Clouds at Barbados are representative of clouds across the trade wind regions in observations and climate models

Abstract

Trade wind regions cover most of the tropical oceans, and the prevailing cloud type is shallow cumulus. These small clouds are parameterized by climate models, and changes in their radiative effects strongly and directly contribute to the spread in estimates of climate sensitivity. This study investigates the structure and variability of these clouds in observations and climate models. The study builds upon recent detailed model evaluations using observations from the island of Barbados. Using a dynamical regimes framework, satellite and reanalysis products are used to compare the Barbados region and the broader tropics. It is shown that clouds in the Barbados region are similar to those across the trade wind regions, implying that observational findings from the Barbados Cloud Observatory are relevant to clouds across the tropics. The same methods are applied to climate models to evaluate the simulated clouds. The models generally capture the cloud radiative effect, but underestimate cloud cover and show an array of cloud vertical structures. Some models show strong biases in the environment of the Barbados region in summer, weakening the connection between the regional biases and those across the tropics. Even bearing that limitation in mind, it is shown that covariations of cloud and environmental properties in the models are inconsistent with observations. The models tend to misrepresent sensitivity to moisture variations and inversion characteristics. These model errors are likely connected to cloud feedback in climate projections, and highlight the importance of the representation of shallow cumulus convection.trade wind clouds | climate models | shallow cumulus | Barbados I s the cloud structure and variability observed at a single location over a long time representative of more general regimes of clouds? This is the conceit of long-term observational "super sites" but has rarely been evaluated (1, 2). This work focuses on clouds found in trade wind regions, which are mostly shallow cumulus, and asks whether findings from the Barbados Cloud Observatory (BCO) are generalizable to the broader tropics in observations and climate models. We will evaluate cloud properties and the environment in which the clouds form across 12 climate models within these regions. This emphasis on the trades is motivated by several factors: (i) Previous studies suggest these are the regions that lead to much of the spread in estimates of climate sensitivity (3, 4), (ii) there is a dearth of evaluation of shallow cumulus in climate models (5), and (iii) space-based and ground-based observations of the trades have recently become available (6-8). This focus on trade wind regions is unique but builds upon previous, broader model evaluations that have, among other things, shown improvements in near-global cloud properties in the latest generation of climate models (9) and provided an overview of the vertical structure of cloud fraction (CF) and cloud water content over tropical oceans (10). Ref. 11 explores low-level, tropical clou...

An A-train and MERRA view of cloud, thermodynamic, and dynamic variability within the subtropical marine boundary layer

“…The width of these layers is similar to the AIRS 700 and 925 hPa temperature and specific humidity weighting functions. Even though the RICO observations do not fall within any of the four regions listed in Table 1, Medeiros and Nuijens (2016) show that the observational site is applicable to the trade regime as a whole across the globe. Thus our approach for the remainder of the investigation is to determine if similar relationships shown in Nuijens et al (2009) exist in cumulus for the regions listed in Table 1.…”

“…Therefore, the assumption that constant RH might hold across the diversity of subtropical cloud regimes with a changing climate is likely not valid. Medeiros and Nuijens (2016) showed that the RH gradient between the MBL and lower free troposphere is widely variable among the CMIP5 models within the trade cumulus regime. Therefore, further examination of cloud variability and the vertical structure of RH with present-day satellite and reanalysis observations is warranted.…”

EUREC4A: A Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation

“…First, shallow cumuli are prominent in this area, especially during winter (Norris 1998;Nuijens et al 2014). Second, the cloudiness in the vicinity of Barbados is representative of clouds across the whole tradewind regions of the tropical ocean, both in models and in observations (Medeiros and Nuijens 2016). Finally, it anchors the measurements to the extensively instrumented Barbados Cloud Observatory which has been monitoring clouds continuously since 2010, and allows it to benefit from the legacy of the NARVAL series of flight campaigns organized in the area in the last few years ).…”

Can We Use Single‐Column Models for Understanding the Boundary Layer Cloud‐Climate Feedback?

“…This site has been selected for several reasons. First, Medeiros and Nuijens () show that the climatological conditions and the cloud properties measured at BCO are representative of the Trades. Second, the cloud fields of this region have been widely investigated in field campaigns (e.g., Nitta & Esbensen, ; Rauber et al, ) and in model studies (e.g., Nuijens et al, ; Siebesma & Cuijpers, ; Vanzanten et al, ).…”

“…We restrict our analysis to the dry season , defined as the period between January and April, following Brueck et al (). The reason for this choice is twofold: on the one hand, the focus of this study is on BL clouds, which are dominant in the dry season (Medeiros & Nuijens, ); on the other, the SCM is not ideally suited for simulating deep convection because, in its standard setup, it does not include the direct feedback of the deep convection on the dynamics. More complex setups allowing for the deep cumulus regime, such as the application of the weak temperature gradient method (e.g., Ruppert, ; Sobel et al, ), will be considered in future work.…”