Diurnal Signal Research Articles

Temporal dynamics of stormwater nutrient attenuation of an urban constructed wetland experiencing summer low flows and macrophyte senescence

The attenuation of urban stormwater nutrient is essential for maintaining ecological health of downstream waterways. Constructed wetlands (CWs) are widely used for stormwater management due to the economic and environmental benefits they create by attenuating nutrients. This research assessed stormwater nutrient attenuation in a surface flow CW located in Western Australia, by assessing data spanning decadal, seasonal and diurnal time-scales. The study site was built in 2004 and restored in 2010, and the significance of such restoration was also assessed in this research. The CW designed to minimize nutrient loads to downstream sensitive waterways despite experiencing Mediterranean climates with strong hydrological seasonality including prolonged low or no flow conditions during the dry summer periods and nutrient-rich pulses during the wet winter periods. Furthermore, seasonal ungauged water inputs and macrophytes senescence made the system complex to optimize nutrient attenuation. The annual average attenuation of nitrogen (N) and phosphorus (P) based nutrients were 2–4 times higher during the post-restoration periods than that observed during the pre-restoration periods. The attenuation of inorganic, organic, dissolved and total N was higher during the dry periods of the post-restoration regimes, while P species showed higher attenuation during the wet periods of the post-restoration regimes. Inorganic N showed greater attenuation than attenuation of inorganic P throughout the year. Baumea articulata and Schoenoplectus validus were the dominant macrophyte species across the site; B. articulata was estimated to store about 3 and 2 times higher N and P pools, respectively than that stored in S. validus. Low dissolved oxygen (DO) occurred in the site and DO showed a decreasing trend over the decade and a distinct diurnal signature of day-time peaks and night-time anoxia. Diurnal pattern in aquatic metabolism, defined as the daily fluctuation of DO levels within the water column of CW, was related to solar radiation and nutrient concentrations, and could be considered as a proxy of CW function.

Statistical analysis of spatial and temporal variations in atmospheric electric fields from a regional array of field mills

AbstractElectric fields and currents of the global electric circuit have been measured at different locations and time periods around the globe, but a long‐term analysis from a regional array of electric field mills has never been performed. Kennedy Space Center (KSC) in Florida has an array of over 30 electric field mills that have been continuously operating and archiving data for over 18 years. KSC is also instrumented with many meteorological towers. With this initial statistical analysis of a long‐term data set four unique observations are reported that encompass global as well as local effects. To quantitatively describe the effects, a model to determine near‐surface electric fields, incorporating space charge, and conductivity perturbations is developed. Statistical autonomous grouping of the mills using the spatial array demonstrates a greater than 50 V/m spatial variation from coastal to inland mills caused by wind‐advected space charge generated near the coast. A temporal analysis identified a strong, globally generated, diurnal signature, but only a weak annual signal is found. A realistic limit on conductivity reductions within clouds is estimated by analyzing overhead cloud cover in relation to near‐surface electric field strengths. The estimated in‐cloud conductivity reduction of approximately 1/3 is similar to observations but appreciably less than values estimated from microphysical cloud estimates. Finally, an enhancement in the electric field at sunrise is statistically well correlated with low wind speeds and high relative humidities. This paper provides a statistical description of local environmental effects on near‐surface electric fields by which to base future studies.

Climatology of Banded Precipitation over the Contiguous United States

Abstract A climatology of banded-precipitation features over the contiguous United States from 2003 to 2014 is constructed. A band is defined as a precipitation feature with a major axis of 100 km or greater and a ratio of major axis length to minor axis length (hereafter, aspect ratio) of 3:1 or greater. By applying an automated feature-based detection algorithm to composite radar imagery, a database of 48 916 844 precipitation features is created, of which 7 213 505 (14.8%) are bands. This algorithm produces the first climatology of precipitation bands over the contiguous United States. Banded-precipitation occurrence is broadly similar to total precipitation occurrence, with a maximum of 175 h of banded precipitation annually over the Ohio River valley. In the warm season, there is a strong diurnal signature associated with convective storm development for both the precipitation feature area and total area covered by precipitation, but little diurnal signature in aspect ratio. Strong west–east gradients in both precipitation occurrence and banded-precipitation occurrence exist, as areas west of the Rockies receive less frequent precipitation, which is much less likely to be banded. East of the Rockies, precipitation features are banded 30% of the time, versus 10%–15% west of the Rockies. Areas downwind of the Great Lakes show prominent late autumn and winter maxima in banded precipitation associated with lake-effect snowbands. Local maxima of banded-precipitation percentage occur in the Dakotas and east of the Colorado Rockies during winter. Although banded-precipitation features compose only 14.8% of all precipitation features, they contribute 21.9% of the annual precipitation occurrence over the contiguous United States.

Using Diurnal Temperature Signals to Infer Vertical Groundwater-Surface Water Exchange.

Heat is a powerful tracer to quantify fluid exchange between surface water and groundwater. Temperature time series can be used to estimate pore water fluid flux, and techniques can be employed to extend these estimates to produce detailed plan-view flux maps. Key advantages of heat tracing include cost-effective sensors and ease of data collection and interpretation, without the need for expensive and time-consuming laboratory analyses or induced tracers. While the collection of temperature data in saturated sediments is relatively straightforward, several factors influence the reliability of flux estimates that are based on time series analysis (diurnal signals) of recorded temperatures. Sensor resolution and deployment are particularly important in obtaining robust flux estimates in upwelling conditions. Also, processing temperature time series data involves a sequence of complex steps, including filtering temperature signals, selection of appropriate thermal parameters, and selection of the optimal analytical solution for modeling. This review provides a synthesis of heat tracing using diurnal temperature oscillations, including details on optimal sensor selection and deployment, data processing, model parameterization, and an overview of computing tools available. Recent advances in diurnal temperature methods also provide the opportunity to determine local saturated thermal diffusivity, which can improve the accuracy of fluid flux modeling and sensor spacing, which is related to streambed scour and deposition. These parameters can also be used to determine the reliability of flux estimates from the use of heat as a tracer.

Idealized Numerical Modeling of the Diurnal Cycle of Tropical Cyclones

Abstract A numerical experiment is performed to evaluate the role of the daily cycle of radiation on axisymmetric hurricane structure. Although a diurnal response in high cloudiness has been well documented previously, the link to tropical cyclone (TC) structure and intensity remains unknown. Previous modeling studies attributed differences in results to experimental setup (e.g., initial and boundary conditions) as well as to radiative parameterizations. Here, a numerically simulated TC in a statistically steady state is examined for 300 days to quantify the TC response to the daily cycle of radiation. Fourier analysis in time reveals a spatially coherent diurnal signal in the temperature, wind, and latent heating tendency fields. This signal is statistically different from random noise and accounts for up to 62% of the variance in the TC outflow and 28% of the variance in the boundary layer. Composite analysis of each hour of the day reveals a cycle in storm intensity: a maximum is found in the morning and a minimum in the evening, with magnitudes near 1 m s−1. Anomalous latent heating forms near the inner core of the storm in the late evening, which persists throughout the early morning. Examination of the radial–vertical wind suggests two distinct circulations: 1) a radiatively driven circulation in the outflow layer due to absorption of solar radiation and 2) a convectively driven circulation in the lower and middle troposphere due to anomalous latent heating. These responses are coupled and are periodic with respect to the diurnal cycle.

Effects of cumulus parameterization closures on simulations of summer precipitation over the continental United States

This study examines the effects of five cumulus closure assumptions on simulations of summer precipitation in the continental U.S. by utilizing an ensemble cumulus parameterization (ECP) that incorporates multiple alternate closure schemes into a single cloud model formulation. Results demonstrate that closure algorithms significantly affect the summer mean, daily frequency and intensity, and diurnal variation of precipitation, with strong regional dependence. Overall, the vertical velocity (W) closure produces the smallest summer mean biases, while the moisture convergence (MC) closure most realistically reproduces daily variability. Both closures have advantages over others in simulating U.S. daily rainfall frequency distribution, though both slightly overestimate intense rain events. The MC closure is superior at capturing summer rainfall amount, daily variability, and heavy rainfall frequency over the Central U.S., but systematically produces wet biases over the North American Monsoon (NAM) region and Southeast U.S., which can be reduced by using the W closure. The instability tendency (TD) and the total instability adjustment (KF) closures are better at capturing observed diurnal signals over the Central U.S. and the NAM, respectively. The results reasonably explain the systematic behaviors of several major cumulus parameterizations. A preliminary experiment combining two optimal closures (averaged moisture convergence and vertical velocity) in the ECP scheme significantly reduced the wet (dry) biases over the Southeast U.S. in the summer of 1993 (2003), and greatly improved daily rainfall correlations over the NAM. Further improved model simulation skills may be achieved in the future if optimal closures and their appropriate weights can be derived at different time scales based on specific climate regimes.

Tropical Cyclone Diurnal Cycle as Observed by TRMM.

Previous work has indicated a clear, consistent diurnal cycle in rainfall and cold cloudiness coverage around tropical cyclones. This cycle may have important implications for structure and intensity changes of these storms and the forecasting of such changes. The goal of this paper is to use passive and active microwave measurements from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Precipitation Radar (PR), respectively, to better understand the tropical cyclone diurnal cycle throughout a deep layer of a tropical cyclone's clouds. The composite coverage by PR reflectivity ≥20 dBZ at various heights as a function of local standard time (LST) and radius suggests the presence of a diurnal signal for radii <500 km through a deep layer (2-10 km height) of the troposphere using 1998-2011 Atlantic tropical cyclones of at least tropical storm strength. The area covered by reflectivity ≥20 dBZ at radii 100-500 km peaks in the morning (0130-1030 LST) and reaches a minimum 1030-1930 LST. Radii between 300-500 km tend to reach a minimum in coverage closer to 1200 LST before reaching another peak at 2100 LST. The inner core (0-100 km) appears to be associated with a single-peaked diurnal cycle only at upper levels (8-10 km) with a maximum at 2230-0430 LST. The TMI rainfall composites suggest a clear diurnal cycle at all radii between 200 and 1000 km with peak rainfall coverage and rain rate occurring in the morning (0130-0730 LST).

Accomplishments of the MUSICA project to provide accurate, long-term, global and high-resolution observations of tropospheric {H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O,&amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;D} pairs – a review

Abstract. In the lower/middle troposphere, {H2O,δD} pairs are good proxies for moisture pathways; however, their observation, in particular when using remote sensing techniques, is challenging. The project MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water) addresses this challenge by integrating the remote sensing with in situ measurement techniques. The aim is to retrieve calibrated tropospheric {H2O,δD} pairs from the middle infrared spectra measured from ground by FTIR (Fourier transform infrared) spectrometers of the NDACC (Network for the Detection of Atmospheric Composition Change) and the thermal nadir spectra measured by IASI (Infrared Atmospheric Sounding Interferometer) aboard the MetOp satellites. In this paper, we present the final MUSICA products, and discuss the characteristics and potential of the NDACC/FTIR and MetOp/IASI {H2O,δD} data pairs. First, we briefly resume the particularities of an {H2O,δD} pair retrieval. Second, we show that the remote sensing data of the final product version are absolutely calibrated with respect to H2O and δD in situ profile references measured in the subtropics, between 0 and 7 km. Third, we reveal that the {H2O,δD} pair distributions obtained from the different remote sensors are consistent and allow distinct lower/middle tropospheric moisture pathways to be identified in agreement with multi-year in situ references. Fourth, we document the possibilities of the NDACC/FTIR instruments for climatological studies (due to long-term monitoring) and of the MetOp/IASI sensors for observing diurnal signals on a quasi-global scale and with high horizontal resolution. Fifth, we discuss the risk of misinterpreting {H2O,δD} pair distributions due to incomplete processing of the remote sensing products.

Actively heated high‐resolution fiber‐optic‐distributed temperature sensing to quantify streambed flow dynamics in zones of strong groundwater upwelling

AbstractZones of strong groundwater upwelling to streams enhance thermal stability and moderate thermal extremes, which is particularly important to aquatic ecosystems in a warming climate. Passive thermal tracer methods used to quantify vertical upwelling rates rely on downward conduction of surface temperature signals. However, moderate to high groundwater flux rates (&gt;−1.5 m d−1) restrict downward propagation of diurnal temperature signals, and therefore the applicability of several passive thermal methods. Active streambed heating from within high‐resolution fiber‐optic temperature sensors (A‐HRTS) has the potential to define multidimensional fluid‐flux patterns below the extinction depth of surface thermal signals, allowing better quantification and separation of local and regional groundwater discharge. To demonstrate this concept, nine A‐HRTS were emplaced vertically into the streambed in a grid with ∼0.40 m lateral spacing at a stream with strong upward vertical flux in Mashpee, Massachusetts, USA. Long‐term (8–9 h) heating events were performed to confirm the dominance of vertical flow to the 0.6 m depth, well below the extinction of ambient diurnal signals. To quantify vertical flux, short‐term heating events (28 min) were performed at each A‐HRTS, and heat‐pulse decay over vertical profiles was numerically modeled in radial two dimension (2‐D) using SUTRA. Modeled flux values are similar to those obtained with seepage meters, Darcy methods, and analytical modeling of shallow diurnal signals. We also observed repeatable differential heating patterns along the length of vertically oriented sensors that may indicate sediment layering and hyporheic exchange superimposed on regional groundwater discharge.

Wintertime water dynamics and moonlight disruption of the acoustic backscatter diurnal signal in an ice‐covered Northeast Greenland fjord

AbstractSix and a half month records from three ice‐tethered Acoustic Doppler Current Profilers deployed in October 2013 in Young Sound fjord in Northeast Greenland are used to analyze the acoustic backscatter signal. The acoustic data suggest a systematic diel vertical migration (DVM) of scatters below the land‐fast ice during polar night. The scatters were likely composed of zooplankton. The acoustic signal pattern typical to DVM persisted in Young Sound throughout the entire winter including the period of civil polar night. However, polynya‐enhanced estuarine‐like cell circulation that occurred during winter disrupted the DVM signal favoring zooplankton to occupy the near‐surface water layer. This suggests that zooplankton avoided spending additional energy crossing the interface with a relatively strong velocity gradient comprised by fjord inflow in the intermediate layer and outflow in the subsurface layer. Instead, the zooplankton tended to remain in the upper 40 m layer where relatively warmer water temperatures associated with upward heat flux during enhanced estuarine‐like circulation could be energetically favorable. Furthermore, our data show moonlight disruption of DVM in the subsurface layer and weaker intensity of vertical migration beneath snow covered land‐fast ice during polar night. Finally, by using existing models for lunar illuminance and light transmission through sea ice and snow cover, we estimated under ice illuminance and compared it with known light sensitivity of Arctic zooplankton species.

Combined use of thermal methods and seepage meters to efficiently locate, quantify, and monitor focused groundwater discharge to a sand‐bed stream

AbstractQuantifying flow of groundwater through streambeds often is difficult due to the complexity of aquifer‐scale heterogeneity combined with local‐scale hyporheic exchange. We used fiber‐optic distributed temperature sensing (FO‐DTS), seepage meters, and vertical temperature profiling to locate, quantify, and monitor areas of focused groundwater discharge in a geomorphically simple sand‐bed stream. This combined approach allowed us to rapidly focus efforts at locations where prodigious amounts of groundwater discharged to the Quashnet River on Cape Cod, Massachusetts, northeastern USA. FO‐DTS detected numerous anomalously cold reaches one to several m long that persisted over two summers. Seepage meters positioned upstream, within, and downstream of 7 anomalously cold reaches indicated that rapid groundwater discharge occurred precisely where the bed was cold; median upward seepage was nearly 5 times faster than seepage measured in streambed areas not identified as cold. Vertical temperature profilers deployed next to 8 seepage meters provided diurnal‐signal‐based seepage estimates that compared remarkably well with seepage‐meter values. Regression slope and R2 values both were near 1 for seepage ranging from 0.05 to 3.0 m d−1. Temperature‐based seepage model accuracy was improved with thermal diffusivity determined locally from diurnal signals. Similar calculations provided values for streambed sediment scour and deposition at subdaily resolution. Seepage was strongly heterogeneous even along a sand‐bed river that flows over a relatively uniform sand and fine‐gravel aquifer. FO‐DTS was an efficient method for detecting areas of rapid groundwater discharge, even in a strongly gaining river, that can then be quantified over time with inexpensive streambed thermal methods.

Inversion of a glacier hydrology model

ABSTRACTThe subglacial hydrologic system exerts strong controls on the dynamics of the overlying ice, yet the parameters that govern the evolution of this system are not widely known or observable. To gain a better understanding of these parameters, we invert a spatially averaged model of subglacial hydrology from observations of ice surface velocity and outlet stream discharge at Kennicott Glacier, Wrangell Mountains, AK, USA. To identify independent parameters, we formally non-dimensionalize the forward model. After specifying suitable prior distributions, we use a Markov-chain Monte Carlo algorithm to sample from the distribution of parameter values conditioned on the available data. This procedure gives us not only the most probable parameter values, but also a rigorous estimate of their covariance structure. We find that the opening of cavities due to sliding over basal topography and turbulent melting are of a similar magnitude during periods of large input flux, though turbulent melting also exhibits the greatest uncertainty. We also find that both the storage of water in the englacial system and the exchange of water between englacial and subglacial systems are necessary in order to explain both surface velocity observations and the relative attenuation in the amplitude of diurnal signals between input and output flux observations.

In vivo Observation of Tree Drought Response with Low-Field NMR and Neutron Imaging.

Using a simple low-field NMR system, we monitored water content in a living tree in a greenhouse over 2 months. By continuously running the system, we observed changes in tree water content on a scale of half an hour. The data showed a diurnal change in water content consistent both with previous NMR and biological observations. Neutron imaging experiments show that our NMR signal is primarily due to water being rapidly transported through the plant, and not to other sources of hydrogen, such as water in cytoplasm, or water in cell walls. After accounting for the role of temperature in the observed NMR signal, we demonstrate a change in the diurnal signal behavior due to simulated drought conditions for the tree. These results illustrate the utility of our system to perform noninvasive measurements of tree water content outside of a temperature controlled environment.

Probing geomagnetic storm-driven magnetosphere–ionosphere dynamics in D-region via propagation characteristics of very low frequency radio signals

The amplitude and phase of VLF/LF radio signals are sensitive to changes in electrical conductivity of the lower ionosphere which imprints its signature on the Earth–ionosphere waveguide. This characteristic makes it useful in studying sudden ionospheric disturbances, especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbance and storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region of the ionosphere. In this paper, using the data of three propagation paths at mid-latitudes (40–54°), we analyse the trend in variation of aspects of VLF diurnal signal under varying solar and geomagnetic space environmental conditions in order to identify possible geomagnetic footprints on the D region characteristics. We found that the trend of variations generally reflected the prevailing space weather conditions in various time scales. In particular, the ‘dipping’ of mid-day signal amplitude peak (MDP) occurs after significant geomagnetic perturbed or storm conditions in the time scale of 1–2days. The mean signal amplitude before sunrise (MBSR) and mean signal amplitude after sunset (MASS) also exhibit storm-induced dipping, but they appear to be influenced by event's exact occurrence time and the highly variable conditions of dusk-to-dawn ionosphere. We also observed few cases of the signals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic event. This effect may be related to storms associated phenomena or effects arising from sources other than solar origin. The magnitude of induced dipping (or rise) significantly depends on the intensity and duration of event(s), as well as the propagation path of the signal. The post-storm day signal (following a main event, with lesser or significantly reduced geomagnetic activity) exhibited a tendency of recovery to pre-storm day level. In the present analysis, we do not see a well-defined trend in the variation of the post-storm sunrise amplitude terminator (SRT) and sunset terminator (SST). The SRT and SST signals show more dipping in GQD-A118 propagation path but generally an increase along DHO-A118 propagation path. Thus the result could be propagation path dependent and detailed modelling is required to understand these phenomena.

Tidal dynamics in a frictionally dominated tropical lagoon

This study examined the dynamics of tidal propagation inside a tropical lagoon. Sea surface elevation (inside) and current profiles (at the inlet) were examined over 60 days at the Chelem lagoon, which is a branched tropical lagoon located in the northern Yucatan Peninsula. Tides were predominantly diurnal with a wavelength at least 20 times longer than the total length of the basin. Spatial variations of sea surface elevation and the longitudinal transport were described in each branch by applying a linear analytical model and the results were compared to observations. Results showed that the coastal lagoon was highly frictional. The tidal signal was attenuated between 30% and 40% toward the lagoon heads, a result of the balance between pressure gradient and frictional forces. A causeway that chokes the western side of the lagoon allowed the propagation of the diurnal signal toward the west head of the basin but damped the semidiurnal signal. The causeway acted as a hydraulic low-pass filter, as in natural choked systems. The causeway's filter effect was included in the analytical model by optimizing the frictional parameters.

High-resolution monitoring of Yok Balum Cave, Belize: An investigation of seasonal ventilation regimes and the atmospheric and drip-flow response to a local earthquake

The nature of cave ventilation is of interest to cavers, speleologists, and paleoclimatologists working with stalagmites. Because cave ventilation systematics may change over the growth span of a stalagmite, understanding what factors affect them is critical for determining events that may have affected climate proxies within the stalagmite. Similarly, understanding how the hydrology of the drips feeding a stalagmite evolves through time is key to building robust records of paleoclimate, particularly because stalagmite records have become critical archives of climate change information of the last 500,000 years. Here we present data from an extensive, on-going monitoring effort at Yok Balum Cave, Belize, initiated in 2011, that characterizes high-resolution ventilation dynamics at this site. Clear seasonal ventilation regimes exist, driven by thermally induced inside-outside air density differences. The winter regime is dominated by air inflow into the cave, decreased drawdown from the epikarst into the cave, and a limited diurnal signal. Conversely, summer ventilation is dominated by air outflow from the cave, greater CO2 drawdown and drip water degassing, and a strong diurnal signal. Active monitoring during a large (M7.4) earthquake in November 2012 provides a unique opportunity to assess the response of the cave atmosphere and hydrology to substantial seismic activity. Cave atmosphere dynamics and hydrology are found to be highly resilient to seismic activity, with no observable disturbance occurring around the earthquake, despite there being considerable evidence of physical disruption in the cave. Monitoring included different kinds of drips, and the earthquake affected none of the monitored drip types. This suggests that stalagmite-derived paleoclimate records are not affected by seismic activity, except in extreme cases where the stalagmite or conjugate stalactite is damaged or reoriented.

Signature of tropical fires in the diurnal cycle of tropospheric CO as seen from Metop-A/IASI

Abstract. Five years (July 2007 to June 2012) of CO tropospheric columns derived from the hyperspectral Infrared Atmospheric Sounding Interferometer (IASI) on-board Metop-A are used to study the impact of fires on the concentrations of CO in the troposphere. Following Chédin et al. (2005, 2008), who found a quantitative relation between the daily tropospheric excess of CO2 and fire emissions, we show that tropospheric CO also displays a diurnal signal with a seasonality that agrees well with the seasonal evolution of fires given by Global Fire Emission Database version 3 (GFED3.1) and Global Fire Assimilation System version 1 (GFAS1.0) emissions and Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 5 burned area product. Unlike day- or night-time CO fields, which mix local emissions with nearby emissions transported to the region of study, the day–night difference of CO allows to highlight the CO signal due to local fire emissions. A linear relationship between CO fire emissions from the GFED3.1 and GFAS1.0 inventories and the diurnal difference of IASI CO was found over various regions in the tropics, with a better agreement with GFAS1.0 (correlation coefficient of R2 ∼ 0.7) than GFED3.1 (R2 ∼ 0.6). Based on the specificity of the two main phases of the combustion (flaming vs. smoldering) and on the vertical sensitivity of the sounder to CO, the following mechanism is proposed to explain such a CO diurnal signal: at night, after the passing of IASI at 21:30 local time (LT), a large amount of CO emissions from the smoldering phase is trapped in the boundary layer before being uplifted the next morning by natural and pyroconvection up to the free troposphere, where it is seen by IASI at 09:30 LT. The results presented here highlight the need to take into account the specificity of both the flaming and smoldering phases of fire emissions in order to fully take advantage of CO observations.

Experimental evaluation of the applicability of phase, amplitude, and combined methods to determine water flux and thermal diffusivity from temperature time series using VFLUX 2

• Recent McCallum et al., 2012 , Luce et al., 2013 methods added to VFLUX. • Recent methods are tested with non-steady flows, and in 3D heterogeneous media. • Spatial or temporal changes in thermal diffusivity output suggests errors in flux. • Amplitude ratio, or combined amplitude ratio and phase shift methods recommended. Vertical fluid exchange between surface water and groundwater can be estimated using diurnal signals from temperature time series methods based on amplitude ratios ( A r ), phase shifts (Δ ϕ ), or combined use of both ( A r Δ ϕ ). The A r , Δ ϕ , and A r Δ ϕ methods are typically applied in conditions where one or more of their underlying assumptions are violated, and the reliability of the various methods in response to non-ideal conditions is unclear. Additionally, A r Δ ϕ methods offer the ability to estimate thermal diffusivity ( κ e ) without assuming any thermal parameters, although the value of such output has not been broadly tested. The A r , Δ ϕ , and A r Δ ϕ methods are tested under non-steady, 1D flows in sand column experiments, and multi-dimensional flows in heterogeneous media in numerical modeling experiments. Results show that, in non-steady flow conditions, estimated κ e values outside of a plausible range for streambed materials (0.028–0.180 m 2 d −1 ) coincide with time periods with erroneous flux estimates. In heterogeneous media, sudden changes of κ e with depth also coincide with erroneous flux estimates. When (known) fluxes are variable in time, poor identification of Δ ϕ leads to poor flux estimates from Δ ϕ and A r Δ ϕ methods. However, when fluxes are steady, or near zero, A r Δ ϕ methods provide the most accurate flux estimates. This comparison of A r , Δ ϕ and A r Δ ϕ methods under non-ideal conditions provides guidance on their use. In this study, A r Δ ϕ methods have been coded into a new version of VFLUX, allowing users easy access to recent advances in heat tracing.

Performance of the White method for estimating groundwater evapotranspiration under conditions of deep and fluctuating groundwater

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration (ETg) from groundwater level diurnal signals. Because of a lack of direct measurements of ETg, it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2-year field observation in a hyper-arid riparian tamarisk (Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration (ETs) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield (Sy) and recharge rate of groundwater (r) in the White method were investigated. The results showed that the readily available Sy determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant Sy value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000 h to 0600 h for r estimation for the White method worked best than other time spans. A 2-day moving average of r values further enhance ETg estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ETg estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ETg in desert riparian forests with deep groundwater depth. Copyright © 2015 John Wiley & Sons, Ltd.

An equilibrium cloud-resolving modeling study of diurnal variation of tropical rainfall

The diurnal variation of tropical rainfall is examined through the analysis of an equilibrium cloud-resolving model experiment. Model domain mean rain rate is defined as a product of rain intensity and fractional rainfall coverage. The diurnal variation of the mean rain rate is associated with that of fractional rainfall coverage because the diurnal variation of rain intensity is significantly weakened through the decrease in rainfall in early morning hours. The decrease in rainfall corresponds to the reduction in secondary circulations through the barotropic conversion from the perturbation kinetic energy to the mean kinetic energy under the imposed negative vertical gradient of westerly winds. The fractional rainfall coverage shows the diurnal signal with the maximum in the early morning hours primarily due to nocturnal infrared radiative cooling.