**1. Introduction**

Albedo, also known as hemispherical reflectance, is a fundamental radiative parameter for energy partition of incoming solar radiation [1]. Albedo controls the temperature of the Earth's surface in concert with the effect of greenhouse gases. Around 30% of the total incoming radiation is reflected back into space and this is known as the planetary albedo [2]. Most of this radiation is reflected by clouds, snow and ice. Around 4% of the incoming solar irradiance is reflected by the land and ocean surface, which is some 13% of the total radiation reflected in the shortwave [3].

Systematic measurements of albedo have been acquired since the 1940s [4], although the instruments in use today were invented earlier, in the 1920s. Albedometers use a pair of calibrated pyranometers, one looking skywards and the other groundward. In order for an albedometer to cover a large enough region of the Earth's land surface, it is usually mounted at the top of a tower, which can vary in height from 10 m up to over a hundred metres. Such towers were extremely rare until the mid 1990s, so most albedo measurements covered only a small patch of ground and, almost invariably, only snow and ice or grass or concrete or tarmac from a height of a few to 10 m. This meant that such albedo measurements could not be employed to study any long-term trends, as the spatial representativeness of such measurements is very limited. Systematic observations from albedometers from the Baseline Surface Radiation Network (BSRN) network [5] started in 1992 from 10-m high towers. Towers are extremely expensive to construct and maintain and the associated electrical power and/or telecommunications infrastructure requirements makes them fairly rare. As part of the National Oceanic and Atmospheric Administration (NOAA) contribution to BSRN, the surface radiation budget network (SURFRAD) [6] tower-based radiation sensor network was founded in 1995, which now includes seven 10-metre towers. BSRN include measurements of total, direct and diffuse downward and upward radiation, mostly in the shortwave region from 300–3000 nm. FLUXNET is a "global network of regional networks" created by scientists across the world to coordinate regional and global observations from micrometeorological tower sites. These flux tower sites use eddy covariance methods to measure the exchanges of carbon dioxide (CO2), water vapour and energy between terrestrial ecosystems and the atmosphere. FLUXNET include total albedometer measurements, but only a limited number include measurement of the diffuse component. Most FLUXNET sites are located over forests.

Since the earliest days of satellite radiometer observations, methods to retrieve surface albedo from visible and near-infrared (NIR) geostationary images [7] and polar orbiting images have been developed [8]. These early images typically had pixel resolution around 4–5 km and relied on intercomparison with other satellite data products [9], with little, if any, intercomparison using field-measured albedos. The first example of broadband (shortwave) albedo validation using tower albedometer measurements, upscaled by Landsat 30-m inferred albedos, were made in the early 2000s [10]. After this initial work, effort was focused on finding homogeneous sites to directly compare tower albedometer measurements with satellite-derived albedos from 500 m–3 km [11] without the need for upscaling. Historically, there has been a lack of an appropriate upscaling method for comparing multi-scale albedo measurements. In this work, we develop a general framework based on analysing time series of tower albedometers to retrieve bi-directional reflectance distribution function (BRDF) along with bi-hemispherical reflectance (BHR), called in one particular theoretical case "white sky albedo", with uniform sky irradiance alongside direct hemispherical reflectance (DHR), usually referred to as "black sky albedo" [12]. The materials used in this study include tower measurements derived from the FLUXNET, SURFRAD, and BSRN tower sites, and satellite data products derived by the Copernicus Global Land Service (CGLS) from VEGETATION-2 and Proba-V [13], moderate resolution imaging spectroradiometer (MODIS) [14] and multi-angle imaging spectroradiometer (MISR) [15]. The aim is to develop a new method for comparing ground-level, in situ measurements derived from a tower albedometer against coarse resolution albedos derived from repeat-pass or near-simultaneous multi-angle spaceborne observations. Specific objectives include developing a new method for deriving DHR and BHR from tower albedometer measurements; comparing these

derived in situ DHR and BHR tower albedos against CGLS, MODIS and MISR products through a pixel-to-point analysis over a long time-series. A new technique is also developed for upscaling albedo from tower to a coarse resolution based on atmospherically corrected BRFs from high-resolution Earth observation (EO) data, combined with downscaled MODIS BRDF climatology over a larger area. A pixel-to-pixel comparison is presented between DHR and BHR retrieved from CGLS products and DHR and BHR upscaled from in situ measurements using this proposed upscaling technique. The sites are both homogeneous and heterogeneous in land cover and reflectance, and are located on all the continents including Antarctica.
