cloudy conditions while actual conditions are cloud-free and vice versa.…”

“…The downwelling solar radiation received at ground level is known to be an essential variable in many domains , including weather and climate, but also life on Earth for animals, plants (Bois et al, 2008;Colombo et al, 2009;Wagner et al, 2012) and humans (Juzeniene et al, 2011), or solar energy. The solar irradiance E is the density of power received from the sun on a horizontal surface at ground level and per unit surface.…”

“…Each report contains the results of the comparison against measurements made at several stations in Europe, Africa and South America. Validations of CAMS-Rad and HC3v5 have been performed at various measuring stations in the tropical Atlantic Ocean (Trolliet et al, 2018), Brazil (Thomas et al, 2016a), Egypt (Eissa et al, 2015), Europe (Thomas et al, 2016b), Morocco (Marchand et al, 2018), and Oman (Marchand et al, 2017).…”