Characterization and Comparison of Biodegradable Printed Capacitive Humidity Sensors

Flexible and biodegradable sensors are advantageous for their versatility in a range of areas from smart packaging to agriculture. In this work, we characterize and compare the performance of interdigitated electrode (IDE) humidity sensors printed on different biodegradable substrates. In these IDE capacitive devices, the substrate acts as the sensing layer. The dielectric constant of the substrate increases as the material absorbs water from the atmosphere. Consequently, the capacitance across the electrodes is a function of environmental relative humidity. Here, the performance of polylactide (PLA), glossy paper, and potato starch as a sensing layer is compared to that of nonbiodegradable polyethylene terephthalate (PET). The capacitance across inkjet-printed silver electrodes is measured in environmental conditions ranging from 15 to 90% relative humidity. The sensitivity, response time, hysteresis, and temperature dependency are compared for the sensors. The relationship between humidity and capacitance across the sensors can be modeled by exponential growth with an R2 value of 0.99, with paper and starch sensors having the highest overall sensitivity. The PET and PLA sensors have response and recovery times under 5 min and limited hysteresis. However, the paper and starch sensors have response and recovery times closer to 20 min, with significant hysteresis around 100%. The PET and starch sensors are temperature independent, while the PLA and paper sensors display thermal drift that increases with temperature.

Influences of environmental relative humidity and horizontal scale of sub-cloud ascent on deep convective initiation

This study examines two factors impacting initiation of moist deep convection: free tropospheric environmental relative humidity (ϕE) and horizon scale of sub-cloud ascent (Rsub), the latter exerting a dominant control on cumulus cloud width. A simple theoretical model is used to formulate a “scale selection” hypothesis: that a minimum Rsub is required for moist convection to go deep, and that this minimum scale decreases with increasing ϕE. Specifically, the ratio of to saturation deficit (1–ϕE) must exceed a certain threshold value that depends on cloud-layer environmental lapse rate. Idealized, large-eddy simulations of moist convection forced by horizontally-varying surface fluxes show strong sensitivity of maximum cumulus height to both ϕE and Rsub consistent with the hypothesis. Increasing Rsub by only 300-400 m can lead to a large increase (> 5 km) in cloud height. A passive tracer analysis shows that the bulk fractional entrainment rate decreases rapidly with Rsub but depends little on ϕE. However, buoyancy dilution increases as either Rsub or ϕE decreases; buoyancy above the level of free convection is rapidly depleted in dry environments when Rsub is small. While deep convective initiation occurs with an increase in relative humidity of the near environment from moistening by earlier convection, the importance of this moisture preconditioning is inconclusive as it is accompanied by an increase in Rsub. Overall, it is concluded that small changes to Rsub driven by external forcing or by convection itself could be a dominant regulator of deep convective initiation.

Thermal Chains and Entrainment in Cumulus Updrafts. Part II: Analysis of Idealized Simulations

Research has suggested that the structure of deep convection often consists of a series of rising thermals, or “thermal chain,” which contrasts with existing conceptual models that are used to construct cumulus parameterizations. Simplified theoretical expressions for updraft properties obtained in Part I of this study are used to develop a hypothesis explaining why this structure occurs. In this hypothesis, cumulus updraft structure is strongly influenced by organized entrainment below the updraft’s vertical velocity maximum. In a dry environment, this enhanced entrainment can locally reduce condensation rates and increase evaporation, thus eroding buoyancy. For moderate-to-large initial cloud radius R, this breaks up the updraft into a succession of discrete pulses of rising motion (i.e., a thermal chain). For small R, this leads to the structure of a single, isolated rising thermal. In contrast, moist environments are hypothesized to favor plume-like updrafts for moderate-to-large R. In a series of axisymmetric numerical cloud simulations, R and environmental relative humidity (RH) are systematically varied to test this hypothesis. Vertical profiles of fractional entrainment rate, passive tracer concentration, buoyancy, and vertical velocity from these runs agree well with vertical profiles calculated from the theoretical expressions in Part I. Analysis of the simulations supports the hypothesized dependency of updraft structure on R and RH, that is, whether it consists of an isolated thermal, a thermal chain, or a plume, and the role of organized entrainment in driving this dependency. Additional three-dimensional (3D) turbulent cloud simulations are analyzed, and the behavior of these 3D runs is qualitatively consistent with the theoretical expressions and axisymmetric simulations.

Thermal Chains and Entrainment in Cumulus Updrafts. Part I: Theoretical Description

Recent studies have shown that cumulus updrafts often consist of a succession of discrete rising thermals with spherical vortex-like circulations. In this paper, a theory is developed for why this “thermal chain” structure occurs. Theoretical expressions are obtained for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. Analysis of these expressions suggests that the thermal chain structure arises from enhanced lateral mixing associated with intrusions of dry environmental air below an updraft’s vertical velocity maximum. This dry-air entrainment reduces buoyancy locally. Consequently, the updraft flow above levels of locally reduced buoyancy separates from below, leading to a breakdown of the updraft into successive discrete thermals. The range of conditions in which thermal chains exist is also analyzed from the theoretical expressions. A transition in updraft structure from isolated rising thermal, to thermal chain, to starting plume occurs with increases in updraft width, environmental relative humidity, and/or convective available potential energy. Corresponding expressions for the bulk fractional entrainment rate ε are also obtained. These expressions indicate rather complicated entrainment behavior of ascending updrafts, with local enhancement of ε up to a factor of ~2 associated with the aforementioned environmental-air intrusions, consistent with recent large-eddy simulation (LES) studies. These locally large entrainment rates contribute significantly to overall updraft dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Part II of this study compares results from the theoretical expressions to idealized numerical simulations and LES.

The vertical moisture structure and precipitation intensity distributions associated with tropical convective systems

<p>In the tropics, the majority of high-intensity precipitation comes from the organization of multiple convective cells into mesoscale convective systems (MCS). Here, we use a synthesis of multi-decade satellite and reanalysis data to investigate relationships between the column water vapor content (CWVC), instability (CAPE), and precipitation from MCS. We find a linear relationship between MCS maximum precipitation intensity and CAPE and a sharp, nonlinear relationship between this maximum precipitation intensity and CWVC. The latter suggests that a deep layer of inflow to the MCS dominates buoyancy and precipitation production. From these multidecade data, we can also illustrate robust shifts in the probability distributions of precipitation intensity with the El Niño Southern Oscillation. El Niño-La Niña relative gains and losses in precipitation intensity can be understood with a vertical momentum budget and the role of environmental relative humidity and large-scale circulation therein. Understanding the associated vertical moisture structure and instability is essential to better predict future variability in tropical precipitation.</p>