High‐resolution estimates of total solar and PAR surface fluxes over large‐scale BOREAS study area from GOES measurements

Abstract

Abstract. A published physical algorithm designed for estimating total solar and photosynthetically active radiation (PAR) fluxes at the Earth's surface from GOES visible imagery has been modified for the Boreal Ecosystem-Atmosphere Study (BOREAS) applications at high space and time resolutions (1 km/half-hourly). Substantive changes to the algorithm are described, along with descriptions of various additional features needed to apply the algorithm over the boreal forest. Because of the propensity of forest fire smoke to impact the BOREAS study area during the summer period, particular attention has been given to the treatment of aerosol effects. To validate the algorithm, instantaneous estimates of downwelling total solar and PAR fluxes at half-hourly time steps obtained from a GOES 7 data set are compared with 15-min averaged in situ radiometer measurements obtained during the second summer intensive field campaign of 1994 from the array of BOREAS automatic meteorological stations. The validation results have been stratified according to sky conditions to help understand the detailed nature of algorithm performance and to identify weaknesses in the current algorithm design. A variety of sensitivity tests have also been conducted to help evaluate the algorithm's strengths and weaknesses. In addition, a large-scale analysis of the retrievals over the five 1994 field campaigns has been carried out to provide background information for modelers on the nature of the solar component of the surface radiation budget across the boreal forest zone. The overall accuracies of the algorithm are 1.6% and 6.5% for total solar and PAR fluxes, with relative precisions of -20% considering all days, including those with extensive cloud cover and/or high concentrations of forest fire smoke. Such precisions are consistent with current published expectations at an hourly timescale. Better precisions of around 7% are found for clear, relatively aerosol-free days, which represents noteworthy algorithm performance in terms of expectations at these timescales. IntroductionThe Boreal Ecosystem-Atmosphere Study (BOREAS), by design, is to understand the exchanges of radiative energy, sensible heat, water, CO2, and additional trace gases between the boreal forest and the atmosphere. Of paramount importance to understanding these exchanges are quantifying the flux of total solar and photosynthetically active radiation (PAR) that is assimilated by the surface [Hall et al., 1992]. This is because the former controls the total energy exchange between the atmosphere and the surface, while the latter regulates the rate of photosynthesis.Earlier studies of the surface radiation budget (SRB) for the BOREAS region may be found in the work of Moats et al. [1994]. Their results (covering the period from 1985 to 1989) contain only the total solar component, obtained from Earth Radiation Budget Experiment (ERBE) satellite measurements and International Satellite Cloud Climatology Project (ISCCP) meteorological satellite retrievals. These results were e...

Retrieval of Daytime Surface Net Longwave Flux over BOREAS from GOES Estimates of Surface Solar Flux and Surface Temperature

“…In fact, we will show that in terms of the algorithm, this complementarity amounts to redundancy. In considering these two parameters, we have taken advantage of the fact that both are directly retrievable from GOES visible radiances at tolerable levels of uncertainty even in the presence of clouds (Gu and Smith, 1997). Furthermore, various techniques have been developed to retrieve PW and Ts over land surfaces from infrared split window temperature differences (Choudhury and DiGirolamo, 1995;Prata, 1994).…”

“…Therefore these two quantities are not perfectly intercorrelated. However, given the marginal improvement obtained by including K* in the regression algorithm, and considering the potential for significant errors in retrieving As (Gu and Smith, 1997), it is reasonable to eliminate K* from the final algorithm. Similarly, we eliminate w, leading to the following two-variable multiple linear regression:…”

“…Figure 1 indicates the locations of these sites, while Table 2 provides a summary of the dominant vegetation covers. The surface downward solar fluxes required to obtain TR were obtained from visible channel data on an 8 x 8 km spatial scale [see Gu and Smith, (1997) for details of the retrieval algorithm]. The Ts values were obtained from the available in situ AMS surface temperature measurements, consisting of within canopy air temperatures taken a few meters above the ground at the eight tree-sites and 10-cm depth soil temperatures at the two grasslandsites.…”

“…In this study, we use downward solar flux estimates derived from visible channel radiances obtained from the older GOES 7 satellite system which was operating during the 1994 BOREAS intensive field campaigns (IFCs) (for which this study is directed), and surface temperatures available from in situ ground sensors. The surface solar fluxes are estimated from a physical retrieval algorithm developed by Gu and Smith (1997), designed after the first-generation GOES algorithm of Gautier et al (1980), and similar in philosophy with the most recent version of the Gautier algorithm (Gautier and Landsfeld, 1997).…”

Remote sensing in BOREAS: Lessons learned

“…This latitude -time diagram is based on an analysis of GOES satellite measurements according to a retrieval methodology described in the studies of Gu and Smith (1997, this issue). Between the f 54 -56jN latitude zone bounded by the BOREAS SSA and NSA, where nearly all recent understanding of the relatively dark albedo properties of the boreal forest has been drawn, wintertime albedos are at their minimum, ranging from approximately 0.2 -0.3 (see also figure 12c in Gu & Smith, 1997). (Note that Betts & Ball, 1997 found individual site wintertime albedos as low as 0.11 for conifers.)…”