DOI: 10.5281/zenodo.15531691
title: Spectral Surface Albedo Observations from NOAA GML/GRAD and CIRES during the SPLASH Campaign

description: Overview

The NOAA Global Monitoring Laboratory (GML), Radiation, Aerosols, and Cloud Division (G-RAD), in collaboration with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, conducted spectral surface albedo measurements at two field sites — Kettle Ponds (CKP) and Brush Creek (CBC) — as part of the NOAA Study of Precipitation, the Lower Atmosphere, and Surface for Hydrometeorology (SPLASH) campaign.

Spectral albedo values were derived from combined upwelling and downwelling hemispheric irradiance measurements in seven narrow spectral bands centered at nominal wavelengths of 415, 500, 670, 870, 940, and 1625 nm.





Downwelling irradiance was measured using an upward-facing Multifilter Rotating Shadowband Radiometer (MFRSR).




Upwelling irradiance was measured using a downward-facing Multifilter Radiometer (MFR) mounted approximately 2 meters above ground level.



Measurements were recorded at a 20-second interval. Due to the sequential nature of MFRSR observations — which reconstruct downwelling irradiance from irradiances during various shadowband positions — each timestamp represents neither a purely instantaneous nor a time-averaged measurement. The full measurement cycle is completed within eight seconds prior to each reported timestamp.

Calibration Approach

Instrument responsivity calibrations were performed using:





Laboratory lamp calibrations for the MFRSR (downwelling)




In-field side-by-side calibration for the MFR (upwelling)



Relative calibration between the MFR and MFRSR reduced propagated uncertainty in the albedo product. Surface albedo was computed as the simple ratio of the calibrated upwelling to downwelling irradiance at each wavelength.

Note: The NOAA/GML/GRAD calibration facility does not currently support absolute responsivity calibration at 1625 nm. Consequently, irradiance values at this wavelength carry an unknown scaling factor. However, a side-by-side calibration was performed, enabling valid and consistent albedo retrievals at 1625 nm.

Data Quality Considerations

During winter, both instruments were occasionally buried in snow when snow depths exceeded their 2-meter installation height. These periods — more frequent at CKP than CBC — were not removed from the dataset and may be associated with degraded data quality. Users should exercise caution when analyzing data from winter periods, particularly at CKP, due to potential signal degradation caused by snow coverage over the instruments.

Data from the CKP site are affected by several quality issues, particularly prior to 2022-11-03. These include intermittent data loss, occasional jumps in the MFRSR signal at 670 nm, and oscillations in the MFR signal, most notably at 1625 nm. While much of the data remains usable, users are advised to apply increased scrutiny when using CKP observations before 2022-11-03. Data quality improved significantly after 2022-11-03.

Data Availability

Spectral surface albedo data are available for the following date ranges:





CKP: 2021-12-20 through 2023-08-15




CBC: 2021-10-16 through 2023-06-22



File Naming Convention

Files are named as follows:

<site>.<product>.<YYYYMMDD>.nc

Example:cbc.spectral_surface_albedo.20230623.nc





cbc – Site identifier




spectral_surface_albedo – Data product descriptor




20230623 – Data collection date (YYYYMMDD)



File Format

All files are in NetCDF format and conform to the CF-1.8 standard. Each file includes the following key variables:



surface_albedo — Spectral surface albedo derived from the ratio of upwelling and downwelling irradiance. Unit: 1.

downwelling_hemispheric_irradiance — Global downwelling irradiance from MFRSR. Unit: W m-2 nm-1.

upwelling_hemispheric_irradiance — Upwelling irradiance from MFR. Unit: W m-2 nm-1. 

downwelling_hemispheric_irradiance_1625nm — Relative downwelling irradiance at 1625 nm. Unit: 1.

upwelling_hemispheric_irradiance_1625nm — Relative upwelling irradiance at 1625 nm. Unit: 1.

solar_zenith_angle — Solar zenith angle in degrees.

solar_azimuth_angle — Solar azimuth angle in degrees.

channel_wavelength_mfr — MFR channel central wavelengths in nanometers.

channel_wavelength_mfrsr — MFRSR channel central wavelengths in nanometers.