Mitigating parameter bias in hydrological modelling due to uncertainty in covariates     

Shahadat Chowdhury  Ashish Sharma   《Journal of Hydrology》2007,340(3-4):197-204

The uncertainty in hydrological model covariates, if ignored, introduces systematic bias in the parameters estimated. We introduce here a method to determine the true value of parameters given uncertainty in model inputs. This method, known as simulation extrapolation (SIMEX) operates on the basis of an empirical relationship between parameters and the level of input noise (or uncertainty). The method starts by generating a series of alternate model inputs by artificially adding white noise in increasing multiples of the known error variance. The resulting parameter sets allow us to formulate an empirical relationship between their values and the level of noise present. SIMEX is based on theory that the trend in alternate parameters can be extrapolated back to the notional error free zone.

We illustrate the strength of SIMEX in improving skills of predictive models that use uncertain sea surface temperature anomaly (SSTA) data over the NINO3 region as predictor to the southern oscillation index (SOI), an alternate measure of the strength of the El Nino southern oscillation. Our hypothesis is that the higher magnitude of noise in the pre 1960 data period introduces bias to model parameters where SSTA is the input variable. The relatively error invariant southern oscillation index (SOI) is regressed over SSTA and calibrated using a subset of the series from 1900 to 1960. We validate the resulting models using the less erroneous 1960–2003 data period. Overall the application of SIMEX is found to reduce the residual predictive errors during the validation period.  相似文献   

Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 2: 2D NMR spectroscopy     

Ashish P. Deshmukh  Michael B. Hay 《Geochimica et cosmochimica acta》2007,71(14):3533-3544

Carboxyl groups are abundant in natural organic molecules (NOM) and play a major role in their reactivity. The structural environments of carboxyl groups in IHSS soil and river humic samples were investigated using 2D NMR (heteronuclear and homonuclear correlation) spectroscopy. Based on the ¹H-¹³C heteronuclear multiple-bond correlation (HMBC) spectroscopy results, the carboxyl environments in NOM were categorized as Type I (unsubstituted and alkyl-substituted aliphatic/alicyclic), Type II (functionalized carbon substituted), Type IIIa, b (heteroatom and olefin substituted), and Type IVa, b (5-membered heterocyclic aromatic and 6-membered aromatic). The most intense signal in the HMBC spectra comes from the Type I carboxyl groups, including the ²J_CH and ³J_CH couplings of unsubstituted aliphatic and alicyclic acids, though this spectral region also includes the ³J_CH couplings of Type II and III structures. Type II and III carboxyls have small but detectable ²J_CH correlations in all NOM samples except for the Suwannee River humic acid. Signals from carboxyls bonded to 5-membered aromatic heterocyclic fragments (Type IVa) are observed in the soil HA and Suwannee River FA, while correlations to 6-membered aromatics (Type IVb) are only observed in Suwannee River HA. In general, aromatic carboxylic acids may be present at concentrations lower than previously imagined in these samples. Vibrational spectroscopy results for these NOM samples, described in an accompanying paper [Hay M. B. and Myneni S. C. B. (2007) Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 1: Infrared spectroscopy. Geochim. Cosmochim. Acta (in press)], suggest that Type II and Type III carboxylic acids with α substituents (e.g., -OH, -OR, or -CO₂H) constitute the majority of carboxyl structures in all humic substances examined. Furoic and salicylic acid structures (Type IV) are also feasible fragments, albeit as minor constituents. The vibrational spectroscopy results also suggest that much of the “Type I” signal observed in the HMBC spectrum is due to carboxylic acid esters and possibly α-substituted alicyclic acids.  相似文献   

New atmospheric pCO₂ estimates from palesols during the late Paleocene/early Eocene global warming interval     

Ashish Sinha  Lowell D. Stott 《Global and Planetary Change》1994,9(3-4)

The late Paleocene to early Eocene was one of the warmest intervals in Earth's history. Superimposed on this long-term warming was an abrupt short-term extreme warm event at or near the Paleocene/Eocene boundary and centered in the higher latitudes. This short-term climate warming was associated with a major benthic foraminiferal extinction and a dramatic 3–4% drop in the ocean's carbon isotopic composition. It has been suggested that the late paleocene/early Eocene global warming was caused by an enhanced greenhouse effect associated with higher levels of atmospheric CO₂ relative to present levels. We present carbon isotopic data from the co-existing paleosols organic matter and carbonates from a terrestrial sequence in the Paris Basin, France that contradict the notion that an increase in atmospheric CO₂ level was the cause of extreme warming for this time interval. Atmospheric pCO₂ estimates for the Late Paleocene/early Eocene estimated from the terrestrial carbon isotopic record spanning the Paleocene/Eocene transition, are indistinguishable from each other and were generally between 300 and 700 ppm.  相似文献   

Understanding Interannual Variations of the Local Rainy Season over the Southwest Indian Ocean     

Hanying LI  Peng HU  Qiong ZHANG  Ashish SINHA  Hai CHENG 《大气科学进展》2021,38(11):1852-1862

Located at the southern boundary of the tropical rainfall belt within the South Africa monsoon regime, Rodrigues Island, ～2500 km east of East Africa, is ideally located to investigate climatic changes over the southwest Indian Ocean(SWIO). In this study, we investigate the climatic controls of its modern interannual rainfall variability in terms of teleconnection and local effects. We find that increased rainfall over the SWIO tends to occur in association with anomalously warm(cold) SSTs over the equatorial central Pacific(Maritime Continent), resembling the central Pacific El Ni?o, closely linked with the Victoria mode in the North Pacific. Our analyses show that the low-level convergence induced by warm SST over the equatorial central Pacific leads to anomalous low-level divergence over the Maritime Continent and convergence over a large area surrounding the Rodrigues Island, which leads to increased rainfall over the SWIO during the rainy season. Meanwhile, the excited Rossby wave along the tropical Indian Ocean transports more water vapor from the tropical convergence zone into the SWIO via intensified northwest wind. Furthermore, positive feedback induced by the Rossby wave response to the increased rainfall in the region contributes to the large interannual variations over the SWIO.  相似文献   

Study of variation of aerosol optical properties over a high altitude station in Indian Western Himalayan region,palampur using raman lidar system     

Singh  Shishir Kumar  Radhakrishnan  S. R.  Jaswant  Mishra  Sumit Kumar  Shukla  Devesh Kumar  Ranjan  Ashish  Sharma  Chhemendra 《Journal of Atmospheric Chemistry》2022,79(2):117-139

A Raman lidar system was operated along with the Microtops sunphotometer measurements to carry out the study of the variation of the optical properties of aerosols over Palampur (32.11° N and 76.53° E), India from 17th April to 11th May 2019. The lidar system is furnished with Raman (N₂) channel and depolarization channel allowing independent measurement of Lidar Ratio (LR) and linear depolarization ratio. The study reveals that the majority of the aerosols approximately were restricted within the planetary boundary layer (PBL) and very less loading was present in the free troposphere over the study location. The particle loading over the study period was found to be very less with aerosol backscatter coefficient (at 355 nm) ranging from ～0.13 Mm^?1sr^?1 to ～7.25 Mm^?1sr^?1 with mean value of 2.67?±?0.82 Mm^?1sr^?1 and it is well supplemented by the mean aerosol optical depth (AOD) of 0.37?±?0.13 obtained from Microtops Sunphotometer. The average lidar ratio values for 0-1 km altitude (L1) 72?±?13sr, for 1-2 km (L2) altitude 55?±?8sr, for 2-3 km (L3) 54?±?15sr were observed as suggesting dominance of the biomass burning aerosols and anthropogenic aerosols. The particle depolarization ratio (355 nm) values were found from approximately 4.8?±?2.7% to 11.5?±?1.9% with the mean value of 7?±?1.3% suggesting the presence of non-spherical particles. To trace the sources of the pollution, we derived the HYSPLIT trajectory which shows the majority of the movement was from local sources.

  相似文献   

Using second-order approximation to incorporate GCM uncertainty in climate change impact assessments     

Sajjad Eghdamirad  Fiona Johnson  Ashish Sharma 《Climatic change》2017,142(1-2):37-52

This study presents a method to incorporate uncertainty of climate variables in climate change impact assessments, where the uncertainty being considered refers to the divergence of general circulation model (GCM) projections. The framework assesses how much bias occurs when the uncertainties of climate variables are ignored. The proposed method is based on the second-order expansion of Taylor series, called second-order approximation (SOA). SOA addresses the bias which occurs by assuming the expected value of a function is equal to the function of the expected value of the predictors. This assumption is not valid for nonlinear systems, such as in the case of the relationship of climate variables to streamflow. To investigate the value of SOA in the climate change context, statistical downscaling models for monthly streamflow were set up for six hydrologic reference stations in Australia which cover contrasting hydro-climate regions. It is shown that in all locations SOA makes the largest difference for low flows and changes the overall mean flow by 1–3%. Another advantage of the SOA approach is that the individual contribution of each climate variable to the total difference can be estimated. It is found that geopotential height and specific humidity cause more bias than wind speeds in the downscaling models considered here.  相似文献   

Evaluation of cloud properties in the NCEP CFSv2 model and its linkage with Indian summer monsoon     

Anupam Hazra  Hemantkumar S. Chaudhari  Ashish Dhakate 《Theoretical and Applied Climatology》2016,124(1-2):31-41

  相似文献   

Estimation of runoff for hilly catchment using satellite data     

Ashish Pandey  P. P. Dabral 《Journal of the Indian Society of Remote Sensing》2004,32(2):235-240

  相似文献   

Understanding fluid mud in a dynamic environment   总被引：1，自引：0，他引：1  

Ashish J. Mehta 《Geo-Marine Letters》1991,11(3-4):113-118

  相似文献   

Mineralogy and geochemistry of picro-dolerite dykes from the central Deccan Traps flood basaltic province,India, and their geodynamic significance     

Dongre  Ashish  Viljoen  K. S.  Rathod  A. 《Mineralogy and Petrology》2018,112(2):267-277

Mineralogy and Petrology - Constituent mineral compositions and whole rock major element geochemistry of picro-dolerite dykes from the central part of the Deccan flood basalt province are presented...  相似文献