Monthly Time Scales Research Articles

Quantitative assessments of moisture sources and temperature governing rainfall δ18O from 20 years' monitoring records in SW-France: Importance for isotopic-based climate reconstructions

In the mid-high latitude region, variations of stable isotopic compositions of atmospheric precipitation (δ18Op and δDp) were commonly regarded as reflecting the “temperature effect”. However, some studies have indicated that changes in moisture sources are important controlling factors for δ18Op. To clarify whether there are connections between δ18Op and variations of moisture sources in Southwest France (SW-France), whose implications for speleothem δ18O are of great importance, we have used among the longest isotopic time-series from SW-France (Le Mas and Villars stations) and a 5 days’ reconstruction of air mass history during the 1997–2016 A.D period based on the HYSPLIT tracking model. We found the percentage of initial moisture sources (PIMS) as important factors controlling the oxygen isotope composition of precipitation in SW-France, whether monthly or inter-annual timescale was considered. Additionally, we observed that the δ18Op preserved the signal of local temperature, supporting the “temperature effect”, while no evidence for its “amount effect” has been observed. These quantified links between PIMS/local-temperature and δ18Op appear useful references to understand the link between stable oxygen isotopes and climate parameters. Our long-term monitoring of δ18Op, d-excess, and moisture sources reveals decadal trends, highlighting a tight coupling in hydrologic systems and relatively fast changes on rainfall sources controlled by atmospheric circulations in SW-France.

The Variable and Non-variable X-Ray Absorbers in Compton-thin Type II Active Galactic Nuclei

Abstract We have conducted an extensive X-ray spectral variability study of a sample of 20 Compton-thin type II galaxies using broadband spectra from XMM-Newton, Chandra, and Suzaku. The aim is to study the variability of the neutral intrinsic X-ray obscuration along the line of sight and investigate the properties and location of the dominant component of the X-ray-obscuring gas. The observations are sensitive to absorption columns of ∼ 1020.5–24 cm−2 of fully and partially covering neutral and/or lowly ionized gas on timescales spanning days to well over a decade. We detected variability in the column density of the full-covering absorber in 7/20 sources, on timescales of months to years, indicating a component of compact-scale X-ray-obscuring gas lying along the line of sight of each of these objects. Our results imply that torus models incorporating clouds or over-dense regions should account for line-of-sight column densities as low as ∼a few ×1021 cm−2. However, 13/20 sources yielded no detection of significant variability in the full-covering obscurer, with upper limits of ΔN H spanning 1021–23 cm−2. The dominant absorbing media in these systems could be distant, such as kiloparsec-scale dusty structures associated with the host galaxy, or a homogeneous medium along the line of sight. Thus, we find that overall, strong variability in full-covering obscurers is not highly prevalent in Compton-thin type IIs, at least for our sample, in contrast to previous results in the literature. Finally, 11/20 sources required a partial-covering, obscuring component in all or some of their observations, consistent with clumpy near-Compton-thick compact-scale gas.

X-Ray Properties of TDEs

Observational astronomy of tidal disruption events (TDEs) began with the detection of X-ray flares from quiescent galaxies during the ROSAT all-sky survey of 1990–1991. The flares complied with theoretical expectations, having high peak luminosities ( $L_{\mathrm{x}}$ up to $\ge 4\times 10^{44}~\text{erg/s}$ ), a thermal spectrum with $kT\sim \text{few} \times 10^{5}~\text{K}$ , and a decline on timescales of months to years, consistent with a diminishing return of stellar debris to a black hole of mass $10^{6\text{--}8}~M_{\odot }$ . These measurements gave solid proof that the nuclei of quiescent galaxies are habitually populated by a super-massive black hole. Beginning in 2000, XMM-Newton, Chandra and Swift have discovered further TDEs which have been monitored closely at multiple wavelengths. A general picture has emerged of, initially near-Eddington accretion, powering outflows of highly-ionised material, giving way to a calmer sub-Eddington phase, where the flux decays monotonically, and finally a low accretion rate phase with a harder X-ray spectrum indicative of the formation of a disk corona. There are exceptions to this rule though which at the moment are not well understood. A few bright X-ray TDEs have been discovered in optical surveys but in general X-ray TDEs show little excess emission in the optical band, at least at times coincident with the X-ray flare. X-ray TDEs are powerful new probes of accretion physics down to the last stable orbit, revealing the conditions necessary for launching jets and winds. Finally we see that evidence is mounting for nuclear and non-nuclear intermediate mass black holes based on TDE flares which are relatively hot and/or fast.

Comparison of spatial interpolation methods for the estimation of precipitation patterns at different time scales to improve the accuracy of discharge simulations

Abstract Interpolating precipitation data is of prime importance to hydrological design, modeling, and water resource management. Various models have been developed that estimate spatial precipitation patterns. The purpose of this study is to analyze different precipitation interpolation schemes at different time scales in order to improve the accuracy of discharge simulations. The study was carried out in the upstream area of the Changjiang River basin. The performance of all selected methods was assessed using cross-validation schemes, with the mixed methods ultimately displaying the best performance at all three time scales. However, the differences in performance between the spatial interpolation methods decreased with increasing time scales. The unifying catchment Soil and Water Assessment Tool (SWAT), ‘abcd’, and the Budyko equation were employed at the daily, monthly, and annual scales, respectively, to simulate discharge. The performance of the discharge simulation at the monthly and annual time scales was consistent with their ranks of spatial precipitation estimation. For coarse, or long period, precipitation, there were no significant differences. However, the mixed methods performed better than the single model for the daily, or short, time scale with respect to the accuracy of the discharge simulation.

Performance evaluation of CHIRPS satellite precipitation estimates over Turkey

Satellite-based precipitation data can be a valuable source for performing hydro-climatological analyses: such as drought and water balance estimations with long and temporally consistent data. In this study, the Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS) precipitation estimations were compared with station-based precipitation measurements at daily, dekadal, and monthly time scales using statistical and categorical validation measures. The assessment study includes 77 ground-based meteorological stations distributed throughout Turkey, and the study period corresponds to years 2008 through 2018. Overall evaluation indicates that CHIRPS estimates exhibit high correlation on dekadal and monthly time scales; however correlation decreases for daily estimates. A positive bias is found over Turkey, and a particular overestimation was detected between the precipitation amounts of 0–25 mm/dekad and 0–80 mm/month. CHIRPS tends to underestimate high precipitation amounts of 25–80 mm/dekad and 150–300 mm/month. CHIRPS estimations show the best performance in winter when Turkey’s precipitation regime is dominated by cyclones and the lowest performance in the spring season. The study area was delineated into six regions according to their precipitation climatology and hydrological basins borders. The best performance for monthly CHIRPS estimates was in western Anatolia (r = 0.88). For all regions, precipitation detection capability is significantly high not only for monthly but also for dekadal CHIRPS estimates. Probability of detection was found between 0.83 and 0.98, and the false alarm rate changes between 0.30 and 0.13 for the thresholds of 50 mm/month and 5 mm/month, respectively. In rainfall categories of 30, 40, and 50 mm / month the capacity to determine rain and no rain was significantly high. CHIRPS estimations for 54 meteorological stations were reasonably biased (between ± 20%) with ground-based observations. Results show that CHIRPS estimations over Western Anatolia, Southern Anatolia, and Marmara regions are more consistent than the mountainous eastern and northeastern parts of the country. CHIRPS estimates can be used for hydro-climatological studies such as drought and water balance modeling over Turkey considering the error characteristics presented in this study.

Orbital Motion, Variability, and Masses in the T Tauri Triple System

Abstract We present results from adaptive optics imaging of the T Tauri triple system obtained at the Keck and Gemini Observatories in 2015−2019. We fit the orbital motion of T Tau Sb relative to Sa and model the astrometric motion of their center of mass relative to T Tau N. Using the distance measured by Gaia, we derived dynamical masses of M ⊙ and M Sb = 0.43 ± 0.06 . The precision in the masses is expected to improve with continued observations that map the motion through a complete orbital period; this is particularly important as the system approaches periastron passage in 2023. Based on published properties and recent evolutionary tracks, we estimate a mass of ∼2 M ⊙ for T Tau N, suggesting that T Tau N is similar in mass to T Tau Sa. Narrowband infrared photometry shows that T Tau N remained relatively constant between late 2017 and early 2019 with an average value of K = 5.54 ± 0.07 mag. Using T Tau N to calibrate relative flux measurements since 2015, we found that T Tau Sa varied dramatically between 7.0 and 8.8 mag in the K band over timescales of a few months, while T Tau Sb faded steadily from 8.5 to 11.1 mag in the K band. Over the 27 yr orbital period of the T Tau S binary, both components have shown 3–4 mag of variability in the K band, relative to T Tau N.

Ultrasensitive ambient-stable SnSe2-based broadband photodetectors for room-temperature IR/THz energy conversion and imaging

The advent of tin diselenide (SnSe2) enables novel pathways for optoelectronics, due to its reduced cost, ultralow thermal conductivity and high potential for thermoelectricity. To date, SnSe2-based optoelectronic devices have been focused on the visible and infrared range of the electromagnetic spectrum, with efficiency sharply decreasing at longer wavelength. Here, we present SnSe2 photodetectors with exfoliated SnSe2 nanosheets extended in the range of THz frequency, exhibiting high responsivity (170 V W−1), fast speed (2.2 μs), as well as room-temperature operation, based on efficient production of hotelectrons under deep-subwavelength electromagnetic focus, which outperform thermal-based photodetectors. Our SnSe2-based detectors show high-contrast imaging from terahertz (THz) up to visible. The outstanding ambient stability of our broadband photodetectors in a timescale of months is due to the chemical inertness of stoichiometric SnSe2 crystals, validated by surface-science experiments. Our results demonstrate the suitability of SnSe2for multispectral sensing and real-time imaging.

A unified framework of water balance models for monthly, annual, and mean annual timescales

At present, many conceptual water balance models have been proposed on monthly, annual, and mean annual timescales. With the increasing applications of these models, an emerging question is whether the water balance models on different timescales have some commonalities and can be integrated. To this end, this study established and applied a practical unified framework of water balance models that can be applied to different timescales. The framework was first developed on monthly timescale, where the water balance was captured by combining the “abcd” model and Budyko hypothesis with four parameters. Then, this framework was extended to annual and mean annual timescales under which certain parameters can be eliminated or assumed to be certain values. To be specific, on the annual timescale, the water balance was simulated by a Budyko-type model incorporating the water storage change; on the mean annual timescale the water balance was further simplified into a Budyko-type model which neglects the water storage change. Thus, all of the three typical water balance models on different timescales were unified to a general framework. The models were applied to 437 catchments in the contiguous United States with satisfying results achieved. Considering the water storage changes can improve the performance of annual water balance models in regions with clear interannual carrying water storage. Furthermore, the water balance models on different timescales share common supply and demand limits which is similar to Budyko hypothesis.

Identification of uncertainty sources in quasi-global discharge and inundation simulations using satellite-based precipitation products

Predicting river discharge and inundation is crucial for water resources management and flood hazard reduction; however, it is still unclear to what extent their variabilities can be captured on global scale. This study evaluates uncertainty sources in the quasi-global river discharge and inundation simulations using the Variable Infiltration Capacity (VIC) macroscale hydrologic model and the Catchment-based Macroscale Floodplain (CaMa-Flood) hydrodynamic model, forced with five high-resolution satellite precipitation datasets. The simulated discharge is first evaluated against more than 2852 sites selected from the Global Streamflow Indices and Metadata Archive (GSIM) dataset, and then the simulated inundation is compared with complementary multiple satellite observations. Globally, about 38% – 43% of the stations produce reasonable discharge simulations with positive Kling-Gupta Efficiency (KGE) on monthly time scale. The simulations show good agreement for flood fractions with mean correlations ranging from 0.47 to 0.62 for satellite detected events. The potential uncertainties sources of discharge and inundation simulation related to physics setting and forcing datasets, such as precipitation, land surface model, routing model, and observation from site and satellite are discussed, as well as future directions for improving large-scale model applications. By using default model settings, we hope our study can offer valuable insights into the applicability of flood simulations and provide guides for model development.

Diffusion Timescales of Magmatic Processes in the Moinui Lava Eruption at Mauna Loa, Hawai`i, as Inferred from Bimodal Olivine Populations

Abstract The 2·1 ka Moinui lava flow field, erupted from the southwest rift zone of Mauna Loa, Hawai`i, exhibits striking textural and geochemical variations, that can be used to interpret magma processes pre-, syn- and post-eruption. From this lava flow, the duration of magma storage and storage conditions, the timescales over which magma is transported to the surface, and flow emplacement mechanisms at Mauna Loa are determined. Electron microprobe analysis (EMPA) and diffusion chronometry of olivine crystals identify two distinct crystal populations: a primitive, polyhedral olivine population with core compositions of Fo90–88 and a more evolved, platy olivine population with core compositions of Fo83–82. Fe–Mg diffusion modelling of these olivine populations gives distinct timescales for each population; platy olivines yield timescales of days up to a few weeks, while polyhedral olivines yield timescales of months to years. Despite the nature of a well-insulated pāhoehoe flow, meaning that post-emplacement diffusion continues for some time, a wealth of time information can be retrieved concerning pre-eruptive magmatic processes as well as the processes associated with the lava extrusion. The short timescales obtained from the platy olivine crystals and the observed equilibrium between its cores and ambient melt suggest late-stage nucleation and crystal growth in the shallow conduit and during lava emplacement. Conversely, the longer timescales and olivine-melt disequilibrium of the polyhedral olivine crystals suggests accumulation from a deeper source and subsequent transportation to shallow magma storage beneath the summit of Mauna Loa months, or even years before eruption. The chemical and textural details of the Moinui lava reflect the mode of flow emplacement and may have implications for the interpretation of the distribution of spinifex and cumulate olivine within komatiites; high-temperature, low-viscosity lavas, common in the Archean.

Spatiotemporal ozone pollution LUR models: Suitable statistical algorithms and time scales for a megacity scale

Abstract Ambient air ozone (O3), a secondary photochemical pollutant, is seriously harmful to human health. Accurate estimation of O3 exposure requires the ability to monitor O3 surface concentration with a high spatiotemporal resolution. Several spatiotemporal land use regression (LUR) models have integrated meteorological factors based on different statistical algorithms to support such epidemiological studies. From among such various existing statistical algorithms, we aim to identify a high-efficiency modeling method, as well as the most suitable lengths of the modeling period (time scale). Three types of typical spatiotemporal LUR models based on parametric, semi-parametric, and non-parametric statistic methods, respectively, are considered to predict daily ground-level O3 in the megacity of Tianjin, China. Based on monthly, seasonal (cold and warm), and annual time scales, these models include: a series of monthly hybrid LUR (Two-stage) models consisting of two sub-models based on the multiple linear regression (MLR) algorithm, general additive mixed models (GAMMs), and land use random forest (LURF) models. Leave-one-out cross-validation was performed to evaluate the temporal and spatial predictive accuracy of each model using the adjusted coefficient of determination (adjR2CV) and root mean square error (RMSECV). In the GAMMs and LURF models, models using a shorter time scale (monthly models) outperformed those using a longer one. In monthly models, the GAMMs performed the best, with the highest average adjR2CV (0.747) and the lowest average RMSECV (15.721 μg/m3), followed by the LURF models (average adjR2CV = 0.695, average RMSECV = 16.405), and the Two-stage models (average adjR2CV = 0.466, average RMSECV = 23.934). Thus, the modeling format consisting of a shorter time scale and the GAMM algorithm performs relatively well in predicting daily O3 pollution on a megacity scale. These findings can be used to select appropriate modeling methods for epidemiological research of O3 pollution.

A stochastic rainfall model that can reproduce important rainfall properties across the timescales from several minutes to a decade

A stochastic rainfall model that can reproduce various rainfall characteristics at timescales between 5 min and one decade is introduced. The model generates the fine-scale rainfall time series using a randomized Bartlett-Lewis rectangular pulse model. Then the rainstorms are shuffled such that the correlation structure between the consecutive storms are preserved. Finally, the time series is rearranged again at the monthly timescale based on the result of the separate coarse-scale monthly rainfall model. The method was tested using the 69 years of 5-minute rainfall data recorded at Bochum, Germany. The mean, variance, covariance, skewness, and rainfall intermittency were well reproduced at the timescales from 5 min to a decade without any systematic bias. The extreme values were also well reproduced at timescales from 5 min to 3 days. The past-7-day rainfall before an extreme rainfall event, which is highly associated with the extreme flow discharge was reproduced well too. The rainstorm shuffling approaches introduced here may be adopted as a standard procedure in combination with any Poisson cluster rainfall model. The methods are simple and parsimonious, yet significantly reduce the systematic underestimation of rainfall variance at coarse scales, and improve the reproduction of skewness, and extreme rainfall depths values at a range of time-scales, thereby addressing well-known shortcomings of Poisson cluster rainfall models.

Early volumetric changes of hippocampus and medial prefrontal cortex following medial temporal lobe resection.

Previous studies have shown that cognitive demands and physical exercise stimulate adult neurogenesis in the dentate gyrus and hippocampus. Recent observations in healthy humans and patients with mild cognitive impairment moreover suggest that training-induced increases in hippocampal volume may be associated with improved memory performance. The corresponding plasticity processes in hippocampal volume may occur on timescales of months to years. For patients with focal lesions in this region, previous functional imaging studies suggest that increased recruitment of the contralateral hippocampus and extratemporal regions may be an important part of the reorganization of episodic memory. However, it is currently unclear whether focal damage to the medial temporal lobe (MTL) induces gray matter (GM) volume changes in the intact contralateral hippocampus and in connected network regions on a shorter timescale. We therefore investigated whether unilateral resection of the MTL, including the hippocampus, induces measurable volumetric changes in the contralateral hippocampus and in the default mode network (DMN). We recruited 31 patients with unilateral left (N=19) or right (N=12) hippocampal sclerosis undergoing MTL resection for treatment of drug-resistant epilepsy. Structural MRI was acquired immediately before and 3months after surgery. Longitudinal voxel-based morphometry (VBM) analysis revealed a significant increase of right hippocampal volume following resection of the left anterior MTL. Furthermore, this patient group showed GM volume increases in the DMN. These results demonstrate significant structural plasticity of the contralateral hippocampus, even in patients with a long-standing unilateral hippocampal dysfunction and structural reorganization processes extending to distant, but functionally connected brain regions.

Variability of the Great Disk Shadow in Serpens

Abstract We present dual-epoch Hubble Space Telescope imaging of the great disk shadow in the Serpens star-forming region. The near-infrared images show strong variability of the disk shadow, revealing dynamics of the inner disk on timescales of months. The Great Shadow is projected onto the Serpens reflection nebula by an unresolved protoplanetary disk surrounding the young intermediate-mass star SVS2/CK3/EC82. Since the shadow extends out to a distance of at least 17,000 au, corresponding to a light-travel time of 0.24 yr, the images may reveal detailed changes in the disk scale height and position angle on timescales as short as a day, corresponding to the angular resolution of the images, and up to the 1.11 yr span between two observing epochs. We present a basic retrieval of temporal changes in the disk density structure, based on the images. We find that the inner disk changes position angle on timescales of months, and that the change is not axisymmetric, suggesting the presence of a non-axisymmetric dynamical forcing on ∼1 au size scales. We consider two different scenarios, one in which a quadrupolar disk warp orbits the central star, and one in which an unequal-mass binary orbiting out of the disk plane displaces the photocenter relative to the shadowing disk. Continued space-based monitoring of the great disk shadow is required to distinguish between these scenarios, and could provide unique and detailed insight into the dynamics of inner protoplanetary disks not available through other means.

Recurrent low-level luminosity behaviour after a giant outburst in the Be/X-ray transient 4U 0115+63

In 2017, the Be/X-ray transient 4U 0115+63 exhibited a new type II outburst that was two times fainter than its 2015 giant outburst (in the Swift/BAT count rates). Despite this difference between the two bright events, the source displayed similar X-ray behaviour after these periods. Once the outbursts ceased, the source did not transit towards quiescence directly, but was detected about a factor of 10 above its known quiescent level. It eventually decayed back to quiescence over timescales of months. In this paper, we present the results of our Swift monitoring campaign, and an XMM-Newton observation of 4U 0115+63 during the decay of the 2017 type II outburst and its subsequent low-luminosity behaviour. We discuss the possible origin of the decaying source emission at this low-level luminosity, which has now been shown as a recurrent phenomenon, in the framework of the two proposed scenarios to explain this faint state: cooling from an accretion-heated neutron star crust or continuous low-level accretion. In addition, we compare the outcome of our study with the results we obtained from the 2015/2016 monitoring campaign on this source.

VERITAS Discovery of VHE Emission from the Radio Galaxy 3C 264: A Multiwavelength Study

Abstract The radio source 3C 264, hosted by the giant elliptical galaxy NGC 3862, was observed with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2017 February and 2019 May. These deep observations resulted in the discovery of very high energy (VHE; E > 100 GeV) γ-ray emission from this active galaxy. An analysis of ∼57 hr of quality-selected live time yields a detection at the position of the source, corresponding to a statistical significance of 7.8 standard deviations above background. The observed VHE flux is variable on monthly timescales, with an elevated flux seen in 2018 observations. The VHE emission during this elevated state is well characterized by a power-law spectrum with a photon index Γ = 2.20 ± 0.27 and flux F(>315 GeV) = ( cm−2 s−1, or approximately 0.7% of the Crab Nebula flux above the same threshold. 3C 264 (z = 0.0217) is the most distant radio galaxy detected at VHE, and the elevated state is thought to be similar to that of the famously outbursting jet in M87. Consequently, extensive contemporaneous multiwavelength data were acquired in 2018 at the time of the VHE high state. An analysis of these data, including Very Long Baseline Array, Very Large Array, Hubble Space Telescope, Chandra, and Swift observations in addition to the VERITAS data, is presented, along with a discussion of the resulting spectral energy distribution.

A New Look at the Variance of Summertime Temperatures over Land

AbstractThe increasing frequency of very high summertime temperatures has motivated growing interest in the processes determining the probability distribution of surface temperature over land. Here, we show that on monthly time scales, temperature anomalies can be modeled as linear responses to fluctuations in shortwave radiation and precipitation. Our model contains only three adjustable parameters, and, surprisingly, these can be taken as constant across the globe, notwithstanding large spatial variability in topography, vegetation, and hydrological processes. Using observations of shortwave radiation and precipitation from 2000 to 2017, the model accurately reproduces the observed pattern of temperature variance throughout the Northern Hemisphere midlatitudes. In addition, the variance in latent heat flux estimated by the model agrees well with the few long-term records that are available in the central United States. As an application of the model, we investigate the changes in the variance of monthly averaged surface temperature that might be expected due to anthropogenic climate change. We find that a climatic warming of 4°C causes a 10% increase in temperature variance in parts of North America.

Impact of time-dependent wettability alteration on the dynamics of capillary pressure

Wettability is a pore-scale property that has an important impact on capillarity, residual trapping, and hysteresis in porous media systems. In many applications, the wettability of the rock surface is assumed to be constant in time and uniform in space. However, many fluids are capable of altering the wettability of rock surfaces permanently and dynamically in time. Experiments have shown wettability alteration (WA) can significantly decrease capillarity in CO2 storage applications. For these systems, the standard capillary-pressure model that assumes static wettability is insufficient to describe the physics. In this paper, we develop a new dynamic capillary-pressure model that takes into account changes in wettability at the pore-level by adding a dynamic term to the standard capillary pressure function. We assume a pore-scale WA mechanism that follows a sorption-based model that is dependent on exposure time to a WA agent. This model is coupled with a bundle-of-tubes (BoT) model to simulate time-dependent WA induced capillary pressure data. The resulting capillary pressure curves are then used to quantify the dynamic component of the capillary pressure function. This study shows the importance of time-dependent wettability for determining capillary pressure over timescales of months to years. The impact of wettability has implications for experimental methodology as well as macroscale simulation of wettability-altering fluids.

Monthly Reservoir Inflow Forecasting for Dry Period Using Teleconnection Indices: A Statistical Ensemble Approach

Reliable long-range reservoir inflow forecast is essential to successfully manage water supply from reservoirs. This study aims to develop statistical reservoir inflow forecast models for a reservoir watershed, based on hydroclimatic teleconnection between monthly reservoir inflow and climatic variables. Predictability of such a direct relationship has not been assessed yet at the monthly time scale using the statistical ensemble approach that employs multiple data-driven models as an ensemble. For this purpose, three popular data-driven models, namely multiple linear regression (MLR), support vector machines (SVM) and artificial neural networks (ANN) were used to develop monthly reservoir inflow forecasting models. These models have been verified using leave-one-out cross-validation with expected error S as a measure of forecast skill. The S values of the MLR model ranged from 0.21 to 0.55, the ANN model ranged from 0.20 to 0.52 and the SVM from 0.21 to 0.56 for different months. When used as an ensemble, Bayesian model averaging was more accurate than simple model averaging and naïve forecast for four target years tested. These were considered to be decent prediction skills, indicating that teleconnection-based models have the potential to be used as a tool to make a decision for reservoir operation in preparing for droughts.

Assessment of satellite precipitation product estimates over Bali Island

Satellite precipitation product estimates (SPPEs) provide rainfall data on regional and global scales and have the potential to be applied in various fields. Several satellite precipitation estimates are available for their various features in retrieval algorithms, used sensor instrument, spatial–temporal resolution, and coverage area. Global Satellite Mapping of Precipitation (GSMaP), Integrated MultisatellitE Retrievals (IMERG), and Climate Hazards Group Infrared Precipitation with Station (CHIRPS) are global coverage precipitation datasets with high spatial resolution (0.1°–0.05°) and high temporal resolution (from 30 min to daily updates). The objective of this study was to assess the performance of GSMaP, IMERG, and CHIRPS over Bali Island from 2015 to 2017 in terms of ground rain gauge data over a high density of rain gauge stations (27 in-situ rain gauges) and at various elevations, rainfall intensities, and temporal scales (i.e., daily, penta-days, monthly, and seasonal). A traditional point-to-pixel-based method along with a new introduced continuous, categorical, and volumetric statistical indices comparison approach were implemented to evaluate satellite products. The assessment results demonstrated that IMERG products achieved the highest performance on daily, penta-day, and seasonal time scales, whereas CHIRPS outperformed the other two products on the monthly time scale. Moreover, IMERG was more efficient in detecting rainfall events at different altitudes, but it tended to overestimate rainfall events at high altitudes. With respect to their abilities to detect rainfall events, GSMaP, IMERG, and CHIRPS tended to underestimate the frequency of light rainfall events (0–1 mm/day) and heavy rainfall events (>50 mm/day) but overestimate the frequency of moderate rainfall events (5–10 mm/day). Our result not only highlight IMERG products exhibited better performance in comparison to GSMaP and CHIRPS in Bali Island but also recommend that further improvement on the precipitation estimate algorithm is required by considering complex terrain over small island in the maritime continent area.