ALBERT

Description: If an industrial civilization had existed on Earth many millions of years prior to ourown era, what traces would it have left and would they be detectable today? We summarize the likely geological fingerprint of the Anthropocene, and demonstrate that while clear, it will not differ greatly in many respects from other known events in the geological record. We then propose tests that could plausibly distinguish an industrial cause from an otherwise naturally occurring climate event.

Description: To overcome a deficiency in the standard Ku- and Ka-band dual-wavelength radar technique, a modified version of the method is introduced. The deficiency arises from ambiguities in the estimate of the massweighted diameter Dm of the raindrop size distribution (DSD) derived from the differential frequency ratio (DFR), defined as the difference between the radar reflectivity factors (dB) at Ku and Ka band Z(sub Ku) - Z(sub Ka). In particular, for DFR values less than zero, there are two possible solutions of Dm, leading to ambiguities in the retrieved DSD parameters. It is shown that the double solutions to Dm are effectively eliminated if the DFR is modified from Z(sub Ku) - Z(sub Ka) to Z(sub Ku) - gZ(sub Ka). (dB), where g is a constant with a value less than 0.8. An optimal radar algorithm that uses the modified DFR for the retrieval of rain and Dm profiles is described. The validity and accuracy of the algorithm are tested by applying it to radar profiles that are generated from measured DSD data. Comparisons of the rain rates and Dm estimated from the modified DFR algorithm to the same hydrometeor quantities computed directly from the DSD spectra (or the truth) indicate that the modified DFR-based profiling retrievals perform fairly well and are superior in accuracy and robustness to retrievals using the standard DFR.

Description: Blowing snow is a frequent and ubiquitous phenomenon over most over Antarctica. The transport and sublimation of blowing snow are important for the mass balance of the Antarctic ice sheet and the latter is a major contributor to the hydrological cycle in high latitude regions. While much is known about blowing snow from surface observations, our knowledge of the thermodynamic structure of deep blowing snow layers is lacking. Here dropsonde measurements are used to investigate the temperature, moisture and wind structure of deep blowing snow layers over Antarctica. The temperature lapse rate within the blowing snow layer is found to be at times close to dry adiabatic and on average between dry and moist adiabatic. Initiation of blowing snow causes the surface temperature to increase to a degree proportional to the depth of the blowing snow layer. The relative humidity is generally largest near the surface (but less than 100%) and decreases with height reaching a minimum near the top of the layer. These findings are at odds with accepted theory which assumes blowing snow sublimation will cool and eventually saturate the layer. The observations support the conclusion that high levels of wind shear induced turbulence causes mixing and entrainment of warmer and drier air from above the blowing snow layer which suppresses humidity and produces the observed well-mixed temperature structure within the layer. The results may have important consequences for Antarctic ice sheet mass balance and the moisture budget of the atmosphere in high latitudes.

Description: Heterogeneous ice nucleation initiated by particles immersed within droplets is likely the main pathway of ice formation in the atmosphere. Theoretical models commonly used to describe this process assume that it mimics ice formation from the vapor, neglecting interactions unique to the liquid phase. This work introduces a new approach that accounts for such interactions by linking the ability of particles to promote ice formation to the modification of the properties of water near the particle-liquid interface. It is shown that the same mechanism that lowers the thermodynamic barrier for ice nucleation also tends to decrease the mobility of water molecules, hence the ice-liquid interfacial flux. Heterogeneous ice nucleation in the liquid phase is thus determined by the competition between thermodynamic and kinetic constraints to the formation and propagation of ice. At the limit, ice nucleation may be mediated by kinetic factors instead of the nucleation work. This new ice nucleation regime is termed spinodal ice nucleation. Comparison of predicted nucleation rates against published data suggests that some materials of atmospheric relevance may nucleate ice in this regime.

Description: The radiative flux data and other meteorological data in the BSRN archive start from 1992, but the RadFlux data, the clearsky radiative fluxes at the BSRN sites derived through regression analyses of actually observed clearsky fluxes, did not come into existence until the early 2000s, and at first, they were limited to the 7 NOAA SURFRAD and 4 DOE ARM sites, a subset of the BSRN sites. Recently, the RadFlux algorithm was applied more extensively to the BSRN sites for the production of clearsky groundbased fluxes. At the time of this writing, there are 7119 site-months of clearsky fluxes at 42 BSRN sites spanning the time from 1992 to late 2017. These data provide an unprecedented opportunity to validate the satellite based clearsky fluxes. In this paper, the GEWEX SRB GSW(V3.0) shortwave downward fluxes spanning 24.5 years from 198307 to 200712, the CERES SYN1deg(Ed4A) and EBAF(Ed4.0) shortwave fluxes spanning 200003 to mid2017 are compared with their RadFlux counterparts on the hourly, 3hourly, daily and monthly time scales. All the three datasets show reasonable agreement with their groundbased counterparts. Comparison of the satellitebased surface shortwave clearsky radiative fluxes to the BSRN RadFlux analysis shows negative biases. Further analysis shows that the satellitebased atmosphere contains greater aerosol optical paths as well as more precipitable water than RadFlux analysis estimates.

Description: Observations from a geostationary satellite are used to describe the lifecycle of mesoscale convective systems (MCS), their associated anvil clouds, and their effects on the radiation balance over the warm pool of the tropical west Pacific Ocean. In their developing stages, MCS primarily consist of clouds that are optically thick and have a negative net cloud radiative effect (CRE). As MCS age, ice crystals in the anvil become larger, the cloud top lowers somewhat, and clouds with neutral and positive net CRE become more common. Shading from anvils causes cool anomalies in the underlying sea surface temperature (SST) of up to -0.6 C. MCS often occur in clusters that are embedded within large westward-propagating disturbances, so shading from anvils can cool SSTs over regions spanning hundreds of kilometers. Triggering of convection is more likely to follow a warm SST anomaly than a cold SST anomaly on timescales of several days. This information is used to test hypotheses on why, over the warm pool, the average shortwave and longwave CRE are individually large but nearly cancel. The results are consistent with the hypothesis that the cancelation in CRE is caused by feedbacks between cloud albedo, large-scale circulation, and SST.

Description: Previous multi-model intercomparisons have shown that chemistryclimate models exhibit significant biases in tropospheric ozone compared with observations. We investigate annual-mean tropospheric column ozone in 15 models participating in the SPARCIGAC (Stratospheretroposphere Processes And their Role in ClimateInternational Global Atmospheric Chemistry) Chemistry-Climate Model Initiative (CCMI). These models exhibit a positive bias, on average, of up to 40 %50 % in the Northern Hemisphere compared with observations derived from the Ozone Monitoring Instrument and Microwave Limb Sounder (OMI/MLS), and a negative bias of up to 30 % in the Southern Hemisphere. SOCOLv3.0 (version 3 of the Solar-Climate Ozone Links CCM), which participated in CCMI, simulates global-mean tropospheric ozone columns of 40.2 DU approximately 33 % larger than the CCMI multi-model mean. Here we introduce an updated version of SOCOLv3.0, SOCOLv3.1, which includes an improved treatment of ozone sink processes, and results in a reduction in the tropospheric column ozone bias of up to 8 DU, mostly due to the inclusion of N2O5 hydrolysis on tropospheric aerosols. As a result of these developments, tropospheric column ozone amounts simulated by SOCOLv3.1 are comparable with several other CCMI models. We apply Gaussian process emulation and sensitivity analysis to understand the remaining ozone bias in SOCOLv3.1. This shows that ozone precursors (nitrogen oxides (NO(sub x)), carbon monoxide, methane and other volatile organic compounds, VOCs) are responsible for more than 90 % of the variance in tropospheric ozone. However, it may not be the emissions inventories themselves that result in the bias, but how the emissions are handled in SOCOLv3.1, and we discuss this in the wider context of the other CCMI models. Given that the emissions data set to be used for phase 6 of the Coupled Model Intercomparison Project includes approximately 20 % more NO(sub x) than the data set used for CCMI, further work is urgently needed to address the challenges of simulating sub-grid processes of importance to tropospheric ozone in the current generation of chemistryclimate models.

Description: Efforts to limit global warming to below 2C in relation to the preindustrial level are under way, in accordance with the 2015 Paris Agreement. However, most impact research on agriculture to date has focused on impacts of warming 〉2C on mean crop yields, and many previous studies did not focus sufficiently on extreme events and yield interannual variability. Here, with the latest climate scenarios from the Half a degree Additional warming, Prognosis and Projected Impacts (HAPPI) project, we evaluated the impacts of the 2015 Paris Agreement range of global warming (1.5 and 2.0C warming above the preindustrial period) on global wheat production and local yield variability. A multicrop and multiclimate model ensemble over a global network of sites developed by the Agricultural Model Intercomparison and Improvement Project (AgMIP) for Wheat was used to represent major rainfed and irrigated wheat cropping systems. Results show that projected global wheat production will change by 2.3% to 7.0% under the 1.5C scenario and 2.4% to 10.5% under the 2.0C scenario, compared to a baseline of 19802010, when considering changes in local temperature, rainfall, and global atmospheric CO2 concentration, but no changes in management or wheat cultivars. The projected impact on wheat production varies spatially; a larger increase is projected for temperate high rainfall regions than for moderate hot low rainfall and irrigated regions. Grain yields in warmer regions are more likely to be reduced than in cooler regions. Despite mostly positive impacts on global average grain yields, the frequency of extremely low yields (bottom 5 percentile of baseline distribution) and yield interannual variability will increase under both warming scenarios for some of the hot growing locations, including locations from the second largest global wheat producerIndia, which supplies more than 14% of global wheat. The projected global impact of warming 〈2C on wheat production is therefore not evenly distributed and will affect regional food security across the globe as well as food prices and trade.

Description: The purpose of this study was to evaluate snow and snowmelt simulated by version 4 of the Community Land Model (CLM4). We performed uncoupled CLM4 simulations, forced by ModernEra Retrospective Analysis for Research and Applications Landonly meteorological fields. GlobSnow snow cover fraction, snow water equivalent (SWE), and satellitebased passive microwave snowmeltoff day of year (MoD) data were used to evaluate snow cover fraction, SWE, and snowmelt simulations. Simulated runoff was then fed into a river routing scheme and evaluation was performed at 408 snowdominated catchments using gauge observations. CLM4 and GlobSnow snow cover extent showed a strong agreement, especially during the peak snow cover months. Overall there was a good correlation between simulated and observed SWE (correlation coefficient, R = 0.6). Simulated and observed SWE were similar over areas with relatively flat terrain and moderate forest density. The simulated MoD agreed (MoD differences [CLM4passive microwave] = 7 days) with observations over 39.4% of the study domain. Snowmeltoff occurred earlier in the model compared to the observations over 39.5% of the domain and later over 21.1% of the domain. Large differences of MoD were seen in the areas with complex terrain and dense forest cover. We also found that, although streamflow seasonal phase was accurately modeled (R = 0.9), the peaks controlled by snowmelt were underestimated. Routed CLM4 streamflow tended to occur early (by 10 days on average).

Description: In the early Pleistocene, global temperature cycles predominantly varied with ~41kyr (obliquityscale) periodicity. Atmospheric greenhouse gas concentrations likely played a role in these climate cycles; marine sediments provide an indirect geochemical means to estimate early Pleistocene CO2. Here we present a boron isotopebased record of continuous highresolution surface ocean pH and inferred atmospheric CO2 changes. Our results show that, within a window of time in the early Pleistocene (1.381.54 Ma), pCO2 varied with obliquity, confirming that, analogous to late Pleistocene conditions, the carbon cycle and climate covaried at ~1.5 Ma. Pairing the reconstructed early Pleistocene pCO2 amplitude (92 13 atm) with a comparably smaller global surface temperature glacial/interglacial amplitude (3.0 0.5 K) yields a surface temperature change to CO2 radiative forcing ratio of S[CO2]~0.75 (0.5) C(sup -1)W(sup -1)m(sup -2), as compared to the late Pleistocene S[CO2] value of ~1.75 (0.6) C(sup -1)W(sup -1)m(sup -2). This direct comparison of pCO2 and temperature implicitly incorporates the large ice sheet forcing as an internal feedback and is not directly applicable to future warming. We evaluate this result with a simple climate model and show that the presumably thinner, though extensive, northern hemisphere ice sheets would increase surface temperature sensitivity to radiative forcing. Thus, the mechanism to dampen actual temperature variability in the early Pleistocene more likely lies with Southern Ocean circulation dynamics or antiphase hemispheric forcing. We also compile this new carbon dioxide record with published PlioPleistocene (sup 11)B records using consistent boundary conditions and explore potential reasons for the discrepancy between Pliocene pCO2 based on different planktic foraminifera.