We analyse a multi-model ensemble at convection-resolving resolution based on the DYAMOND models, and a resolution ensemble based on the limited area model COSMO over 40 days to study how tropical and subtropical marine low clouds are represented at kilometer-scale resolution. The analysed simulations produce low cloud fields that look in general realistic in comparison to satellite images. The evaluation of the radiative balance, however, reveals substantial inter-model differences and an underestimated low cloud cover in most models. Models that simulate increased low cloud cover are found to have a deeper marine boundary layer (MBL), stronger entrainment, and an enhanced latent heat flux. These findings demonstrate that some of the fundamental relations of the MBL are systematically represented by the model ensemble which implies that the relevant dynamical processes start to become resolved on the model grid at kilometer-scale resolution. A sensitivity experiment with the COSMO model suggests that differences in the strength of turbulent vertical mixing may contribute to the inter-model spread in cloud cover.
Clouds over tropical oceans are among the most uncertain factors controlling Earth's temperature response to anthropogenic greenhouse gas emissions (Forster et al., 2021). They form along the branches of the Hadley circulation (HC, e.g., Held & Hou, 1980), for instance, in the form of deep convection at the intertropical convergence zone (ITCZ) (Waliser & Gautier, 1993) and shallow convection in the marine boundary layer (MBL) in the Trades (e.g., Stevens, 2007;Vial et al., 2017;Wood, 2012). Tropical clouds have the potential for a strong radiative feedback in a warming climate (Bony & Dufresne, 2005;Zelinka et al., 2016). Yet, their evolution with climate change is uncertain (e.g., Bretherton, 2015), making them a prime focus of current climate change research.Model intercomparison projects of global climate models (GCMs) such as the fifth or sixth phase of the Coupled Model Intercomparison Project (CMIP5, CMIP6, Eyring et al., 2016;Taylor et al., 2012) allow for an assessment of the magnitude and inter-model variability of cloud changes in a large ensemble of state-of-the-art GCMs. With respect to tropical deep convection at the ITCZ, many GCMs project that the upper part of the clouds (i.e., the anvils) will rise in a warming atmosphere and remain at approximately the same temperature, according to the fixed anvil temperature (FAT) hypothesis (Hartmann & Larson, 2002). As the anvils rise, they find themselves in
Abstract. The term “pseudo-global warming” (PGW) refers to a simulation strategy in regional climate modeling. The strategy consists of directly imposing large-scale changes in the climate system on a control regional climate simulation (usually representing current conditions) by modifying the boundary conditions. This differs from the traditional dynamic downscaling technique where output from a global climate model (GCM) is used to drive regional climate models (RCMs). The PGW climate changes are usually derived from a transient global climate model (GCM) simulation. The PGW approach offers several benefits, such as lowering computational requirements, flexibility in the simulation design, and avoiding biases from global climate models. However, implementing a PGW simulation is non-trivial, and care must be taken not to deteriorate the physics of the regional climate model when modifying the boundary conditions. To simplify the preparation of PGW simulations, we present a detailed description of the methodology and provide the companion software PGW4ERA5 facilitating the preparation of PGW simulations. In describing the methodology, particular attention is devoted to the adjustment of the pressure and geopotential fields. Such an adjustment is required when ensuring consistency between thermodynamical (temperature and humidity) changes on the one hand and dynamical changes on the other hand. It is demonstrated that this adjustment is important in the extratropics and highly essential in tropical and subtropical regions. We show that climate projections of PGW simulations prepared using the presented methodology are closely comparable to traditional dynamic downscaling for most climatological variables.
Abstract. The northern basin of the Dead Sea is occupied by a -300-m-deep lake. A series of cores in the deep-water part of the lake provide information about the top 365 cm of the sediments. The cores were correlated with high-resolution 3.5-kHz seismic profiles from this area and provide lithologic and age constraints for the high-resolution seismic reflection data. Visual comparison of the two data sets shows that strong surface and shallow subsurface reflectors (A and B) correlate to the massive salt at the seafloor surface and the indurated salt at the base of the cores, respectively. Calculations of an average seismic velocity based on the interval between these reflectors and the corresponding sedimentary thickness yield an average 3500 m/s velocity. This agrees closely with velocities determined from direct measurements of compressional velocities for sediment samples. Ultrasonic wave velocity measurements of salt samples from the cores and dry rock salt cores from the southern basin of the Dead Sea indicate that wave velocities are independent of the burial depth at shallow depths; however, velocities show strong dependence on porosity. At low hydrostatic pressure a reduction in porosity as well as closure of microcracks in the crystals cause an increase in the velocities. This increase disappears at higher stress levels. Synthetic seismograms of the upper 3 ms and the entire 25 ms penetrated by the seismic profiles reinforce the lithologic and seismic stratigraphic correlation and confirm that prominent reflectors in the basin represent the top boundary of halite layers which are separated by laminated sequences of evaporites and clastics. The salt in the upper salt sequence is deposited at a very fast rate of more than 20 mm/yr. However, at shallow depths, considerable compaction takes place. Variations in appearance and velocities of the upper salt sequence and middle salt sequence indicate that the porous, granular, and fine-grained precipitates of the surface salts are diagenetically altered to a coarse and compact crystalline aggregate by re-solution and crystallization with burial. The sedimentary sequences recovered in the cores suggest that significant lake level fluctuations took place in the past in response to climatic changes. The detailed correlation of the cores and seismic profiles makes it possible to extrapolate climatic data from earlier periods beneath the maximum core penetration by analyzing the seismic stratigraphic sequences of the seismic reflection data. IntroductionThe Dead Sea basin is one of the deepest active strike-slip basins on Earth. It is located along the Dead Sea transform, a plate boundary which separates the Arabian plate from the African and Sinai plates. The Dead Sea is a topographic depression over 110 km long and more than 400 m below sea
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