Cloud Radiative Effect over 2001-2020 according to the energy and radiation balance applied for De Bilt, Netherlands

The Cloud Radiative Effect (CRE) is a crucial parameter in climate studies as it quantifies the influence of clouds on the radiation balance. Clouds affect both shortwave (solar) and longwave (thermal) radiation, and the CRE expresses the net effect by comparing the average cloudy situation with a cloud-free situation. This study determines the CRE at the surface and at the top of the atmosphere for De Bilt, Netherlands, over the period from 2001 to 2020.

To investigate this, we combined solar surface radiation and temperature data from the Royal Netherlands Meteorological Institute (KNMI) with satellite measurements of radiation and cloud cover at the top of the atmosphere from the CERES satellite. The study solved for the longwave downward radiation using balance equations. The atmosphere’s near surface emissivity is approximated by a power function of the cloud fraction. Energy balance calibration involved adjusting the temperature difference between the surface and the 2-meter height temperature to match observed temperature differences and tuning the balance-calculated ratio of total forcing differences to temperature differences per season to align with the derivative of the Stefan-Boltzmann radiation law.

Key findings related to the cloud radiative effect over the period from 2001 to 2020 for De Bilt, Netherlands, include:

• Total Cloud Effect at the Surface: The cloud effect calculated from surface data is a clearly cooling effect, averaging -24 W/m². In line with an decrease in cloud cover the cooling effect has  decreased over time (2,5 Wm-2.dec-1) with rising temperatures (ΔTCE/ΔTa: 5,7 W.m⁻².K⁻¹).
• Total Cloud Effect at the Top of the Atmosphere: Similar to the surface, the TCE at the top of the atmosphere is also cooling, averaging -21 W/m². With a decrease in cloud cover, the cooling effect decreases (ΔTCE +2.3 W/m²).

The study also discusses

-The significant increase in clear sky Reflected Solar Radiation and clear sky Solar Surface Radiation over the period, which could indicate a decrease in aerosols.

-The total cloud effect at the top of the atmosphere becomes less cooling with decreasing cloud cover, which could lead to the incorrect conclusion that clouds at the top of the atmosphere contribute to additional warming (ΔTCE/ΔTa = 5.2 W/m².K⁻¹ with a 0.4 K increase in surface temperature).

In summary, it is possible to coherently present the Cloud Radiative Effect over the period from 2001 to 2020 using the energy and radiation balance for De Bilt. The findings provide insight that clouds in temperate regions generally contribute to cooling both at the surface and at the top of the atmosphere.