Spin-up Period Research Articles

Impact of Ozone Valley over the Tibetan Plateau on the South Asian High in CAM5

Local climate effects of Tibetan Plateau Ozone Valley (OVTP) were investigated by numerical simulations using Community Atmosphere Model version 5.1.1 (CAM5). After a 20-year spin-up period, two additional 10-year experiments were conducted. CAM5 was driven by monthly mean climatological ozone in control experiment (CE) and OVTP in the sensitivity experiment (SE) was removed from May to September. After the removal of OVTP, South Asian High (SAH) becomes more robust and colder from June to August, especially in June. The reason for enhancement of SAH is that removal of OVTP increasing ozone in 200–30 hPa leads to significant enhancement of longwave and shortwave radiative heating rate in SAH region in June, and then enhancement of horizontal divergence resulting from the radiative warming leads to strengthening of SAH influenced by the Coriolis force, while the colder SAH is primarily caused by dynamic processes. Adiabatic expansion and ascending movement mainly bring about temperature decrease in SAH after OVTP removal, but the thermodynamic process related to radiative heating offsets part of the cooling response.

Sensitivity of Axisymmetric Tropical Cyclone Spinup Time to Dry Air Aloft

AbstractThe sensitivity of tropical cyclone spinup time to the initial entropy deficit of the troposphere is examined in an axisymmetric hurricane model. Larger initial entropy deficits correspond to less moisture above the initial lifting condensation level of a subcloud-layer parcel. The spinup time is quantified in terms of thresholds of integrated horizontal kinetic energy within a radius of 300 km and below a height of 1.5 km. The spinup time increases sublinearly with increasing entropy deficit, indicating the greatest sensitivity lies with initial moisture profiles closer to saturation. As the moisture profile approaches saturation, there is a large increase in the low-level, area-averaged, vertical mass flux over the spinup period because of the predominance of deep convection. Higher entropy deficit experiments have a greater amount of cumulus congestus and reduced vertical mass flux over a longer duration. Consequently, the secondary circulation takes longer to build upward, and the radial influx of angular momentum is reduced. There is also a reduction in the conversion of potential available enthalpy to horizontal kinetic energy, as a result of reduced flow down the radial pressure gradient early in the spinup period. Later in the spinup period, the low-level vortex spins up relatively quickly near the nascent radius of maximum wind in the high-entropy deficit experiments, whereas the low-level vortex spins up over a wider area in the low-entropy deficit experiments.

SeaRISE experiments revisited: potential sources of spread in multi-model projections of the Greenland ice sheet

Abstract. The present paper revisits the future surface-climate experiments on the Greenland ice sheet proposed by the Sea-level Response to Ice Sheet Evolution (SeaRISE; Bindschadler et al., 2013) study. The projections of the different SeaRISE participants show dispersion, which has not been examined in detail to date. A series of sensitivity experiments are conducted and analyzed using the ice-sheet model for integrated Earth-system studies (IcIES) by replacing one or more formulations of the model parameters with those adopted in other model(s). The results show that large potential sources of the dispersion among the projections of the different SeaRISE participants are differences in the initialization methods and in the surface mass balance methods, and both aspects have almost equal impact on the results. The treatment of ice-sheet margins in the simulation has a secondary impact on the dispersion. We conclude that spinning up the model using fixed topography through the spin-up period while the temperature is allowed to evolve according to the surface temperature history is the preferred representation, at least for the experiment configuration examined in the present paper. A benchmark model experimental setup that most of the numerical models can perform is proposed for future intercomparison projects, in order to evaluate the uncertainties relating to pure ice-sheet model flow characteristics.

Assessment of a Nonlinear Ensemble Transform Filter for High-Dimensional Data Assimilation

Abstract This work assesses the large-scale applicability of the recently proposed nonlinear ensemble transform filter (NETF) in data assimilation experiments with the NEMO ocean general circulation model. The new filter constitutes a second-order exact approximation to fully nonlinear particle filtering. Thus, it relaxes the Gaussian assumption contained in ensemble Kalman filters. The NETF applies an update step similar to the local ensemble transform Kalman filter (LETKF), which allows for efficient and simple implementation. Here, simulated observations are assimilated into a simplified ocean configuration that exhibits globally high-dimensional dynamics with a chaotic mesoscale flow. The model climatology is used to initialize an ensemble of 120 members. The number of observations in each local filter update is of the same order resulting from the use of a realistic oceanic observation scenario. Here, an importance sampling particle filter (PF) would require at least 106 members. Despite the relatively small ensemble size, the NETF remains stable and converges to the truth. In this setup, the NETF achieves at least the performance of the LETKF. However, it requires a longer spinup period because the algorithm only relies on the particle weights at the analysis time. These findings show that the NETF can successfully deal with a large-scale assimilation problem in which the local observation dimension is of the same order as the ensemble size. Thus, the second-order exact NETF does not suffer from the PF’s curse of dimensionality, even in a deterministic system.

Modelling soil organic carbon 1. Performance of APSIM crop and pasture modules against long-term experimental data

We used the APSIM model to explore the ability to simulate soil organic carbon (SOC) from three long term agricultural experiments (LTAE) over 24–44years in eastern Australia (Hamilton Victoria, Wagga Wagga New South Wales and Warwick Queensland). The model was initialized for each location soil type using a 20-year spin-up period to reach stable SOC fractions. The model was not tuned to any of the data at each site. The cropping systems model simulated reasonably well the calculated changes in SOC over the 24–44-year period at the three different sites under various agronomic management practices. At Hamilton calculated SOC did not change significantly over 32years and the APSIM-Agpasture model represented this satisfactorily. Root mean square error was 4.9tCha−1 (0–30cm) (5.2%) ranging from 1.5 to 10.9tCha−1 (1.7 to 11.8%) over time. Similarly, over twenty-four years of continuous cropping at Wagga Wagga under a Wheat-Lupin crop rotation using APSIM-Wheat and APSIM-Lupin under stubble retention and zero tillage showed a largely neutral trend over time with RMSE of 3.4tCha−1 (0–30cm) (9.5%) ranging from 1.4 to 7.8tCha−1 (4 to 21%) over time. The SOC was much lower at Wagga Wagga compared to that at Hamilton. Forty-four years of continuous cropping at the Queensland site resulted in significant declines in SOC irrespective of the farming practices applied. However, the rates of decline were different with the least decline achieved from high N application with stubble retention. The modelled changes, using APSIM-Wheat and APSIM-Barley, in SOC matched reasonably well the calculated behaviour with the greatest decline under zero N application with overall RMSE of 4.1tCha−1 (0–30cm) (4.5%) ranging from 1.4 to 8.7tCha−1 (2 to 7%) over time. The biophysical simulation model exemplified by the APSIM model explained well the observed changes in SOC at the various locations without specifically fitting the model to the observed data, despite large variation in the calculated data. This indicates that the SOC model was robust over the moderate to long term period. As such the model is suitable to extrapolate a simulated response beyond these locations under various treatment combinations that should predict realistic SOC stocks within the expected mean error of less than 10% (3 to 6tCha−1 0–30cm).

An Adaptive Compensatory Approach of the Fixed Localization in the EnKF

Abstract Although the fixed covariance localization in the ensemble Kalman filter (EnKF) can significantly increase the reliability of background error covariance, it has been demonstrated that extreme impact radii can cause the EnKF to lose some useful information. Tuning an optimal impact radius, on the other hand, is always difficult for a general circulation model. The EnKF multiscale analysis (MSA) approach was presented to make up for the above-mentioned drawback of the fixed localization. As a follow-up, this study presents an adaptive compensatory approach to further improve the performance of the EnKF-MSA. The new method adaptively triggers a multigrid analysis (MGA) to extract multiscale information from the observational residual after the EnKF without inflation is completed at each analysis step. Within a biased twin experiment framework consisting of a barotropic spectral model and an idealized observing system, the performance of the adaptive method is examined. Results show that the MGA reduces the computational cost of the MSA by 93%. On the assimilation quality, the adaptive method has an incremental improvement over the EnKF-MSA. That is, the adaptive EnKF-MGA reduces to the EnKF without inflation, which is better than the EnKF-MSA, for moderate impact radii. The proposed scheme works for a broader range of impact radii than the standard EnKF (i.e., the EnKF with inflation). For extreme impact radii, the adaptive EnKF-MGA can produce smaller assimilation errors than the standard EnKF and shorten the spinup period by 53%. In addition, the computational cost of the MGA is negligible relative to that of the standard EnKF.

Assimilation of surface AWS using 3DVAR and LAPS and their effects on short-term high-resolution weather forecasts

The progress in data assimilation techniques that incorporate weather observations into high-resolution numerical weather prediction models is challenging because of handling surface data in terrain misrepresentation, balance approximations, instrument errors and sensor representativeness. In the framework of operational numerical weather prediction, two data assimilation systems are compared using conventional observations from surface Automatic Weather Stations (AWS), a three-dimensional variational analysis (3DVAR) and the Local Analysis and Prediction System (LAPS). The goal is to study the ability of these two systems to assimilate data from AWS and to assess which performs better for near-surface wind and temperature fields to initialize a short-range 1-km resolution forecast with the Weather Research and Forecasting (WRF) model. Results show that the 3DVAR assimilation patterns are unrealistic given the inhomogeneous nature of the near-surface fields in complex terrains. In contrast, LAPS analyses show a heterogeneous assimilation pattern, more consistent with the complexity of the terrain and the observations. During the model spin-up period, simulations initialized using both data assimilation methods approach rapidly the control simulation, initialized without assimilation. However, the 1km resolution forecasts initialized with LAPS exhibit a significant improvement, particularly for the wind field module.

Concurrent correction method for modeling morphological response to dredging an offshore sandpit

In modeling the morphological impact of an offshore dredging pit using a process-based model, the hydrodynamic conditions, often derived from regional scale models and sometimes incomplete measurements, may not be in equilibrium with the initial bathymetry profile before dredging. This lack of equilibrium causes a fast profile adjustment toward an equilibrium, which may bias the model results representing the direct effect of the pit.In this study, a method is described which uses the parallel computing technique and evaluates only the morphological changes due to a single factor, i.e., the dredging of a pit. The hydrodynamics and morphodynamics are assumed to be in equilibrium before initial dredging of the pit. The morphological evolution is computed based on the change of sediment flux due to the presence of the pit. The model can avoid the computational demanding spin-up period necessary to reach equilibrium conditions. In particular the concurrent correction method is efficient for scenario testing with a large amount of morphological settings and hydrodynamic conditions.

Technical Note: Reducing the spin-up time of integrated surface water–groundwater models

Abstract. One of the main challenges in the application of coupled or integrated hydrologic models is specifying a catchment's initial conditions in terms of soil moisture and depth-to-water table (DTWT) distributions. One approach to reducing uncertainty in model initialization is to run the model recursively using either a single year or multiple years of forcing data until the system equilibrates with respect to state and diagnostic variables. However, such "spin-up" approaches often require many years of simulations, making them computationally intensive. In this study, a new hybrid approach was developed to reduce the computational burden of the spin-up procedure by using a combination of model simulations and an empirical DTWT function. The methodology is examined across two distinct catchments located in a temperate region of Denmark and a semi-arid region of Australia. Our results illustrate that the hybrid approach reduced the spin-up period required for an integrated groundwater–surface water–land surface model (ParFlow.CLM) by up to 50%. To generalize results to different climate and catchment conditions, we outline a methodology that is applicable to other coupled or integrated modeling frameworks when initialization from an equilibrium state is required.

Wind climate estimation using WRF model output: method and model sensitivities over the sea

High-quality tall mast and wind lidar measurements over the North and Baltic Seas are used to validate the wind climatology produced from winds simulated by the Weather, Research and Forecasting (WRF) model in analysis mode. Biases in annual mean wind speed between model and observations at heights around 100 m are smaller than 3.2% at offshore sites, except for those that are affected by the wake of a wind farm or the coastline. These biases are smaller than those obtained by using winds directly from the reanalysis. We study the sensitivity of the WRF-simulated wind climatology to various model setup parameters. The results of the year-long sensitivity simulations show that the long-term mean wind speed simulated by the WRF model offshore in the region studied is quite insensitive to the global reanalysis, the number of vertical levels, and the horizontal resolution of the sea surface temperature used as lower boundary conditions. Also, the strength and form (grid vs spectral) of the nudging is quite irrelevant for the mean wind speed at 100 m. Large sensitivity is found to the choice of boundary layer parametrization, and to the length of the period that is discarded as spin-up to produce a wind climatology. It is found that the spin-up period for the boundary layer winds is likely larger than 12 h over land and could affect the wind climatology for points offshore for quite a distance downstream from the coast.

An Observational Study of Tropical Cyclone Spinup in Supertyphoon Jangmi (2008) from 24 to 27 September

AbstractAn observational study of tropical cyclone intensification is performed using dropsondes, in situ flight-level data, satellite imagery, and Electra Doppler Radar (ELDORA) during the spinup of Tropical Storm Jangmi (2008) in the western North Pacific. This event was observed with research aircraft during the Tropical Cyclone Structure 2008 (TCS08) field experiment over the course of 3 days as Jangmi intensified rapidly from a tropical storm to a supertyphoon. The dropsonde analysis indicates that the peak azimuthally averaged storm-relative tangential wind speed occurs persistently within the boundary layer throughout the spinup period and suggests that significant supergradient winds are present near and just within the radius of maximum tangential winds. An examination of the ELDORA data in Tropical Storm Jangmi reveals multiple rotating updrafts near the developing eye beneath cold cloud top temperatures ≤−65°C. In particular, there is a 12-km-wide, upright updraft with a peak velocity of 9 m s−1 with collocated strong low-level (z < 2 km) convergence of 2 × 10−3 s−1 and intense relative vorticity of 4 × 10−3 s−1. The analysis of the corresponding infrared satellite imagery suggests that vortical updrafts are common before and during rapid intensification. The findings of this study support a recent paradigm of tropical cyclone intensification in which rotating convective clouds are important elements in the spinup process. In a system-scale view of this process, the maximum tangential wind is found within the boundary layer, where the tangential wind becomes supergradient before the air ascends into the eyewall updraft.

Asymmetric and axisymmetric dynamics of tropical cyclones

Abstract. We present the results of idealized numerical experiments to examine the difference between tropical cyclone evolution in three-dimensional (3-D) and axisymmetric (AX) model configurations. We focus on the prototype problem for intensification, which considers the evolution of an initially unsaturated AX vortex in gradient-wind balance on an f plane. Consistent with findings of previous work, the mature intensity in the 3-D model is reduced relative to that in the AX model. In contrast with previous interpretations invoking barotropic instability and related horizontal mixing processes as a mechanism detrimental to the spin-up process, the results indicate that 3-D eddy processes associated with vortical plume structures can assist the intensification process by contributing to a radial contraction of the maximum tangential velocity and to a vertical extension of tangential winds through the depth of the troposphere. These plumes contribute significantly also to the azimuthally averaged heating rate and the corresponding azimuthal-mean overturning circulation. The comparisons show that the resolved 3-D eddy momentum fluxes above the boundary layer exhibit counter-gradient characteristics during a key spin-up period, and more generally are not solely diffusive. The effects of these eddies are thus not properly represented by the subgrid-scale parameterizations in the AX configuration. The resolved eddy fluxes act to support the contraction and intensification of the maximum tangential winds. The comparisons indicate fundamental differences between convective organization in the 3-D and AX configurations for meteorologically relevant forecast timescales. While the radial and vertical gradients of the system-scale angular rotation provide a hostile environment for deep convection in the 3-D model, with a corresponding tendency to strain the convective elements in the tangential direction, deep convection in the AX model does not suffer this tendency. Also, since during the 3-D intensification process the convection has not yet organized into annular rings, the azimuthally averaged heating rate and radial gradient thereof is considerably less than that in the AX model. This lack of organization results broadly in a slower intensification rate in the 3-D model and leads ultimately to a weaker mature vortex after 12 days of model integration. While azimuthal mean heating rates in the 3-D model are weaker than those in the AX model, local heating rates in the 3-D model exceed those in the AX model and at times the vortex in the 3-D model intensifies more rapidly than AX. Analyses of the 3-D model output do not support a recent hypothesis concerning the key role of small-scale vertical mixing processes in the upper-tropospheric outflow in controlling the intensification process. In the 3-D model, surface drag plays a particularly important role in the intensification process for the prototype intensification problem on meteorologically relevant timescales by helping foster the organization of convection in azimuth. There is a radical difference in the behaviour of the 3-D and AX simulations when the surface drag is reduced or increased from realistic values. Borrowing from ideas developed in a recent paper, we give a partial explanation for this difference in behaviour. Our results provide new qualitative and quantitative insight into the differences between the asymmetric and symmetric dynamics of tropical cyclones and would appear to have important consequences for the formulation of a fluid dynamical theory of tropical cyclone intensification and mature intensity. In particular, the results point to some fundamental limitations of strict axisymmetric theory and modelling for representing the azimuthally averaged behaviour of tropical cyclones in three dimensions.

Improving the spin-up of regional EnKF for typhoon assimilation and forecasting with Typhoon Sinlaku (2008)

The Running-In-Place (RIP) method is implemented in the framework of the Local Ensemble Transform Kalman Filter (LETKF) coupled with the Weather Research and Forecasting (WRF) model. RIP aims at accelerating the spin-up of the regional LETKF system when the WRF ensemble is initialised from a global analysis, which is obtained at a coarser resolution and lacks features related to the underlying mesoscale evolution. The RIP method is further proposed as an outer-loop scheme to improve the nonlinear evolution of the ensemble when the characteristics of the error statistics change rapidly owing to strong nonlinear dynamics. The impact of using RIP as an outer-loop for the WRF-LETKF system is evaluated for typhoon assimilation and prediction with Typhoon Sinlaku (2008) as a case study. For forecasts beyond one day, the typhoon track prediction is significantly improved after RIP is applied, especially during the spin-up period of the LETKF assimilation when Sinlaku is developing rapidly from a severe tropical storm to a typhoon. The impact of the dropsondes is significantly increased by RIP at early assimilation cycles. Results suggest that these improvements are because of the positive impact on the environmental condition of the typhoon. Results also suggest that using the RIP scheme adaptively allows RIP to be used as an outer-loop for the WRF-LETKF with further improvements.

A down-valley low-level jet event during T-REX 2006

A prolonged down-valley flow and low-level jet were observed throughout the Enhanced Observing Period 4 (April 28–29) of the 2006 Terrain-induced Rotor Experiment, held in the Owens Valley of California near the town of Independence. The low-level jet was strongest during the nocturnal hours, and special field observations captured important details of the event lifecycle. High-resolution simulations using the Weather Research and Forecasting numerical weather prediction model were generated, with underlying assumptions being that model resolution, boundary layer physics, and nesting configuration would be dominant controlling factors in reproducing the jet. The large-scale conditions were dry throughout the event, so moist physics were not a significant forcing consideration. For the control simulation, a two-nest (4.5 and 1.5 km grid spacing) configuration with 90 vertical levels was applied. Additionally, the Quasi-Normal Scale Elimination planetary boundary and surface layer option were selected due to its published performance under conditions of stable stratification. Three other sensitivity simulations were run for comparison, differing from the control just in the choice of vertical resolution (60 versus 90 levels with Quasi-Normal Scale Elimination) and planetary boundary/surface layer physics (90 levels/Mellor-Yamada-Jancic; 90 levels/Yonsei State University). Although the gross evolution (location, height, and timing) of the low-level jet is captured by all model runs (with the 1.5 km inner nest providing the more accurate details), there were at times large underestimations of the nocturnal jet speed max in each simulation (approaching 100 % error, or up to almost 10 m s−1). Overall, the variations of vertical resolution and planetary boundary/surface physics against the control seemed to (1) yield little overall improvement to statistical or subjective evaluations; (2) do little to improve deficiencies in reproducing the magnitude strength of the nocturnal down-valley low-level jet. Since the cold-start simulations spanned 36 h (including a 12-h spin-up period), it was suspected that the lateral boundary conditions imposed on the outermost 4.5 km nest might negatively impact the interior model solutions in the Owens Valley. To investigate this possibility, an additional simulation was executed by adding two extra nests to the control configuration: an outer 13.5 km and an inner 500 m. This simulation produced a better evolution of the nocturnal low-level jet and especially the speed max. The addition of the larger 13.5 km nest appears more critical to this improvement than that of the extra spatial resolution provided by the inner 500 m nest, which supports the idea that accurate capturing of the large-scale synoptic condition was critical in reproducing important details of this down-valley low-level jet event. The extra 500-m resolution did seem to improve the morning valley cold pool forecast.

Application of an operational storm coastal impact forecasting system

The National Oceanography Centre (NOC), in collaboration with the UK Met Office, has developed an operational suite of models for the Irish Sea. This work has extended this suite to include a coastal vulnerability early warning system for the Sefton Coast in Liverpool Bay. The system is designed to forecast when the offshore hydrodynamic and wave conditions exceed thresholds for low, moderate and high hazard of storm impact and also gives an indication of the extent of dune erosion. The model simulates a 72h period (yesterday:today:tomorrow), allowing a 24h spin-up period and 48h forecast. The main idea of the system is to use forecasts for impending storms to assist with management decisions.

The linear spin-up of a stratified, rotating fluid in a square cylinder

AbstractHere we present experimental and theoretical results for how a stratified fluid, initially rotating as a solid body with constant angular velocity, $\Omega $, within a closed cylinder of square cross-section, is spun up when subject to a small, impulsive increase, $ \mrm{\Delta} \Omega $, in the cylinder’s rotation rate. The fluid’s adjustment to the new state of solid rotation can be characterized by: (a) an inviscid, horizontal starting flow which conserves the vorticity of the initial condition; (b) the eruption of Ekman layer fluid from the perimeter region of the cylinder’s base and lid; (c) horizontal-velocity Rayleigh layers that grow into the interior from the container’s sidewalls; and (d) the formation and decay of columnar vortices in the vertical corner regions. Asymptotic results describe the inviscid starting flow, and the subsequent interior spin-up that occurs due to the combined effects of Ekman suction through the base and lid Ekman layers, and the growth of the sidewall Rayleigh layers. Attention is focused on the flow development over the spin-up time scale ${T}_{s} = {E}^{\ensuremath{-} 1/ 2} {\Omega }^{\ensuremath{-} 1} $, where $E$ is the Ekman number. (The spin-up process over the much longer diffusive time scale, ${T}_{d} = {E}^{\ensuremath{-} 1} {\Omega }^{\ensuremath{-} 1} $, is not considered here.) Experiments were performed using particle imaging velocimetry (PIV) to measure horizontal velocity components at fixed heights within the flow interior and at regular stages during the spin-up period. The velocity data obtained are shown to be in excellent agreement with the asymptotic theory.

Accelerating the EnKF Spinup for Typhoon Assimilation and Prediction

Abstract A mesoscale ensemble Kalman filter (EnKF) for a regional model is often initialized from global analysis products and with initial ensemble perturbations constructed based on the background error covariance used in the three-dimensional variational data assimilation (3DVar) system. Because of the lack of proper mesoscale information, a long spinup period of typically a few days is required for the regional EnKF to reach its asymptotic level of accuracy, and thus, the impact of observations is limited during the EnKF spinup. For the case of typhoon assimilation, such spinup usually corresponds to the stages of generation and development of tropical cyclones, when observations are important but limited over open waters. To improve the analysis quality during the spinup, the “running in place” (RIP) method is implemented within the framework of the local ensemble transform Kalman filter (LETKF) coupled with the Weather Research and Forecasting model (WRF). Results from observing system simulation experiments (OSSEs) for a specific typhoon show that the RIP method is able to accelerate the analysis adjustment of the dynamical structures of the typhoon during the LETKF spinup, and improves both the accuracy of the mean state and the structure of the ensemble-based error covariance. These advantages of the RIP method are found not only in the inner-core structure of the typhoon but also identified in the environmental conditions. As a result, the LETKF-RIP analysis leads to better typhoon prediction, particularly in terms of both track and intensity.

A 3DVAR-Based Dynamical Initialization Scheme for Tropical Cyclone Predictions*

Abstract A combined tropical cyclone dynamic initialization–three-dimensional variational data assimilation scheme (TCDI–3DVAR) is proposed. The specific procedure for the new initialization scheme is described as follows. First, a first-guess vortex field derived from a global analysis will be spun up in a full-physics mesoscale regional model in a quiescent environment. During the spinup period, the weak vortex is forced toward the observed central minimum sea level pressure (MSLP). The so-generated balanced TC vortex with realistic MSLP and a warm core is then merged into the environmental field and used in the subsequent 3DVAR data assimilation. The observation system simulation experiments (OSSEs) demonstrate that this new TC initialization scheme leads to much improved initial MSLP, warm core, and asymmetric temperature patterns compared to those from the conventional 3DVAR scheme. Forecasts of TC intensity with the new initialization scheme are made, and the results show that the new scheme is able to predict the “observed” TC intensity change, compared to runs with the conventional 3DVAR scheme or the TCDI-only scheme. Sensitivity experiments further show that the intensity forecasts with knowledge of the initial MSLP and wind fields appear more skillful than do the cases where the initial MSLP, temperature, and humidity fields are known. The numerical experiments above demonstrate the potential usefulness of the proposed new initialization scheme in operational applications. A preliminary test of this scheme with a navy operational model shows encouraging results.

Effects of short spin-up periods on soil moisture simulation and the causes over New Zealand

[1] A spin-up is needed for a land surface model (LSM) to simulate reliable soil moisture. The best spin-up is to integrate a LSM for many consecutive years before the start of an experimental period. Alternatively, a one-year spin-up period (defined in the text) can be used to spin-up a LSM if longer periods of forcing data are not available. However, for special cases with only a few months of observations, can one use a few months to spin-up a LSM? If yes, what are the conditions? To answer these questions, we have run a LSM during 1 August 2008–31 July 2010 at 4 sites in New Zealand with widely different annual rainfall amount and soil textures. Two test groups were conducted at each station. Each group had 6 spin-up tests differing only in spin-up periods, ranging from 2 months to 1 year (CTRL). For the 4 stations, a few months had the same effect as CTRL except at Winchmore, where significant differences in simulated soil moisture between a test and CTRL still occurred at or after the equilibrium state. The results at Winchmore repeated at the other 3 stations with reduced rainfall rates, and vice versa. Sensitivity tests with decreased or increased rainfall rates showed that quite large differences in simulated soil moisture at the equilibrium states between CTRL and a test occurred in an annual rainfall window from 350–1000 mm. Further analyses suggested a E/R ratio (the ratio of evaporation accumulation to rainfall accumulation during the spin-up period at the last spin-up cycle) to determine whether or not a few months can be used to spin-up a LSM. A simple rule was suggested for application of these results in New Zealand and other areas with almost similar climates to those of New Zealand.

A new scheme for initializing process-based ecosystem models by scaling soil carbon pools

Process-based ecosystem models are useful tools, not only for understanding the forest carbon cycle, but also for predicting future change. In order to apply a model to simulate a specific time period, model initialization is required. In this study, we propose a new scheme of initialization for forest ecosystem models, which we term a “slow-relaxation scheme”, that entails scaling of the soil carbon and nitrogen pools slowly during the spin-up period. The proposed slow-relation scheme was tested with the CENTURY version 4 ecosystem model. Three different combinations of scaled soil pools were also tested, and compared to the results from a fast-relaxation regime. The fast-relaxation of soil pools produced unstable, transient model behaviour whereas slow-relaxation overcame this instability. This approach holds promise for initializing ecosystem models, and for starting simulations with more realistic initial conditions.