Diurnal Water Cycle Research Articles

Wintertime column modeling in Jezero crater, Mars: Period of near-fog and a dust event

The diurnal water cycle was studied at a Jezero crater base site using M2020 observations and adsorptive column modeling in a cool midwinter period just before a dust peak and at the peak. During low-dust sols 566–571 the observed air relative humidity (RH) at 1.45 m height was high, 40–90% near dawn. For the typical nocturnal wind speed of 2 m/s at 1.45 m and column water of 9.7 μm, the model's diurnal air temperatures and RH were within observations. For weaker winds model-RH increased as in some of the observed sols. In a nearly calm experiment fog formed from 1.45 m upward. Ground frost appeared independently of wind, but only if adsorption was disabled. The dust peak at around sol 577 was also well simulated with the same wind and absolute humidity as before the event, but relative humidities dropped dramatically due to the dust-enhanced higher nocturnal temperatures.

Extrapolating continuous vegetation water content to understand sub-daily backscatter variations

Abstract. Microwave observations are sensitive to vegetation water content (VWC). Consequently, the increasing temporal and spatial resolution of spaceborne microwave observations creates a unique opportunity to study vegetation water dynamics and its role in the diurnal water cycle. However, we currently have a limited understanding of sub-daily variations in the VWC and how they affect microwave observations. This is partly due to the challenges associated with measuring internal VWC for validation, particularly non-destructively, and at timescales of less than a day. In this study, we aimed to (1) use field sensors to reconstruct diurnal and continuous records of internal VWC of corn and (2) use these records to interpret the sub-daily behaviour of a 10 d time series of polarimetric L-band backscatter with high temporal resolution. Sub-daily variations in internal VWC were calculated based on the cumulative difference between estimated transpiration and sap flow rates at the base of the stems. Destructive samples were used to constrain the estimates and for validation. The inclusion of continuous surface canopy water estimates (dew or interception) and surface soil moisture allowed us to attribute hour-to-hour backscatter dynamics either to internal VWC, surface canopy water, or soil moisture variations. Our results showed that internal VWC varied by 10 %–20 % during the day in non-stressed conditions, and the effect on backscatter was significant. Diurnal variations in internal VWC and nocturnal dew formation affected vertically polarized backscatter most. Moreover, multiple linear regression suggested that the diurnal cycle of VWC on a typical dry day leads to a 2 (HH, horizontally, and cross-polarized) to almost 4 (VV, vertically, polarized) times higher diurnal backscatter variation than the soil moisture drydown does. These results demonstrate that radar observations have the potential to provide unprecedented insight into the role of vegetation water dynamics in land–atmosphere interactions at sub-daily timescales.

Groundwater net discharge rates estimated from lake level change in Badain Jaran Desert, Northwest China

Over a hundred sizable lakes, some even larger than ~1 km2, are distributed across the lowlands between megadunes in the southeastern Badain Jaran Desert (BJD), northwest China. With rather limited precipitation and hardly any surface runoff, these lakes are fed mainly by groundwater. However, the source of the groundwater and the groundwater discharge rates to these lakes are poorly understood. Water level and temperature of four representative lakes in the southeastern BJD were monitored continuously between 2013 and 2017. Water surface evaporation and rainfall in this area were also measured. Combining these acquired data, groundwater discharge rates were estimated using water balance models. The results show that the four lakes have similar recharge and discharge patterns, but at different average net groundwater discharge rates ranging from 1.79 to 3.09 mm d−1. The lake level variation mainly depends on groundwater discharge and lake surface evaporation. We found that diurnal lake level variation may be controlled by earth tide and atmospheric pressure change, and is five times greater than the evaporation. The desert precipitation and deep confined groundwater with high temperature were found also recharging the desert lakes. An Empirical Mode Decomposition (EMD) method was used to identify the lake level trend. All lake levels increased over the last four years, except one decreased in 2015 and 2016. The lake levels’ increasing trend is synchronously similar with the precipitation in north China. This study analyzes annual and seasonal lake level variations, and also finds the diurnal water cycle between the groundwater and lake water for the first time.

G‐CLASS: geosynchronous radar for water cycle science – orbit selection and system design

The mission geosynchronous – continental land atmosphere sensing system (G-CLASS) is designed to study the diurnal water cycle, using geosynchronous radar. Although the water cycle is vital to human society, processes on timescales less than a day are very poorly observed from space. G-CLASS, using C-band geosynchronous radar, could transform this. Its science objectives address intense storms and high resolution weather prediction, and significant diurnal processes such as snow melt and soil moisture change, with societal impacts including agriculture, water resource management, flooding, and landslides. Secondary objectives relate to ground motion observations for earthquake, volcano, and subsidence monitoring. The orbit chosen for G-CLASS is designed to avoid the geosynchronous protected region and enables integration times of minutes to an hour to achieve resolutions down to ∼20 m. Geosynchronous orbit (GEO) enables high temporal resolution imaging (up to several images per hour), rapid response, and very flexible imaging modes which can provide much improved coverage at low latitudes. The G-CLASS system design is based on a standard small geosynchronous satellite and meets the requirements of ESA's Earth Explorer 10 call.

Ladakh: diverse, high-altitude extreme environments for off-earth analogue and astrobiology research

AbstractThis paper highlights unique sites in Ladakh, India, investigated during our 2016 multidisciplinary pathfinding expedition to the region. We summarize our scientific findings and the site's potential to support science exploration, testing of new technologies and science protocols within the framework of astrobiology research. Ladakh has several accessible, diverse, pristine and extreme environments at very high altitudes (3000–5700 m above sea level). These sites include glacial passes, sand dunes, hot springs and saline lake shorelines with periglacial features. We report geological observations and environmental characteristics (of astrobiological significance) along with the development of regolith-landform maps for cold high passes. The effects of the diurnal water cycle on salt deliquescence were studied using the ExoMars Mission instrument mockup: HabitAbility: Brines, Irradiance and Temperature (HABIT). It recorded the existence of an interaction between the diurnal water cycle in the atmosphere and salts in the soil (which can serve as habitable liquid water reservoirs). Life detection assays were also tested to establish the best protocols for biomass measurements in brines, periglacial ice-mud and permafrost melt water environments in the Tso-Kar region. This campaign helped confirm the relevance of clays and brines as interest targets of research on Mars for biomarker preservation and life detection.

Exposed bright features on the comet 67P/Churyumov–Gerasimenko: distribution and evolution

Context. Since its arrival at the comet 67P/Churyumov–Gerasimenko in August 2014, the Rosetta spacecraft followed the comet as it went past the perihelion and beyond until September 2016. During this time there were many scientific instruments operating on board Rosetta to study the comet and its evolution in unprecedented detail. In this context, our study focusses on the distribution and evolution of exposed bright features that have been observed by OSIRIS, which is the scientific imaging instrument aboard Rosetta. Aims. We envisage investigating various morphologies of exposed bright features and the mechanisms that triggered their appearance. Methods. We co-registered multi-filter observations of OSIRIS images that are available in reflectance. The Lommel–Seeliger disk function was used to correct for the illumination conditions and the resulting colour cubes were used to perform spectrophotometric analyses on regions of interest. Results. We present a catalogue of 57 exposed bright features observed on the nucleus of the comet, all of which are attributed to the presence of H2O ice on the comet. Furthermore, we categorise these patches under four different morphologies and present geometric albedos for each category. Conclusions. Although the nucleus of 67P/Churyumov–Gerasimenko appears to be dark in general, there are localised H2O ice sources on the comet. Cometary activity escalates towards the perihelion passage and reveals such volatile ices. We propose that isolated H2O ice patches found in smooth terrains in regions, such as Imhotep, Bes, and Hapi, result from frost as an aftermath of the cessation of the diurnal water cycle on the comet as it recedes from perihelion. Upon the comet’s return to perihelion, such patches are revealed when sublimation-driven erosion removes the thin dust layers that got deposited earlier. More powerful activity sources such as cometary outbursts are capable of revealing much fresher, less contaminated H2O ice that is preserved with consolidated cometary material, as observed on exposed patches resting on boulders. This is corroborated by our albedo calculations that attribute higher albedos for bright features with formations related to outbursts.

The temporal evolution of exposed water ice-rich areas on the surface of 67P/Churyumov-Gerasimenko: spectral analysis

Water ice-rich patches have been detected on the surface of comet 67P/Churyumov-Gerasimenko by the VIRTIS hyperspectral imager on-board the Rosetta spacecraft, since the orbital insertion in late August 2014. Among those, three icy patches have been selected, and VIRTIS data are used to analyse their properties and their temporal evolution while the comet was moving towards the Sun. We performed an extensive analysis of the spectral parameters, and we applied the Hapke radiative transfer model to retrieve the abundance and grain size of water ice, as well as the mixing modalities of water ice and dark terrains on the three selected water ice rich areas. Study of the spatial distribution of the spectral parameters within the ice-rich patches has revealed that water ice follows different patterns associated to a bimodal distribution of the grains: ~50 {\mu}m sized and ~2000 {\mu}m sized. In all three cases, after the first detections at about 3.5 AU heliocentric distance, the spatial extension and intensity of the water ice spectral features increased, it reached a maximum after 60-100 days at about 3.0 AU, and was followed by an approximately equally timed decrease and disappearanceat about ~2.2 AU, before perihelion. The behaviour of the analysed patches can be assimilated to a seasonal cycle. In addition we found evidence of short-term variability associated to a diurnal water cycle. The similar lifecycle of the three icy regions indicates that water ice is uniformly distributed in the subsurface layers, and no large water ice reservoirs are present.

The diurnal water cycle at Curiosity: Role of exchange with the regolith

Hourly temperatures T, relative humidities RH and mass mixing ratios of moisture q at 1.6m derived from the Mars Science Laboratory REMS-H and REMS-P measurements are shown for MSL sols 15–17 (Curiosity at Bradbury) and 80–82 (Curiosity at Rocknest). They are compared to column model simulations with and without adsorption to porous regolith. The observed mixing ratio is small at night (10–30ppmm). It increases rapidly to 50–80ppmm after sunrise and decreases slowly during the evening. The model gives a good account of the observed T and, with adsorption and realistic precipitable water content (PWC), reproduces the diurnal cycles of both RH and q relatively well. The suggested regolith thermal inertia is 300tiu and porosity 35–40% for both sites. According to the simulations moisture is adsorbed and diffused to the cooling regolith in the evening from the lowest very stable air layer. It is then desorbed in the morning from the rapidly warming regolith and mixed throughout the growing convective boundary layer. Estimates of PWC based on night-time near-surface moistures (assuming an even distribution with height) might therefore be on the low side in areas of porous adsorbing regolith.

NARR’s Atmospheric Water Cycle Components. Part II: Summertime Mean and Diurnal Interactions

Abstract Summertime interactions in the North American Regional Reanalysis (NARR) atmospheric water cycle are examined from a user’s perspective over the 1980–99 period with a particular emphasis on the diurnal cycle, the nocturnal maximum of precipitation over the Midwest, and the impacts of precipitation assimilation. NARR’s full-year mean atmospheric water cycle and its annual variations are examined in Part I of this study. North American summertime (June–August) features substantial convective activity that is often organized on a diurnal scale, although diverse regional diurnal features are evident to various extents in high-resolution precipitation products. NARR’s hourly assimilation of precipitation observations over the continental United States allows it to resolve diurnal effects on the water cycle, but in other regions the diurnal cycle of precipitation is imposed from an external reanalysis model. The prominent nocturnal maximum in precipitation across the upper Midwest is captured in NARR, but different precipitation assimilation sources disrupt the propagation of convective systems across the Canadian border. Normalized covariances of NARR’s diurnal water cycle component interactions in the nocturnal maximum region reveal a strong relationship between moisture convergence and precipitation, and also measure the way in which the precipitable water column holds a lagged response between evaporation and precipitation. In many regions the diurnal cycle of rainfall is driven by interactions with water cycle components that differ from those driving the seasonal cycle. A comparison between NARR’s precipitation and an estimate of the model precipitation prior to precipitation assimilation distinguishes the portion of the water cycle captured in full by the model and that which is value added by the assimilation routine. The nocturnal rainfall maximum is not present in the model precipitation estimate, leading to diurnal-scale biases in the evaporation and moisture flux convergence fields that are not directly modified by precipitation assimilation.

Dominant Balances and Exchanges of the Atmospheric Water Cycle in the Reanalysis 2 at Diurnal, Annual, and Intraseasonal Time Scales

Abstract Output from the National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 (R2) is passed through a broadband filter to determine the normalized covariances that describe the variance of the atmospheric water cycle at diurnal, annual, and intraseasonal (∼7–80 days) time scales. Vapor flux convergence is residually defined to close the water cycle between successive 3-hourly output times from 2002 to 2004, resulting in a balance between precipitation, evaporation, precipitable water tendency, and vertically integrated vapor flux convergence. The same balance holds at each time scale, allowing 100% of each variable’s temporal variance to be described by its covariance with other water cycle components in the same variance category. Global maps of these normalized covariances are presented to demonstrate the unique balances and exchanges that govern temporal variations in the water cycle. The diurnal water cycle is found to be dominated by a land–sea contrast, with continents controlled thermodynamically through evaporation and the oceans following dynamic convergence. The annual time-scale features significant meridional structure, with the low latitudes described mostly through variability in convergence and the extratropics governed by the properties of advected continental and maritime air masses. Intraseasonal transients lack direct solar oscillations at the top of the atmosphere and are characterized by propagating dynamic systems that act to adjust the precipitable water content of unsaturated regions or exchange directly with precipitation in saturated areas. By substituting the modeled precipitation with observation-based fields, a detailed description of the water cycle’s exchanges relating to the nocturnal precipitation maximum over the Midwest is obtained.

The martian diurnal water cycle

Near-surface water (both in the regolith and atmosphere) represents a potentially important resource for the exploration of Mars. However, most analyses of these resources (cf. Mellon and Jakosky, 1993, and references therein) hearken back to the seminal discussion of Martian volatiles by Leighton and Murray (1966) which assumed a planetwide average water vapor column of about 10 precipitable micrometers (pr-μm). This ignores considerable latitude, seasonal, and diurnal variations in water vapor. Similarly, previous high-resolution boundary-layer modeling of the diurnal water cycle (Zent et al., 1993) regarded the atmospheric column as controlling the soil conditions. Thus, the soil was initialized with some 2 kg m −3 (corresponding to 20 pr-μm cm −1) of water, causing the assumed atmospheric column to be filled in a matter of days. Global modeling of the Martian water cycle over a period of several decades (Houben et al., 1997), on the other hand, gives strong indications that the regolith water inventory is much smaller and that adsorption of water on this regolith dominates the lower atmospheric column. Presented here are diurnal boundary-layer calculations utilizing the one-dimensional Zent et al. (1993) model but which are initialized with the smaller Houben et al. (1997) estimated soil water inventories, for a variety of locations and seasons on Mars.