4DVAR retrieval of prognostic land surface model variables.

The major findings of the first type of retrieval are: Initial soil moisture contents as well as deep soil temperature can all be successfully retrieved, for realistic initial guess errors; the relative difficulty in retrieving superficial and deep soil moisture contents depends on the vegetation coverage and growth conditions; the revision to the soil temperature equations as documented in a separate paper is found critical in retrieval with real OASIS data; daytime observations are found much more effective because of their higher signal to noise ratio; assimilation window length up to ten days is found to produce the better retrievals. This signifies the value of information contents.For numerical weather prediction models, it is critically important to properly initialize its land surface model component. This study demonstrates successful variational retrievals of land surface states by assimilating either skin temperature or screen-level atmospheric measurements. For this purpose, the land surface model is first validated against the Oklahoma Atmospheric Surface Layer Instrumentation System (OASIS) measurements. Refinements to the model were found necessary.Two distinct retrieval problems are tackled in this study. One uses skin temperature as the observation and one uses observations of near surface atmospheric variables. The former is done using a land surface model and its adjoint in a stand-alone mode, forced by observed meteorological parameters. A 4D variational (4DVAR) retrieval system is developed in which the cost-function is defined as a quadratic measurement of the model forecasting error in ground surface or skin temperature. The latter involves a 1D land surface-atmosphere model, the corresponding adjoint codes, and a definition of cost function that measures misfit between observed and modeled screen-level atmospheric temperature and specific humidity.For the second retrieval problem, the Medium Range Forecast (MRF) PBL model is implemented within the Advanced Regional Prediction System (ARPS), forming a coupled LSM-PBL model. We show that under ideal synoptic conditions, land surface prognostic variables can also be successfully retrieved from the screen-level atmospheric observations. The retrieval scheme is robust when subject to observational errors with magnitudes comparable to instrument accuracy, and for initial guess errors larger than typical model forecast errors. Compared to the early case, the validity period for tangent linearization is shorter due to feedbacks from atmospheric components. There exists an optimal assimilation window length resulting from the contest between degrading forecast accuracy and increasing necessary information content. For a moist period tested, taking the scheme efficiency into consideration, a length of about six hours seems to be a suitable assimilation window length

Climate Warming and Effects on Aviation

The greatest concerns of the aviation industry under a warming climate possibly are the following two questions: first, what are the consequences for maximum payloads? and second, will changed air properties (density, temperature and viscosity) affect fuel efficiency? Here, the effects of climate warming on maximum payload and fuel efficiency are examined using atmospheric parameters from 27 climate models. Historical (20th century) climate simulations credibly reproduce the reanalysis period (1950–2015) of near-surface air density (NSAD). Lower NSAD is a first-order global signal continuing into the future. The NSAD reduction impact on MTOW could be ∼1% over the busy North Atlantic Corridor (NAC), and also varies among aircraft. Furthermore, for the standard 7-stage flight profile, negative effects of warming on fuel efficiency affect civil aviation. The cruising stage consumes most aviation fuel, and as cruising altitude coincides with the tropopause, the tropopause structure in a warming climate supports the conclusions drawn here. Tropopause temperature changes cause only ∼0.08% reduction in thermal efficiency. The net effect on total efficiency is smaller because of improved mechanical efficiency. Work required for a commercial aircraft increases in a warmer climate due to elevated tropopause altitude and increased air drag. The latter outweigh the former by almost an order of magnitude, for international flights

Stress fields in granular material and implications for performance of robot locomotion over granular media

Legged locomotion of robots has advantages in reducing payload in contexts such as travel over deserts or in planet surfaces. A recent study (Li et al. 2013) partially addresses this issue by examining legged locomotion over granular media (GM). However, they miss one extremely significant fact. When the robots wheels (legs) run over GM, the granules are set into motion. Hence, unlike the study of Li et al. (2013), the viscosity of the GM must be included to simulate the kinematic energy loss in striking and passing through the GM. Here the locomotion in their experiments is re-examined using an advanced Navier-Stokes framework with a parameterized granular viscosity. It is found that the performance efficiency of a robot, measured by the maximum speed attainable, follows a six-parameter sigmoid curve when plotted against rotating frequency. A correct scaling for the turning point of the sigmoid curve involves the footprint size, rotation frequency and weight of the robot. Our proposed granular response to a load, or the influencing domain concept points out that there is no hydrostatic balance within granular material. The balance is a synergic action of multi-body solids. A solid (of whatever density) may stay in equilibrium at an arbitrary depth inside the GM. It is shown that there exists only a minimum set-in depth and there is no maximum or optimal depth. The set-in depth of a moving robot is a combination of its weight, footprint, thrusting/stroking frequency, surface property of the legs against GM with which it has direct contact, and internal mechanical properties of the GM. If the vehicles working environment is known, the wheel-granular interaction and the granular mechanical properties can be grouped together. The unitless combination of the other three can form invariants to scale the performance of various designs of wheels/legs. Wider wheel/leg widths increase the maximum achievable speed if all other parameters are unchanged

Reduced viscosity steadily weakens oceanic currents

The viscosity of both air and water is temperature dependent. A rising temperature leads to an increased viscosity for air but a decreased viscosity for water. As climate becomes warmer, this increased air viscosity can partly inhibit the reduction of wind stress over the ocean, and the reduced water viscosity causes less downward momentum and heat transport. As these opposing effects of warming on air and water viscosity are not included in the state-of-the-art climate models, the understanding of their potential impacts on the response of the climate system to the anthropogenic warming is lacking. Here, via analyzing the Simple Ocean Data Assimilation oceanic reanalysis dataset, we show that the ocean heat content increases at a rate of ~1.3 × 1022 J/yr over 35 years, which leads to a continuous reduction of oceanic viscosity. As a result, the ocean vertical shear enhances with a shoaling of the mixed layer depth and a reduced vertical linkage in the ocean. Our calculations show a reduction of the oceanic kinetic energy at a rate of ~2.4 × 1016 J/yr. Potentially, this could generate far-reaching impacts on the energy storage of the climate system and, hence, could pace the global warming. Thus, it is important to include the temperature-dependent viscosity in our climate models. Freshwater discharged from polar ice sheets and mountain glaciers also contributes to the reduction in oceanic viscosity but, at present, to a lesser extent than that in oceanic warming. Reduced oceanic viscosity, therefore, is an important, but hitherto overlooked, response to a warming climate and contributes to many recent weather extremes including heavier rainfall rates in hurricanes, slackening of the polar vortex, and oceanic heat waves

The Gravity Environment of Zhouqu Debris Flow of August 2010 and Its Implication for Future Recurrence

This study investigates the geological background of the August 7-8, 2010 Zhouqu debris flows in the northwestern Chinese province of Gansu, and possible future occurrence of such hazards in the peri-Tibetan Plateau (TP) regions. Debris flows are a more predictable type of landslide because of its strong correlation with extreme precipitation. However, two factors affecting the frequency and magnitude of debris flows: very fine scale precipitation and degree of fracture of bedrock, both defy direct observations. Annual mean Net Primary production (NPP) is used as a surrogate for regional precipitation with patchiness filtered out, and gravity satellite measured regional mass changes as an indication of bedrock cracking, through the groundwater as the nexus. The GRACE measurements indicate a region (to the north east of TP) of persistent mass gain (started well before the 2008 Wenchuan earthquake), likely due to increased groundwater percolation. While in the neighboring agricultural region further to the north east, there are signal of decreased fossil water reservoir. The imposed stress fields by large scale increase/decrease groundwater may contribute to future geological instability of this region. Zhouqu locates right on the saddle of the gravity field anomaly. The region surrounding the Bay of Bangle (to the southeast of TP) has a similar situation. To investigate future changes in extreme precipitation, the other key player for debris flows, the “pseudo-climate change” experiments of a weather model forced by climate model provided perturbations on the thermal fields are performed and endangered locations are identified. In the future warmer climate, extreme precipitation will be more severe and debris will be more frequent and severe

The Greenland Ice Sheet Response to Transient Climate Change

ABSTRACT This study applies a multiphase, multiple-rheology, scalable, and extensible geofluid model to the Greenland Ice Sheet (GrIS). The model is driven by monthly atmospheric forcing from global climate model simulations. Novel features of the model, referred to as the scalable and extensible geofluid modeling system (SEGMENT-Ice), include using the full NavierStokes equations to account for nonlocal dynamic balance and its influence on ice flow, and a granular sliding layer between the bottom ice layer and the lithosphere layer to provide a mechanism for possible large-scale surges in a warmer future climate (granular basal layer is for certain specific regions, though). Monthly climate of SEGMENT-Ice allows an investigation of detailed features such as seasonal melt area extent (SME) over Greenland. The model reproduced reasonably well the annual maximum SME and total ice mass lost rate when compared observations from the Special Sensing Microwave Imager (SSM/I) and Gravity Recovery and Climate Experiment (GRACE) over the past few decades. The SEGMENT-Ice simulations are driven by projections from two relatively high-resolution climate models, the NCAR Community Climate System Model, version 3 (CCSM3) and the Model for Interdisciplinary Research on Climate 3.2, highresolution version [MIROC3.2(hires)], under a realistic twenty-first-century greenhouse gas emission scenario. They suggest that the surface flow would be enhanced over the entire GrIS owing to a reduction of ice viscosity as the temperature increases, despite the small change in the ice surface topography over the interior of Greenland. With increased surface flow speed, strain heating induces more rapid heating in the ice at levels deeper than due to diffusion alone. Basal sliding, especially for granular sediments, provides an efficient mechanism for fast-glacier acceleration and enhanced mass loss. This mechanism, absent from other models, provides a rapid dynamic response to climate change. Net mass loss estimates from the new model should reach ;220 km 3 yr 21 by 2100, significantly higher than estimates by the Intergovernmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4) of ;50-100 km 3 yr 21 . By 2100, the perennial frozen surface area decreases up to ;60%, to ;7 3 10 5 km 2 , indicating a massive expansion of the ablation zone. Ice mass change patterns, particularly along the periphery, are very similar between the two climate models. * Current affiliation

Verification of model simulated mass balance, flow fields and tabular calving events of the Antarctic ice sheet against remotely sensed observations

The Antarctic ice sheet (AIS) has the greatestpotential for global sea level rise. This study simulates AISice creeping, sliding, tabular calving, and estimates the totalmass balances, using a recently developed, advanced icedynamics model, known as SEGMENT-Ice. SEGMENTIceis written in a spherical Earth coordinate system.Because the AIS contains the South Pole, a projectiontransfer is performed to displace the pole outside of thesimulation domain. The AIS also has complex ice-watergranularmaterial-bedrock configurations, requiringsophisticated lateral and basal boundary conditions.Because of the prevalence of ice shelves, a ‘girder yield’type calving scheme is activated. The simulations of presentsurface ice flow velocities compare favorably with InSARmeasurements, for various ice-water-bedrock configurations.The estimated ice mass loss rate during 2003–2009agrees with GRACE measurements and provides morespatial details not represented by the latter. The modelestimated calving frequencies of the peripheral ice shelvesfrom 1996 (roughly when the 5-km digital elevation andthickness data for the shelves were collected) to 2009compare well with archived scatterometer images. SEGMENT-Ice’s unique, non-local systematic calving schemeis found to be relevant for tabular calving. However, theexact timing of calving and of iceberg sizes cannot besimulated accurately at present. A projection of the futuremass change of the AIS is made, with SEGMENT-Iceforced by atmospheric conditions from three differentcoupled general circulation models. The entire AIS is estimatedto be losing mass steadily at a rate of*120 km3/a atpresent and this rate possibly may double by year 2100

Storm-triggered Landslides in Warmer Climates

This volume covers the general physics of debris flows and various approaches to modeling - including the SEGMENT-Landslide approach – as well as the pros and cons of these approaches, and how other approaches are sub-sets of the SEGMENT-Landslide approach. In addition, this volume will systematically unify the concepts of vadose zone hydrology and geotechnical engineering, with special emphasis on quantifying ecosystem consequences of storm-triggered landslides in a warmer climate setting. The reader will find a comprehensive coverage of concepts ranging from hillslope hydrology, porous granular material rheology, and the fundamentals of soil properties to state-of-the-art concepts of enhanced hydrological cycle with climate warming, finishing with a discussion of new approaches for future research