Tropopause sharpening by data assimilation

Data assimilation was recently suggested to smooth out the sharp gradients that characterize the tropopause inversion layer (TIL) in systems that did not assimilate TIL-resolving observations. We investigate whether this effect is present in the ERA-Interim reanalysis and the European Centre for Medium-Range Weather Forecasts (ECMWF) operational forecast system (which assimilate high-resolution observations) by analyzing the 4D-Var increments and how the TIL is represented in their data assimilation systems. For comparison, we also diagnose the TIL from high-resolution GPS radio occultation temperature profiles from the COSMIC satellite mission, degraded to the same vertical resolution as ERA-Interim and ECMWF operational analyses. Our results show that more recent reanalysis and forecast systems improve the representation of the TIL, updating the earlier hypothesis. However, the TIL in ERA-Interim and ECMWF operational analyses is still weaker and farther away from the tropopause than GPS radio occultation observations of the same vertical resolution

Winter marine atmospheric conditions over the Japan Sea

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C12011, doi:10.1029/2001JC001197.Four basic types of synoptic-scale conditions describe the atmospheric structure and variability observed over the Japan Sea during the 1999/2000 winter season: (1) flow of cold Asian air from the northwest, (2) an outbreak of very cold Siberian air from the north and northeast, (3) passage of a weak cyclone over the southern Japan Sea with a cold air outbreak on the backside of the low, and (4) passage of a moderate cyclone along the northwestern side of the Japan Sea. In winter, the Russian coastal mountains and a surface-air temperature inversion typically block cold surface continental air from the Japan Sea. Instead, the adiabatic warming of coastal mountain lee-side air results in small air-sea temperature differences. Occasional outbreaks of very cold Siberian air eliminate the continental surface-based inversion and stability, allowing very cold air to push out over the Japan Sea for 1–3 days. During these outbreaks, the 0°C surface air isotherm extends well southward of 40°N, the surface heat losses in the center of the Japan Sea can exceed 600 W m−2, and sheet clouds cover most of the Japan Sea, with individual roll clouds extending from near the Russian coast to Honshu. During December through February, 1991–2002, these strong cold-air outbreak conditions occur 39% of the time and contribute 43% of the net heat loss from the Japan Sea. The average number of strong cold-air events per winter (November–March) season is 13 (ranging from 5 to 19); the 1999/2000 winter season covered in our measurements was normal.Support for this program was provided by the Office of Naval Research through grants N00014-98-1-0345, N00014-99-1- 0205, and N00014-98-1-0210

Flow of bottom water in the northwestern Weddell Sea

The Weddell Sea is known to feed recently formed deep and bottom water into the Antarctic circumpolar water belt, from whence it spreads into the basins of the world ocean. The rates are still a matter of debate. To quantify the flow of bottom water in the northwestern Weddell Sea data obtained during five cruises with R/V Polarstern between October 1989 and May 1998 were used. During the cruises in the Weddell Sea, five hydrographic surveys were carried out to measure water mass properties, and moored instruments were deployed over a time period of 8.5 years to obtain quasi-continuous time series. The average flow in the bottom water plume in the northwestern Weddell Sea deduced from the combined conductivity-temperature-depth and moored observations is 1.3±0.4 Sv. Intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed. The variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large-scale circulation. Taking into account entrainment of modified Warm Deep Water and Weddell Sea Deep Water during the descent of the plume along the slope, between 0.5 Sv and 1.3 Sv of surface-ventilated water is supplied to the deep sea. This is significantly less than the widely accepted ventilation rates of the deep sea. If there are no other significant sources of newly ventilated water in the Weddell Sea, either the dominant role of Weddell Sea Bottom Water in the Southern Ocean or the global ventilation rates have to be reconsidered

Copernicus Climate Change Service, Klimatreanalyser

Climate reanalyses combine past observations with models to generate consistent time series of multiple climate variables. Reanalyses are among the most-used datasets in the geophysical sciences. They provide a comprehensive description of the observed climate as it has evolved during recent decades, on 3D grids at sub-daily intervals.Klimatreanalyser kombinerar tidigare observationer med modeller för att generera konsekventa tidsserier av flera klimatvariabler. Reanalyser är bland de mest använda dataseten inom geofysiska vetenskaper. De ger en omfattande beskrivning av det observerade klimatet som det har utvecklats under de senaste decennierna, på 3D-nät med dagliga intervaller

The credibility challenge for global fluvial flood risk analysis

Quantifying flood hazard is an essential component of resilience planning, emergency response, and mitigation, including insurance. Traditionally undertaken at catchment and national scales, recently, efforts have intensified to estimate flood risk globally to better allow consistent and equitable decision making. Global flood hazard models are now a practical reality, thanks to improvements in numerical algorithms, global datasets, computing power, and coupled modelling frameworks. Outputs of these models are vital for consistent quantification of global flood risk and in projecting the impacts of climate change. However, the urgency of these tasks means that outputs are being used as soon as they are made available and before such methods have been adequately tested. To address this, we compare multi-probability flood hazard maps for Africa from six global models and show wide variation in their flood hazard, economic loss and exposed population estimates, which has serious implications for model credibility. While there is around 30-40% agreement in flood extent, our results show that even at continental scales, there are significant differences in hazard magnitude and spatial pattern between models, notably in deltas, arid/semi-arid zones and wetlands. This study is an important step towards a better understanding of modelling global flood hazard, which is urgently required for both current risk and climate change projections

Using large eddy simulations to reveal the size, strength, and phase of updraft and downdraft cores of an Arctic mixed‐phase stratocumulus cloud

Three‐dimensional large eddy simulations (LES) are used to analyze a springtime Arctic mixed‐phase stratocumulus observed on 26 April 2008 during the Indirect and Semi‐Direct Aerosol Campaign. Two subgrid‐scale turbulence parameterizations are compared. The first scheme is a 1.5‐order turbulent kinetic energy (1.5‐TKE) parameterization that has been previously applied to boundary layer cloud simulations. The second scheme, Cloud Layers Unified By Binormals (CLUBB), provides higher‐order turbulent closure with scale awareness. The simulations, in comparisons with observations, show that both schemes produce the liquid profiles within measurement variability but underpredict ice water mass and overpredict ice number concentration. The simulation using CLUBB underpredicted liquid water path more than the simulation using the 1.5‐TKE scheme, so the turbulent length scale and horizontal grid box size were increased to increase liquid water path and reduce dissipative energy. The LES simulations show this stratocumulus cloud to maintain a closed cellular structure, similar to observations. The updraft and downdraft cores self‐organize into a larger meso‐γ‐scale convective pattern with the 1.5‐TKE scheme, but the cores remain more isotropic with the CLUBB scheme. Additionally, the cores are often composed of liquid and ice instead of exclusively containing one or the other. These results provide insight into traditionally unresolved and unmeasurable aspects of an Arctic mixed‐phase cloud. From analysis, this cloud’s updraft and downdraft cores appear smaller than other closed‐cell stratocumulus such as midlatitude stratocumulus and Arctic autumnal mixed‐phase stratocumulus due to the weaker downdrafts and lower precipitation rates.Plain Language SummaryLow‐lying clouds in the Arctic are ubiquitous and important to understand for the near‐surface energy balance. These clouds are difficult to measure because of the challenging environment in which they reside. High‐resolution models are tools that help fill in knowledge gaps about these clouds. In this work, we compare two different ways to represent fine motion within the cloud and see how the macrophysical properties of the cloud are affected. We found that one representation creates a more energetic cloud, and this type of cloud would exist longer than the other. We also are led to believe in these simulations that these clouds have different internal motions when compared to similar‐looking clouds formed at lower latitudes or formed in a different season in the Arctic.Key PointsTwo subgrid‐scale turbulence schemes, 1.5‐TKE and CLUBB, were compared in simulations of an Arctic mixed‐phase stratocumulus cloudComparing to observations shows that both schemes produce the liquid water profiles within measurement variability but not ice water profilesUpdraft and downdraft cores are different in size, similar in strength, and composed of both liquid and icePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137515/1/jgrd53743_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137515/2/jgrd53743-sup-0001-supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137515/3/jgrd53743.pd

CERA-20C: A Coupled Reanalysis of the Twentieth Century

CERA‐20C is a coupled reanalysis of the twentieth century which aims to reconstruct the past weather and climate of the Earth system including the atmosphere, ocean, land, ocean waves, and sea ice. This reanalysis is based on the CERA coupled atmosphere‐ocean assimilation system developed at ECMWF. CERA‐20C provides a 10 member ensemble of reanalyses to account for errors in the observational record as well as model error. It benefited from the prior experience of the retrospective atmospheric analysis ERA‐20C. The dynamical model and the data assimilation systems initially developed for NWP had been modified to take into account the evolution of the radiative forcing and the observing system. To limit the impact of changes in the observing system throughout the century, only conventional surface observations have been used in the atmosphere. CERA‐20C improves the specification of the background and the observation errors, two key elements to ensure a consistent weighting of the uncertainties across geophysical variables, space, and time. The quality of CERA‐20C has been evaluated against other centennial reanalyses and independent observations. Although CERA‐20C inherits some limitations of ERA‐20C to represent correctly the tropical cyclones in the first part of the century, it shows significant improvements in the troposphere, compared to ERA‐20C and 20CRv2c (the twentieth century reanalysis produced by NOAA/CIRES). A preliminary study of the climate variability in CERA‐20C has been carried out. CERA‐20C improves on the representation of atmosphere‐ocean heat fluxes and mean sea level pressure compared to previous uncoupled ocean and atmospheric historical reanalyses performed at ECMWF.© 2018 The Author