Variation in Radiative Contribution by Clouds to Downward Longwave Flux

Clouds strongly influence downward longwave radiative flux and affect the radiation budget at the surface. We evaluated the cloud radiative effect in both absolute and relative terms on downward longwave radiation at the surface; we considered variations in the cloud radiative effect with changes in cloud amount, precipitable water, and cloud base height, as measured by eight stations of the Baseline Surface Radiation Network. The downward longwave radiation predicted by a radiative transfer model agreed well with observations. The cloud radiative forcing and contribution ranged from –21 to 92 W m–2 and from –6 % to 38 %, respectively. The cloud effect shows a positive correlation to the shortwave diffusivity index (an index of cloud amount) and a negative correlation to precipitable water amount. The absolute effect values are small, depending on site conditions, but the relative effect values are larger under dry conditions than under humid conditions. Under humid conditions, the effect of the shortwave diffusivity index is very small. Under dry and cold conditions, such as those found in polar regions, negative values of cloud radiative contribution appear frequently because clouds absorb the emissions from temperature inversion layers. In comparison with prior research that used the A-Train satellite product, the present study shows a wider distribution and a larger maximum value for cloud forcing from amount of water vapor. Cloud effect has a roughly negative relationship with cloud base height, but a positive correlation with cloud base height occurs under low clouds at Tateno, which is located on the Pacific Ocean side of Japan. This correlation is because of the unusual relationship between cloud base height and cloud effect at Tateno during the summer and winter seasons. These results describe small-scale and near-surface variations in cloud effect, which are difficult to detect by satellite measurements.