Satellite-based albedo data

What is ground albedo

Ground albedo is the fraction of solar irradiance reflected by the Earth's surface, calculated as reflected horizontal irradiance (RHI) divided by global horizontal irradiance (GHI). It is dimensionless, ranging from 0 (perfect absorber) to 1 (perfect reflector). Most natural land surfaces have albedo values between 0.15 and 0.35; water bodies are lower (approximately 0.05), while fresh snow or clean ice can exceed 0.85.

Albedo determines how much reflected solar radiation reaches a PV array, which makes it a critical parameter for calculating global tilted irradiance (GTI) — see Solargis satellite-based model — and for bifacial PV systems, where albedo directly influences the reflected solar radiation reaching the rear side of the modules. The traditional assumption of surface albedo as a constant value has been increasingly challenged with the advent of bifacial photovoltaic (PV) technology, which relies on accurate albedo data to maximize the harnessing of reflected solar energy.

Ground-based albedometer measurements are generally more representative of local surface conditions, but they are typically available only for limited periods. Satellite-derived albedo estimates, such as those available through the MODIS and VIIRS data products, offer a robust alternative with a long historic record and global coverage. Additionally, reanalysis datasets derived from numerical weather prediction (NWP) models provide gap-free albedo inputs, further enhancing data reliability.

Solargis provides ground albedo in two formats: a daily albedo Time Series covering February 2000 to present, more suited to the due diligence phase, and gap-free long-term monthly averages for 2006 to 2015, generally recommended during the pre-feasibility stage.

Figure 1: Spatial representation of satellite-based albedo for a sample site in USA.

How Solargis derives albedo from satellite data

Albedo is estimated based on surface reflectance data captured by instruments aboard satellite platforms. One such instrument is the Moderate Resolution Imaging Spectroradiometer (MODIS), which operates on the Terra and Aqua satellites. These satellites are strategically positioned in complementary orbits: Terra crosses the equator from north to south in the morning, while Aqua follows an opposite path, crossing from south to north in the afternoon. Together, they provide near-global coverage of the Earth's surface every 1 to 2 days.

Since 2012, the MODIS record has been extended and complemented by the Visible Infrared Imaging Radiometer Suite (VIIRS), which operates on the Suomi-NPP, NOAA-20 (JPSS-1) and NOAA-21 (JPSS-2) platforms. VIIRS applies the same BRDF-based retrieval approach as MODIS and delivers both white-sky albedo (WSA) and black-sky albedo (BSA). Operating several platforms in parallel increases the number of valid observations available for each day and safeguards the continuity of the albedo record as individual missions reach the end of their operational life.

Albedo is not typically provided directly as a standalone satellite product, even with MODIS or VIIRS data. Instead, it is derived from two distinct components, black-sky albedo (BSA) and white-sky albedo (WSA), which represent surface reflectance under entirely direct and entirely diffuse incoming solar radiation respectively (see Surface albedo and reflectance: Review of definitions, angular and spectral effects):

To calculate the effective albedo value, the diffuse fraction (D2G) data from the Solargis irradiance database is applied, combining black-sky and white-sky albedo values — for how Solargis separates global irradiance into its direct and diffuse components, see Solar irradiance components. This integration allows for a more accurate representation of albedo under varying solar radiation conditions.

Daily albedo Time Series

The Solargis albedo Time Series provides daily albedo values from February 2000 to present at 0.5 km spatial resolution, derived from MODIS and VIIRS satellite products. It contains data gaps. For how Solargis structures time series and TMY datasets in general, see Time Series and TMY data.

Data sources for the daily albedo Time Series

For the Solargis albedo Time Series, only data sources with a long historic record and a temporal resolution of at least daily values are used. Solargis currently derives the daily albedo Time Series from a combination of MODIS and VIIRS satellite products:

• MODIS (MCD43A3) provides the historical record from February 2000 onward.

• VIIRS extends and complements this record from January 2012 to present through three platforms: Suomi-NPP (VNP43MA3), NOAA-20 (VJ143MA3) and NOAA-21 (VJ243MA3).

All VIIRS products used are the Moderate-resolution (M-band) albedo products at 1 km spatial resolution, providing white-sky albedo (WSA) and black-sky albedo (BSA). However, these products present missing data (data gaps), mainly due to persistent cloudiness.

Note:

• Data quality issues in the MODIS MCD43A3 product have been reported at some locations from 2025 onward and the Suomi-NPP VNP43MA3 product is scheduled for discontinuation in November 2026. The NOAA-20 (VJ143MA3) and NOAA-21 (VJ243MA3) products ensure continuity of the Solargis albedo Time Series beyond that date.

• This does not affect your data since we are constantly harmonizing the data sources to provide you with the best possible albedo Time Series data.

When to use the daily albedo Time Series

Some examples of situations when the use of the daily albedo Time Series is more appropriate than long-term monthly averages include:

• Satellite-derived albedo is to be compared with ground-based albedo measurements.

• Monthly average data may not be sufficient for considering individual stochastic events such as ephemeral snow. Only the averaged effect of such events is considered.

• The land cover of the project location may have changed over time, and an average of years 2006-2015 may not represent the current situation.

• The land cover at the project location is quite heterogeneous, and the higher native resolution of the MODIS-based Time Series (0.5 km) gives more representative results than the 1 km long-term monthly dataset.

Technical specifications of the daily albedo Time Series

Figure 2: Representation of albedo daily time series for a site in Almeria, Spain.

Long-term monthly albedo averages

The Solargis long-term monthly albedo averages provide a gap-free global dataset of 12 monthly means plus one annual mean, at 1 km spatial resolution, covering the 10-year period 2006 to 2015.

How the gap-free monthly dataset is built

Gaps in satellite-derived albedo data are unavoidable, often caused by persistent cloud cover or misdetections, and these gaps are reflected in historical time series datasets. Consequently, temporal and spatial gaps are common in time series data derived from MODIS and VIIRS. The size of these gaps is influenced by the dataset's spatial resolution, while their duration depends on the persistence of cloudy conditions.

Despite these challenges, monthly averages can be calculated to create gap-free datasets. This is achieved by combining pre-processed MODIS products with additional datasets from meteorological reanalysis databases, such as those from the ECMWF ERA-5 and NASA MERRA-2 projects.

The primary advantage of reanalysis-based albedo calculations is their ability to provide consistent, global data for any surface without gaps. However, the trade-off is the lower spatial resolution compared to other methods.

To calculate monthly averages of albedo, gap-filled snow-free MODIS product MCD43GF is used as the primary data source. However, since this MODIS product does not include albedo values for ephemeral snow, additional information from other MODIS products and ERA-5 reanalysis data are combined to prepare a gap-free high-resolution dataset for all kind of land surfaces (including ephemeral snow). Finally, additional information from National Snow and Ice Data Center (NSIDC) is used to add the albedo of ocean ice. To correct imperfections and errors related to imprecisions of the MODIS land-sea mask we use additional layers from Solargis database.

The obtained dataset offers gap-free albedo information with high spatial resolution and global coverage, making it suitable for a variety of applications in solar energy and environmental modeling.

When to use long-term monthly albedo averages

Long-term monthly albedo averages are generally recommended during the pre-feasibility stage, where they offer a broad overview of the albedo conditions at a site. They are also appropriate when:

• A gap-free dataset is required. Unlike the daily albedo Time Series, the long-term monthly averages contain no missing values.

• The location includes surfaces not covered by the daily MODIS and VIIRS land products, such as ocean ice, which is included in the monthly dataset using NSIDC data.

Technical specifications of the long-term monthly albedo averages

Figure 3: Representation of albedo monthly averages for a site in Almeria, Spain.

Validation against ground measurements

Final albedo products from Solargis have been validated using available ground measurements from albedometers. An albedometer comprises two pyranometers mounted in opposite directions, parallel to the horizontal surface, to simultaneously measure global horizontal irradiance (GHI) and reflected horizontal irradiance (RHI). For the validation statistics per station, including bias, MAD, RMSD, STD and correlation coefficient, see Validation of albedo.

To enhance the accuracy of satellite-derived ground albedo data, dedicated ground measurement campaigns are often conducted for projects where albedo significantly impacts the expected energy yield. These campaigns provide localized, high-precision data, complementing satellite-based estimates.

However, due to the inherent temporal variability of albedo, measurement campaigns can yield widely dispersed results, influenced by the duration of the observation period. Short campaigns, lasting only a few days, often introduce greater errors than those found in satellite-based data, highlighting the importance of extended measurement durations for reliable validation.

Where ground measurements of sufficient quality and length are available, the satellite-derived albedo Time Series can be locally corrected to better represent on-site conditions — see Site adaptation of albedo data.

Further reading

• "Surface albedo and reflectance: Review of definitions, angular and spectral effects, and intercomparison of major data sources in support of advanced solar irradiance modeling over the Americas". C. Gueymard, V. Lara-Fanego, M. Sengupta, Y. Xie, 2019, Solar Energy, Volume 182, April 2019, Pages 194-212

• "How to use albedo for more accurate bifacial PV estimates". White Paper. Solargis website, 2022.

• "Prospect map layers"

• "Solargis Glossary"