A Study to Explore the Dew Condensation Potential of Cars

The metal surfaces of a car exhibit favorable properties for the passive condensation of atmospheric water. Under certain nocturnal climatic conditions (high relative humidity, weak windspeed, and total nebulosity), dew is often observed on cars, and it is appropriate to ask the question of using a vehicle as a standard condenser for estimating the dew yield. In order to see whether cars can be used as reference dew condensers, we report a detailed study of radiative cooling and dew formation on cars in the presence of radiating obstacles and for various windspeeds. Measurements of temperature and condensed dew mass on different car parts (rooftop, front and back hoods, windshield, lateral and back windows, inside and outside air) are compared with the same data obtained on a horizontal, thermally isolated planar film. The paper concludes that heat transfer coefficients, evaluated from temperature and dew yield measurements, are found nearly independent of windspeed and tilt angles. Moreover, this work describes the relation between cooling and dew condensation with the presence or not of thermal isolation. This dependence varies with the surface tilt angle according to the angular dependence of the atmosphere radiation. This work also confirms that cars can be used to estimate the dew yields in a given site. A visual observation scale h = Kn, with h the dew yield (mm) and n = 0, 1 2, 3 an index, which depends whether dew forms or not on rooftop, windshield, and lateral windows, is successfully tested with 8 different cars in 5 sites with three different climates, using K = (0.067 ± 0.0036) mm.day−1.

Reconstructing Continuous Vegetation Water Content To Understand Sub-daily Backscatter Variations

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 VWC and how they affect passive and active 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-day time series of polarimetric L-band backscatter with high temporal resolution. Sub-daily variations of 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 to either internal VWC, surface canopy water or soil moisture variations. Our results showed that internal VWC varied with 10–20 % during the day in non-stressed conditions, and the effect on backscatter was significant. Diurnal variations of internal VWC and nocturnal dew formation affected vertically polarized backscatter most. Moreover, on a typical dry day, backscatter variations were 1.5 (HH-pol) to 3 (VV-pol) times more sensitive to VWC than to soil moisture. 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.

Delineation of dew formation zones in Iran using long-term model simulations and cluster analysis

Dew is a non-conventional source of water that has been gaining interest over the last two decades, especially in arid and semi-arid regions. In this study, we performed a long-term (1979–2018) energy balance model simulation to estimate dew formation potential in Iran aiming to identify dew formation zones and to investigate the impacts of long-term variation in meteorological parameters on dew formation. The annual average of dew occurrence in Iran was ∼102 d, with the lowest number of dewy days in summer (∼7 d) and the highest in winter (∼45 d). The average daily dew yield was in the range of 0.03–0.14 L m−2 and the maximum was in the range of 0.29–0.52 L m−2. Six dew formation zones were identified based on cluster analysis of the time series of the simulated dew yield. The distribution of dew formation zones in Iran was closely aligned with topography and sources of moisture. Therefore, the coastal zones in the north and south of Iran (i.e., Caspian Sea and Oman Sea), showed the highest dew formation potential, with 53 and 34 L m−2 yr−1, whereas the dry interior regions (i.e., central Iran and the Lut Desert), with the average of 12–18 L m−2 yr−1, had the lowest potential for dew formation. Dew yield estimation is very sensitive to the choice of the heat transfer coefficient. The uncertainty analysis of the heat transfer coefficient using eight different parameterizations revealed that the parameterization used in this study – the Richards (2004) formulation – gives estimates that are similar to the average of all methods and are neither much lower nor much higher than the majority of other parameterizations and the largest differences occur for the very low values of daily dew yield. Trend analysis results revealed a significant (p<0.05) negative trend in the yearly dew yield in most parts of Iran during the last 4 decades (1979–2018). Such a negative trend in dew formation is likely due to an increase in air temperature and a decrease in relative humidity and cloudiness over the 40 years.

The role of dew and radiation fog inputs in the local water cycling of a temperate grassland during dry spells in central Europe

During dry spells, non-rainfall water (hereafter NRW) mostly formed from dew and fog potentially plays an increasingly important role in temperate grassland ecosystems with ongoing global warming. Dew and radiation fog occur in combination during clear and calm nights, and both use ambient water vapor as a source. Research on the combined mechanisms involved in NRW inputs to ecosystems is rare, and distillation of water vapor from the soil as a NRW input pathway for dew formation has hardly been studied. Furthermore, eddy covariance (EC) measurements are associated with large uncertainties on clear, calm nights when dew and radiation fog occur. The aim of this paper is thus to use stable isotopes as tracers to investigate the different NRW input pathways into a temperate Swiss grassland at Chamau during dry spells in summer 2018. Stable isotopes provide additional information on the pathways from water vapor to liquid water (dew and fog) that cannot be measured otherwise. We measured the isotopic composition (δ18O, δ2H, and d=δ2H-8⋅δ18O) of ambient water vapor, NRW droplets on leaf surfaces, and soil moisture and combined them with EC and meteorological observations during one dew-only and two combined dew and radiation fog events. The ambient water vapor d was found to be strongly linked with local surface relative humidity (r=-0.94), highlighting the dominant role of local moisture as a source for ambient water vapor in the synoptic context of the studied dry spells. Detailed observations of the temporal evolution of the ambient water vapor and foliage NRW isotopic signals suggest two different NRW input pathways: (1) the downward pathway through the condensation of ambient water vapor and (2) the upward pathway through the distillation of water vapor from soil onto foliage. We employed a simple two-end-member mixing model using δ18O and δ2H to quantify the NRW inputs from these two different sources. With this approach, we found that distillation contributed 9 %–42 % to the total foliage NRW, which compares well with estimates derived from a near-surface vertical temperature gradient method proposed by Monteith in 1957. The dew and radiation fog potentially produced 0.17–0.54 mm d−1 NRW gain on foliage, thereby constituting a non-negligible water flux to the canopy, as compared to the evapotranspiration of 2.7 mm d−1. Our results thus underline the importance of NRW inputs to temperate grasslands during dry spells and reveal the complexity of the local water cycle in such conditions, including different pathways of dew and radiation fog water inputs.

An Attempt to Utilize a Regional Dew Formation Model in Kenya

Model evaluation against experimental data is an important step towards accurate model predictions and simulations. Here, we evaluated an energy-balance model to predict dew formation occurrence and estimate its amount for East-African arid-climate conditions against 13 months of experimental dew harvesting data in Maktau, Kenya. The model was capable of predicting the dew formation occurrence effectively. However, it overestimated the harvestable dew amount by about a ratio of 1.7. As such, a factor of 0.6 was applied for a long-term period (1979–2018) to investigate the spatial and temporal variation of the dew formation in Kenya. The annual average of dew occurrence in Kenya was ~130 days with dew yield > 0.1 L/m2/day. The dew formation showed a seasonal cycle with the maximum yield in winter and minimum in summer. Three major dew formation zones were identified after cluster analysis: arid and semi-arid regions; mountain regions; and coastal regions. The average daily and yearly maximum dew yield were 0.05 and 18; 0.9 and 25; and 0.15 and 40 L/m2/day; respectively. A precise prediction of dew occurrence and dew yield is very challenging due to inherent limitations in numerical models and meteorological input parameters.

Field calibration and correction of air water concentration measurements

&lt;p&gt;Infrared gas analyzers (IRGAs) are commonly used in Eddy Covariance (EC) system and are used for, in particular, the ecosystem water cycle. However, they suffer from a measurement drift of absolute concentrations with time, leading to the increasing bias of readings. It is recommended in the manuals to do a factory calibration once every 1-2 years (e.g., LI-6262) or user calibration when considerable drift occurs (e.g., LI-7000). However, our experience shows that a significant drift can occur within a few days already. At our semi-arid EC site of Yatir Forest (31&amp;#730;20'N, 35&amp;#730;03'E, Israel), we are measuring a vertical air humidity profile (absolute humidity, C&lt;sub&gt;w&lt;/sub&gt; in mmol&amp;#215;mol&lt;sup&gt;-1&lt;/sup&gt;, and relative, RH, %),&amp;#160; to study the VPD regime within the canopy and to analyze dew formation events, which requires highly accurate RH measurements, however accurate RH measurements are difficult to achieve.&lt;/p&gt;&lt;p&gt;Air humidity in Yatir is measured by three different instruments: (1) LI-7000 close-pass IRGA above the canopy for EC flux calculations; (2) LI-6262 close-pass IRGA with inlets in 4 different heights from above the ground up to the sonic height, used for humidity profile measurements; (3) Rotronic HC2S3 air humidity (RH) and temperature (T) sensor above the canopy. Both IRGAs are placed within a temperature-controlled box, and calibrated for zero and span with N2, dew point generator and laboratory standard gases every 1-2 weeks. The Rotronic sensor has very low drift and does not require calibration, but is assumed to be less accurate, especially under high and low RH.&lt;/p&gt;&lt;p&gt;To achieve highly accurate measurements on daily time scale we propose a correction routine that rely on the stability of the RH probe, and the accuracy of the IRGAs after calibration. Every time the IRGA is calibrated, a correction-1 to the RH probe is produced. Between calibrations, the trends in the drifting IRGAs data are corrected (correction-2) to the interpolated stable RH probe data.&lt;/p&gt;&lt;p&gt;For the flux measurements, the mean absolute Cw error before correction was 1.0 mmol&amp;#215;mol&lt;sup&gt;-1&lt;/sup&gt;, which translates under average temperature of 25&lt;sup&gt;&amp;#176;&lt;/sup&gt;C and RH of 50% to errors of RH, VPD and dew point of 3.0%, 93.5 Pa and 0.9&lt;sup&gt;&amp;#176;&lt;/sup&gt;C, respectively. Following our correction procedure, reduced the error to 0.5 mmol&amp;#215;mol&lt;sup&gt;-1&lt;/sup&gt;, which decreased the errors in RH, VPD and dew point under the same conditions to 1.5%, 47 Pa and 0.4&lt;sup&gt;&amp;#176;&lt;/sup&gt;C, respectively. For the humidity profile, Cw error after correction decreased from 1.9 mmol&amp;#215;mol&lt;sup&gt;-1&lt;/sup&gt; to 0.5 mmol&amp;#215;mol&lt;sup&gt;-1&lt;/sup&gt;, which decreased the errors in RH, VPD and dew point under the same conditions by 4.1%, 131 Pa and 1.2&lt;sup&gt;&amp;#176;&lt;/sup&gt;C, respectively.&lt;/p&gt;&lt;p&gt;We will describe the method in more detail and demonstrate its application to our field measurements.&lt;/p&gt;

Delineation of Dew Formation Zones in Iran Using Long-Term Model Simulations and Cluster Analysis

Dew is a non-conventional source of water that has been gaining interest over the last two decades, especially in arid and semi-arid regions. In this study, we performed a long-term (1979–2018) energy balance model simulation to estimate dew formation potential in Iran aiming to identify dew formation zones and to investigate the impacts of long-term variation in meteorological parameters on dew formation. The annual average of dew occurrence in Iran was ~ 102 days, with the lowest number of dewy days in summer (~ 7 days) and highest in winter (~ 45 days). The average daily dew yield was in the range of 0.03–0.14 L/m2 and the maximum was in the range of 0.29–0.52 L/m2. Six dew formation zones were identified based on cluster analysis of the timeseries of the simulated dew yield. The distribution of dew formation zones in Iran was closely aligned with topography and sources of moisture. Therefore, the coastal zones in the north and south of Iran (i.e., Caspian Sea and Oman Sea), showed the highest dew formation potential with 53 and 34 L/m2/year, whereas the dry interior regions (i.e., central Iran and the Lut Desert), with the average of 12–18 L/m2/year had the lowest potential for dew formation. Trend analysis results revealed a significance (p