Abstract Vertical decorrelation length ( L cf ) as used to determine overlap of cloudy layers in GCMs was obtained from CloudSat/CALIPSO measurements, made between 2007 and 2010, and analyzed in terms of monthly means. Global distributions of L cf were produced for several cross-sectional lengths. Results show that: L cf over the tropical convective regions typically exceeds 2 km and shift meridionally with season; the smallest L cf ( L cf for mid-to-high latitude continents of the Northern Hemisphere (NH) ranges from 5–6 km during winter to 2–3 km during summer; and there are marked differences between continental and oceanic values of L cf in the mid-latitudes of the NH. These monthly-gridded, observationally-based values of L cf data were then used by the Monte Carlo Independent Column Approximation (McICA) radiation routines within the Beijing Climate Center's GCM (BCC_AGCM2.0.1). Additionally, the GCM was run with two other descriptions of L cf : one varied with latitude only, and the other was simply 2 km everywhere all the time. It is shown that using the observationally-based L cf in the GCM led to local and seasonal changes in total cloud fraction and shortwave (longwave) cloud radiative effects that serve mostly to reduce model biases. This indicates that usage of L cf that vary according to location and time has the potential to improve climate simulations.