Impact Of Lateral Boundary Conditions Research Articles

Why do inverse models disagree? A case study with two European CO2 inversions

Abstract. We present an analysis of atmospheric transport impact on estimating CO2 fluxes using two atmospheric inversion systems (CarboScope-Regional (CSR) and Lund University Modular Inversion Algorithm (LUMIA)) over Europe in 2018. The main focus of this study is to quantify the dominant drivers of spread amid CO2 estimates derived from atmospheric tracer inversions. The Lagrangian transport models STILT (Stochastic Time-Inverted Lagrangian Transport) and FLEXPART (FLEXible PARTicle) were used to assess the impact of mesoscale transport. The impact of lateral boundary conditions for CO2 was assessed by using two different estimates from the global inversion systems CarboScope (TM3) and TM5-4DVAR. CO2 estimates calculated with an ensemble of eight inversions differing in the regional and global transport models, as well as the inversion systems, show a relatively large spread for the annual fluxes, ranging between −0.72 and 0.20 PgC yr−1, which is larger than the a priori uncertainty of 0.47 PgC yr−1. The discrepancies in annual budget are primarily caused by differences in the mesoscale transport model (0.51 PgC yr−1), in comparison with 0.23 and 0.10 PgC yr−1 that resulted from the far-field contributions and the inversion systems, respectively. Additionally, varying the mesoscale transport caused large discrepancies in spatial and temporal patterns, while changing the lateral boundary conditions led to more homogeneous spatial and temporal impact. We further investigated the origin of the discrepancies between transport models. The meteorological forcing parameters (forecasts versus reanalysis obtained from ECMWF data products) used to drive the transport models are responsible for a small part of the differences in CO2 estimates, but the largest impact seems to come from the transport model schemes. Although a good convergence in the differences between the inversion systems was achieved by applying a strict protocol of using identical prior fluxes and atmospheric datasets, there was a non-negligible impact arising from applying a different inversion system. Specifically, the choice of prior error structure accounted for a large part of system-to-system differences.

Dynamical downscaling of temperature extremes over China using the WRF model driven by different lateral boundary conditions

Extreme temperature events have considerable impact on society and natural ecosystems, which receive less attention. This study examined the impact of lateral boundary conditions (LBCs) on the downscaling simulations of the Weather Research and Forecasting (WRF) model for temperature extremes over China for the period 1981–2010. The driving data comprised two reanalysis datasets from two different institutes, ERA-interim and NECP-R2. Twelve extreme temperature indices were calculated from the WRF simulations and compared with both reanalysis products and observations. The comparison includes their climate mean and interannual variation. Results indicated that WRF can skillfully reproduce the spatial distributions of extreme temperature indices, despite some biases in certain regions. The large biases in the warmest and coldest days, warm spell duration and cool days over eastern China from reanalysis data were reduced significantly. The simulation driven by ERA-interim outperforms NECP-R2. In terms of interannual variation, the WRF model was able to capture the observational trends except the number of days with warm spell and days with moderate warm. Furthermore, the accuracy of WRF in simulating interannual variation was consistent with that of the driving data. The WRF simulation driven by ERA-interim appeared best in terms of bias frequency distributions for most indices. Overall, the results revealed that WRF dynamical downscaling of temperature extremes is sensitive to selection of LBCs, and that performance could be improved by adopting driving data such as ERA-interim.

Impact of lateral boundary and initial conditions in the prediction of Bay of Bengal cyclones using WRF model and its 3D-VAR data assimilation system

This study examines the impact of lateral boundary conditions (LBCs) and initial conditions (ICs) in the prediction of land-falling tropical cyclones (TCs) using Weather Research and Forecasting (WRF) model and its three-dimensional variational (3D-VAR) data assimilation system for the Bay of Bengal (BoB) region. The LBCs are derived from the National Centre for Environmental Prediction (NCEP) Final (FNL) analysis and GFS forecasted datasets at different horizontal and temporal resolutions. The assimilated datasets in the model comprises of NCEP PREBUFR (PILOT, SOUND, SYNOP, METAR, SONDE, BUOY, SHIP and QuikSCAT) and satellite radiances from Advanced Microwave Sounding Unit (AMSU-A, AMSU-B), High Resolution Infra-Red Sounder (HIRS), Microwave Humidity Sounder (MHS) at different assimilation cycles. The impact of the LBCs with varying spatial and temporal resolutions are investigated in detail and the results indicate that the forecasted track during the first 48 h are not affected, however the subsequent forecasted track is significantly influenced by the 3 hourly GFS LBCs. The initial intensity, positional errors, and storm structure show improvements with the assimilation of NCEP PREBUFR and radiance observations at different assimilation cycles. Simulations carried out with improved initial conditions indicate that the forecast (in terms of track, intensity, and trends in the intensification and dissipation) of the cyclonic storm Sidr improved significantly. In addition, five severe land-falling BoB cyclones (Phailin, Lehar, Helen, Madi, and Hudhud) during 2013–2014 were studied using 3 hourly GFS LBCs and with assimilation of NCEP PREBUFR and satellite radiance observations. The results clearly signify the ability of the WRF modeling system in forecasting the track, intensity, and landfall of the storms. The forecasted mean track errors during 2013–2014 cyclones are 65 km, 58 km, 99 km, and 103 km from day-1 to day-4 respectively, while the initial positional errors was about 41 km. The mean landfall time and positional errors are about 3 h and 15 km respectively. The forecasted intensity of the storm in terms of maximum surface wind (MSW) is also reasonably well predicted, and these errors are 4 ms-1, 10 ms-1, 6ms-1, and 8 ms-1 from day-1 to day-4 respectively.

Impact of Lateral Boundary Conditions on Regional Analyses

Abstract Regional and global climate models are usually validated by comparison to derived observations or reanalyses. Using a model in data assimilation results in a direct comparison to observations to produce its own analyses that may reveal systematic errors. In this study, regional analyses over North America are produced based on the fifth-generation Canadian Regional Climate Model (CRCM5) combined with the variational data assimilation system of the Meteorological Service of Canada (MSC). CRCM5 is driven at its boundaries by global analyses from ERA-Interim or produced with the global configuration of the CRCM5. Assimilation cycles for the months of January and July 2011 revealed systematic errors in winter through large values in the mean analysis increments. This bias is attributed to the coupling of the lateral boundary conditions of the regional model with the driving data particularly over the northern boundary where a rapidly changing large-scale circulation created significant cross-boundary flows. Increasing the time frequency of the lateral driving and applying a large-scale spectral nudging significantly improved the circulation through the lateral boundaries, which translated in a much better agreement with observations.

Modeling the potential contribution of land cover changes to the late twentieth century Sahel drought using a regional climate model: impact of lateral boundary conditions

This paper investigates the potential impact of “idealized-but-realistic” land cover degradation on the late twentieth century Sahel drought using a regional climate model (RCM) driven with lateral boundary conditions (LBCs) from three different sources, including one re-analysis data and two global climate models (GCMs). The impact of land cover degradation is quantified based on a large number of control-and-experiment pairs of simulations, where the experiment features a degraded land cover relative to the control. Two different approaches of experimental design are tested: in the 1st approach, the RCM land cover degradation experiment shares the same LBCs as the corresponding RCM control, which can be derived from either reanalysis data or a GCM; with the 2nd approach, the LBCs for the RCM control are derived from a GCM control, and the LBCs for the RCM land cover degradation experiment are derived from a corresponding GCM land cover degradation experiment. When the 1st approach is used, results from the RCM driven with the three different sources of LBCs are generally consistent with each other, indicating robustness of the model response against LBCs; when the 2nd approach is used, the RCM results show strong sensitivity to the source of LBCs and the response in the RCM is dominated by the response of the driving GCMs. The spatiotemporal pattern of the precipitation response to land cover degradation as simulated by RCM using the 1st approach closely resembles that of the observed historical changes, while results from the GCMs and the RCM using the 2nd approach bear less similarity to observations. Compared with the 1st approach, the 2nd approach has the advantage of capturing the impact on large scale circulation, but has the disadvantage of being influenced by the GCMs’ internal variability and any potential erroneous response of the driving GCMs to land degradation. The 2nd approach therefore requires a large ensemble to reduce the uncertainties derived from the driving GCMs. All RCM experiments based on the 1st approach produce a predominantly dry signal in West Africa throughout the year, with a dipole pattern found in the peak monsoon season that features a slight increase of precipitation over the Guinea Coast and strong decrease in the north; a similar spatiotemporal distribution is found for temperature changes, with warming (cooling) coinciding with precipitation decrease (increase). The model precipitation changes in West Africa are dominated by evapotranspiration changes in the north and by atmospheric moisture convergence changes in the south; in temperature changes, surface warming due to the decrease of evaporative cooling dominates over the albedo-induced radiative cooling.

Impact of lateral boundary conditions on precipitation and temperature extremes over South Korea in the CORDEX regional climate simulation using RegCM4

In this study, the impact of lateral boundary conditions (LBCs) on the simulation skills of the climate model RegCM4 for extreme climate events (ECEs) over South Korea are investigated using two sets of 20-yr (1989–2008) integration with two LBCs (ERA and R-2). The ECEs were defined by a percentile method and an absolute threshold method. In general, RegCM4 successfully reproduces the spatial distribution and the inter-annual variability (R > 0.76) of ECEs indices (precipitation and temperature extreme 5%) irrespective of LBCs. When driven by ERA, RegCM4 shows better simulation skill for ECEs (PR5%, TX5%, hot days, tropical nights, and wet days) than when driven by R-2. The ERA run simulates more than +7 mm day−1 and +1.1°C for PR5% and TX5% than those by the R-2 run, respectively. Although RegCM4’s simulation skills for the ECEs defined by absolute thresholds are significantly different according to LBCs, RegCM4 captured the interannual variability of frost days, hot days, tropical nights, and dry days well. However, the numbers of hot days and dry days are strongly underestimated and overestimated, respectively, compared to the observations, irrespective of LBCs. The large differences in the simulation skill of RegCM4 for ECEs over South Korea between two simulations can be caused by the differences between two LBCs, ERA and R-2. For the mean climate state, ERA simulates more humidity and warmer temperatures than the R-2, especially in the low-to-mid troposphere, resulting in a warmer and more humid troposphere especially at the central region of the model domain.

Summer Land–Atmosphere Coupling Strength over the United States: Results from the Regional Climate Model RegCM4–CLM3.5

AbstractThis study investigates the land–atmosphere coupling strength during summer over the United States using the Regional Climate Model version 4 (RegCM4)–Community Land Model version 3.5 (CLM3.5). First, a 10-yr simulation driven with reanalysis lateral boundary conditions (LBCs) is conducted to evaluate the model performance. The model is then used to quantify the land–atmosphere coupling strength, predictability, and added forecast skill (for precipitation and 2-m air temperature) attributed to realistic land surface initialization following the Global Land–Atmosphere Coupling Experiment (GLACE) approaches. Similar to previous GLACE results using global climate models (GCMs), GLACE-type experiments with RegCM4 identify the central United States as a region of strong land–atmosphere coupling, with soil moisture–temperature coupling being stronger than soil moisture–precipitation coupling, and confirm that realistic soil moisture initialization is more promising in improving temperature forecasts than precipitation forecasts. At a 1–15-day lead, the added forecast skill reflects predictability (or land–atmosphere coupling strength) indicating that that model can capture the realistic land–atmosphere coupling at a short time scale. However, at a 16–30-day lead, predictability cannot translate to added forecast skill, implying that the coupling at the longer time scale may not be represented well in the model. In addition, comparison of results from GLACE2-type experiments with RegCM4 driven by reanalysis LBCs and those driven by GCM LBCs suggest that the intrinsic land–atmosphere coupling strength within the regional model is the dominant factor for the added forecast skill at a 1–15-day lead, while the impact of LBCs from the GCM may play a dominant role in determining the signal of added forecast skill in the regional model at a 16–30-day lead. It demonstrates the complexities of using regional climate model for GLACE-type studies.

Uncertainty of lateral boundary conditions in a convection-permitting ensemble: a strategy of selection for Mediterranean heavy precipitation events

Abstract. This study examines the impact of lateral boundary conditions (LBCs) in convection-permitting (C-P) ensemble simulations with the AROME model driven by the ARPEGE EPS (PEARP). Particular attention is paid to two torrential rainfall episodes, observed on 15–16 June 2010 (the Var case) and 7–8 September 2010 (the Gard-Ardèche case) over the southeastern part of France. Regarding the substantial computing time for convection-permitting models, a methodology of selection of a few LBCs, dedicated for C-P ensemble simulations of heavy precipitation events is evaluated. Several sensitivity experiments are carried out to evaluate the skill of the AROME ensembles, using different approaches for selection of the driving PEARP members. The convective-scale predictability of the Var case is very low and it is driven primarily by a surface low over the Gulf of Lyon inducing a strong convergent low-level flow, and accordingly advecting strong moisture supply from the Mediterranean Sea toward the flooded area. The Gard-Ardèche case is better handled in ensemble simulations as a surface cold front moved slowly eastwards while increasing the low-level water vapour ahead is well reproduced. The selection based on a cluster analysis of the PEARP members generally better performs against a random selection. The consideration of relevant meteorological parameters for the convective events of interest (i.e. geopotential height at 500 hPa and horizontal moisture flux at 925 hPa) refined the cluster analysis. It also helps in better capturing the forecast uncertainty variability which is spatially more localized at the "high-impact region" due to the selection of more mesoscale parameters.

The impact of chemical lateral boundary conditions on CMAQ predictions of tropospheric ozone over the continental United States

A sensitivity study is performed to examine the impact of lateral boundary conditions (LBCs) on the NOAA-EPA operational Air Quality Forecast Guidance over continental USA. We examined six LBCS: the fixed profile LBC, three global LBCs, and two ozonesonde LBCs for summer 2006. The simulated results from these six runs are compared to IONS ozonesonde and surface ozone measurements from August 1 to 5, 2006. The choice of LBCs can affect the ozone prediction throughout the domain, and mainly influence the predictions in upper altitude or near inflow boundaries, such as the US west coast and the northern border. Statistical results shows that the use of global model predictions for LBCs could improve the correlation coefficients of surface ozone prediction over the US west coast, but could also increase the ozone mean bias in most regions of the domain depending on global models. In this study, the use of the MOZART (Model for Ozone And Related chemical Tracers) prediction for CMAQ (Community Multiscale Air Quality) LBC shows a better surface ozone prediction than that with fixed LBC, especially over the US west coast. The LBCs derived from ozonesonde measurements yielded better O3 correlations in the upper troposphere.

Stationary Rossby wave propagation in a shear flow along a reflective boundary

This study investigates the impact of lateral boundary conditions on the propagation and dispersion of locally excited Rossby waves in a zonally periodic, barotropic, quasigeostrophic channel model on the β-plane. We use basic flows with either a linear meridional shear or a jet-like profile. On the southern boundary of the channel we impose either a rigid wall or a radiation condition, whereas the northern sidewall is permeable for Rossby waves. We compare the numerical solutions found for a reflecting southern boundary in a weakly dissipative flow to the solutions obtained from a WKB-analysis for the corresponding unforced nondissipative situation. Furthermore, we compare the generalized Eliassen-Palm flux vectors to the ray paths of Rossby wave packets, obtained from WKB ray tracing. In particular, we focus our investigation on the two-dimensional structure of trapped modal waves and wave trains in a simple linear numerical model. Summarizing our results, we find that along the reflective wall, trapped modal wave structures as well as reflected wave trains occur with characteristics (e.g., wave numbers, turning latitudes) similar to the ones computed using asymptotic methods. In a linear sheared flow wave packets are trapped for all zonal wave numbers in contrast to a jet-like mean flow which has a selective effect on the waves; i.e., a turning latitudes phenomenon between the coast and the flow maximum occurs for short waves, while long waves can propagate freely across the zonal mean flow. This comes out clearly when studying the stream lines of the Eliassen-Palm flux vectors of the numerical model simulations. Furthermore, due to the reflected wave activity, the dispersion of Rossby waves is influenced by the southern boundary condition not only in the vicinity of the border but also in regions away from the boundary. These results appear to be important on the one hand for the existence of trapped Rossby waves in large-scale oceanic shear flows along a zonally oriented coast. And, on the other hand for large-scale boundary waves in conceptional atmospheric channel models which can lead to unwanted resonance effects.

The Impact of Lateral Boundary Conditions and Horizontal Resolution on North Atlantic Water Mass Transformations and Pathways in an Isopycnic Coordinate Ocean Model

Abstract A series of numerical experiments for the North Atlantic have been carried out with a primitive equation density-coordinate model, incorporating a relaxation zone south of 65°N designed to simulate the production of dense water in regions outside the model domain. The principal experiments vary both the horizontal grid resolution of the model (0.9° and 1/3°) and the width of the buffer zone (4° and 4/3°); an additional model experiment addresses the issue of the forcing used within the restoring region. The North Atlantic water mass transformations simulated in the various experiments and the subsequent pathways of converted water from the northern boundary region into the interior basin are documented both by time-evolution and time-mean analysis of the model results, focusing on the mass content of the isopycnic layers, the spread of water masses tagged with a passive tracer, the meridional overturning streamfunction and associated transport of heat, and mass transport budgets within specified ...