Abstract
Abstract. Based on the decoupling parameterization of the cloud-topped planetary boundary layer, a simple equation is derived to compute the inversion height. In combination with the lifting condensation level and the amount of water vapor in near-surface air, we propose a low-level cloud suppression parameter (LCS) and estimated low-level cloud fraction (ELF), as new proxies for the analysis of the spatiotemporal variation of the global low-level cloud amount (LCA). Individual surface and upper-air observations are used to compute LCS and ELF as well as lower-tropospheric stability (LTS), estimated inversion strength (EIS), and estimated cloud-top entrainment index (ECTEI), three proxies for LCA that have been widely used in previous studies. The spatiotemporal correlations between these proxies and surface-observed LCA were analyzed. Over the subtropical marine stratocumulus deck, both LTS and EIS diagnose seasonal–interannual variations of LCA well. However, their use as a global proxy for LCA is limited due to their weaker and inconsistent relationship with LCA over land. EIS is anti-correlated with the decoupling strength more strongly than it is correlated with the inversion strength. Compared with LTS and EIS, ELF and LCS better diagnose temporal variations of LCA, not only over the marine stratocumulus deck but also in other regions. However, all proxies have a weakness in diagnosing interannual variations of LCA in several subtropical stratocumulus decks. In the analysis using all data, ELF achieves the best performance in diagnosing spatiotemporal variation of LCA, explaining about 60 % of the spatial–seasonal–interannual variance of the seasonal LCA over the globe, which is a much larger percentage than those explained by LTS (2 %) and EIS (4 %). Our study implies that accurate prediction of inversion base height and lifting condensation level is a key factor necessary for successful simulation of global low-level clouds in general circulation models (GCMs). Strong spatiotemporal correlation between ELF (or LCS) and LCA identified in our study can be used to evaluate the performance of GCMs, identify the source of inaccurate simulation of LCA, and better understand climate sensitivity.
Highlights
Clouds belong to the most important but uncertain components of the climate system
Likely that the relative humidity at the base of zinv (RH−inv) is a good proxy for level cloud amount (LCA). Over both the ocean and land, RH−inv shows a stronger correlation with LCA than lower-tropospheric stability (LTS) and estimated inversion strength (EIS) but a weaker correlation than zinv and zLCL. We speculate that this relatively poor performance of RH−inv compared to zinv and zLCL is due in part to the poor estimation of qv−,inv rather than indicating that RH−inv is a poor proxy for LCA
We have presented results based on the analysis of the data satisfying 0 < α < 1 when our decoupling hypothesis can be applied without any conceptual ambiguity
Summary
Clouds belong to the most important but uncertain components of the climate system. Due to their strong shortwave radiative cooling effect on the Earth, low-level clouds have been the focus of various studies in the past few decades, both in the observation and modeling communities. Slingo (1990) estimated that a 4 % increase in the low-level cloud amount (LCA) has the potential to offset global warming associated with a doubled CO2 concentration. Enhanced longwave radiative cooling at the top of MSCs capped by a drier free atmosphere (this process was not included in the PLR04’s model) may increase MSCs by enhancing turbulent vertical moisture transport from the sea surface to overlying MSCs. Based on the decoupling hypothesis suggested by PLR04 and other proceeding works (e.g., Augstein et al, 1974; Albrecht et al, 1979; Betts and Ridgway, 1988; Bretherton, 1992), Wood and Bretherton (2006) (WB06 hereafter) extended KH93’s LTS and suggested an estimated inversion strength (EIS) as a better proxy for LCA in which temperature profiles in the decoupled layer below the inversion and the free troposphere above the inversion are assumed to be close to a moist adiabat that is strongly temperaturedependent.
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