Water Vapor Gradients Research Articles

Systematic errors in atmospheric profiles obtained from Abelian inversion of radio occultation data: Effects of large‐scale horizontal gradients

Reduction of radio occultation data to retrieve atmospheric profiles (T‐p(r)) requires knowledge or assumption of the horizontal structure of the atmosphere. In the case of terrestrial planets the atmosphere in the vicinity of ray periapsides usually is assumed to be spherically symmetric. This assumption leads to an integral transform relationship between the profiles of refractivity versus radius and the total bending angle versus the asymptotic closest approach of rays, where the latter is directly obtainable from occultation frequency data and trajectory information. Occultation studies of the giant planets have demonstrated that departures from spherical symmetry, if not accounted for, can result in serious errors in derived T‐p(r) profiles. We analyze errors in atmospheric profiles due to large‐scale departures from spherical symmetry. For analytic convenience we represent departures from spherical symmetry as locally spherical structures with center of curvature offset in three dimensions from the center of mass, from which follow analytic expressions for errors in bending angle and impact parameter as functions of the offset and trajectory parameters. Since these expressions are not restricted to any specific occultation type, it is easy to identify the geometrical configurations and the specific trajectory parameters that enhance or suppress these errors. Errors in bending angle and impact parameter carry over into the refractivity and radius profiles, while at the same time, new errors are introduced because the bending angle versus impact parameter profile is integrated along a nonvertical path in the presence of large‐scale departures from spherical symmetry, to obtain refractivity and radius profiles. Similarly, refractivity and radius errors propagate into the temperature and pressure profiles, while a nonvertical path of integration in the presence of horizontal gradients provides another opportunity for new errors to be introduced. We estimate that fractional errors in temperature profiles can be as large as a few percent for the Martian atmosphere above 20 km, decreasing in magnitude closer to the surface. For Earth, such errors are estimated to be less than 1% above 30 km. In the lower parts of Earth's atmosphere, however, and especially in the lower troposphere, these errors can be very sensitive to horizontal gradients and hence highly variable; typically, the error magnitude remains less than 2% for the dry regions of Earth's troposphere. We have not addressed the effect on errors of water vapor gradients, or of more extreme structures such as sharp weather fronts. A small variation on this approach can incorporate errors due to imprecise knowledge of the transmitter and receiver trajectories.

Data Assimilation of Satellite-Derived Heating Rates as Proxy Surface Wetness Data into a Regional Atmospheric Mesoscale Model. Part II: A Case Study

Abstract A satellite data assimilation method is applied which incorporates satellite-observed heating infrared rates into a mesoscale atmospheric model to retrieve model soil moisture. In a 3D case study, the method is successful at retrieving realistic spatial representations of the heterogeneous surface soil moisture as compared to microwave surface emissivities, precipitation reports, and radar summaries; however, absolute magnitudes of the derived soil moisture fields are by nature model dependent. From noise sensitivity experiments, satellite instrument noise is not found to be a major factor in the data assimilation method’s performance. The case study presented here over the Great Plains region showed a significant soil moisture gradient where a weak dryline feature formed in the afternoon. The main effect of the soil moisture gradient was a tightening of the water vapor gradient in the boundary layer. However, it was found that this feature was much less important than latent heat release due to ...

Overcoming humidity dependence of the chromium trioxide converter used in luminol-based nitric oxide detection

The surface-atmosphere exchange of gaseous NO x (NO+NO 2) is commonly measured by chamber methods that employ a portable luminol-based NO 2 detector. Detection of NO depends on its prior oxidation to NO 2 by CrO 3 with efficiency that depends on sample relative humidity. We found that inadequate drying of wet chamber air samples, and consequent changes in NO oxidation efficiency, often contributes to observed apparent shifts in the response of commercially available detectors in the field. Our instrument’s performance was improved when the water vapor gradient across its membrane dryer was enhanced by replacing the restrictor on the air inlet of the dryer’s shell with a desiccant column, and then increasing ambient air flow through the shell to approximately equal the sample air flow rate. We also demonstrated that pumice is the preferred CrO 3 support material for luminol-based detection of NO in chamber air samples; advantages include low cost, superior physical properties, and the simplicity of fabricating a CrO 3/pumice converter by the user.

Stomatal responses to changes in air humidity are not necessarily linked to nocturnal CO2 uptake in the CAM plant Plectranthus marrubioides Benth. (Lamiaceae)

ABSTRACTPlants of the crassulacean acid metabolism (CAM) species Plectranthus marrubioides (Lamiaceae) were subjected to short‐ and long‐term changes in air humidity in controlled‐environment experiments. Stomata of well‐watered individuals of this all‐cell leaf‐succulent taxon responded directly, quickly and reversibly to variations of the water vapour gradient between leaf and air (Δw). Mean night‐time leaf conductance to water vapour decreased curvilinearly with increasing Δw but linearly with lowered relative air humidity. Stomatal response was generally independent of the prevailing temperature and was not linked to CO2 uptake rates. Therefore, net night‐time carbon gain, nocturnal malic acid accumulation and, thus, relative carbon recycling were not influenced by changes in air humidity in the temperature range tested. Mean nocturnal molar water use efficiency, however, decreased with decreasing air humidity because of the increased transpirational water loss. If watering was repeatedly withheld for several days during the experiments, employing a temperature regime of 35/30°C day and night, stomatal conductance became low enough to inhibit CO2 uptake, but only at the highest Δw. The results suggest that drought stress was necessary to increase responsiveness of plants to the point where CAM was also inhibited by decreases in air humidity.

Leaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects.

Leaf carbon isotope discrimination (Δ), seasonal estimates of the leaf-to-air water vapor gradient on a molar basis (ω), and leaf nitrogen contents were examined in three riparian tree species (Populus fremontii, P. angustifolia, and Salix exigua) along elevational transects in northern and southern Utah USA (1500-2670 m and 600-1820 m elevational gradients, respectively). The ω values decreased with elevation for all species along transects. Plants growing at higher elevations exhibited lower Δ values than plants at lower elevations (P. fremontii, 22.9‰ and 19.5‰, respectively; P. angustifolia, 23.2‰ and 19.2‰, respectively; and S.␣exigua, 21.1‰ and 19.1‰, respectively). Leaf nitrogen content increased with elevation for all species, suggesting that photosynthetic capacity at a given intercellular carbon dioxide concentration was greater at higher elevations. Leaf Δ and nitrogen content values were highly correlated, implying that leaves with higher photosynthetic capacities also had lower intercellular carbon dioxide concentrations. No significant interannual differences were detected in carbon isotope discrimination.

Long-Term, Hygrothermal Performance of Exterior Insulation and Finish Systems (EIFS)

To control rain ingress, the Exterior Insulation and Finish Systems (EIFS) can be constructed with one of two design principles • a drain screen (rain screen, pressure equalized rain screen-PER) approach • a face sealed (barrier) approach While the barrier approach has an inherently higher risk for moisture damage, it offers econormc advantages and may be successfully used under a multitude of cli matic and service conditions. Its performance evaluation is carried out in three steps: 1. Examine whether this system can be used for a specific set of climatic and use con ditions 2. Formulate necessary measures for material and field quality control in the design specifications 3. Examine the system for integrity under cycling environmental conditions and long-term performance Step 1 of this evaluation is based on the moisture control strategy of the whole wall assembly. Step 2 requires the addition of several small-scale material tests, and Step 3 involves full-scale system testing. The full scale testing is necessary to incor porate several parameters that act on the system simultaneously (thermal stress, thermal and moisture originated expansion and contraction, structural movements, material changes caused by weathering and aging, presence of moisture, etc.). The in teraction among these factors may provide conditions with severity significantly in excess of those achieved when these parameters are tested separately. This discussion paper consists of three parts: Part 1: Deals with the assessment of constituent materials of the EIFS with a view to ensuring (characterizing) hygrothermal performance of the system (by testing and characterization of the constituent materials, one may limit the range of variation in the hygrothermal performance of the whole system). Part 2: Postulates a new test for evaluation of EIFS integrity under simulated expo sure to climatic cycling. Part 3: Recommends EIFS design solutions and construction details for both new and retrofit construction. These details are discussed in the context of specific environmental/climatic conditions. This will be published in a later issue of JTIBE. Testing EIFS cladding systems for integrity under simulated exposure to climatic cycling involves preconditioning under simultaneous exposure to a water vapor gra dient and temperature gradient followed by exposure to severe conditions of one- sided, uni-directional climatic cycling. The testing and characterization of the con stituent materials (Step 1) coupled with the full-scale test for EIFS integrity under exposure to climatic cycling (Step 2) will increase the reliability of the EIFS systems in North America. Part 3 fills the gap between the current understanding of moisture control princi ples and construction practices. Many small changes in materials and technology create a compound effect that increases the frequency of inadequate field perform ance. Though most inherent EIFS flaws may be minor, those, when coupled with a number of failures of wall systems involving EIFS, caused an adverse attitude toward EIFS, perhaps more prevalent in some regions of North America. This paper pro vides a fundamental framework to cement many of the previously postulated correc tive measures into a coherent package aiming at the improvement of the EIFS reli ability in North America.

Mass and energy exchange between the atmosphere and leaf influence gas pressurization in aquatic plants.

A mechanistic model was used to describe how mass and energy exchange between a leaf and air affect internal pressurization in aquatic plants. The core of the model is the calculation of the temperature of a leaf for which the energy balance is zero. The leaf temperature and water vapour gradient between leaf and air are then used to calculate potential pressurization. Simulations with the model were used to demonstrate the sensitivity of gas pressurization in aquatic plants to key environmental and physiological factors including radiation, temperature, humidity, wind speed, leaf size and leaf conductance. The model confirms other published data showing that humidity-induced pressurization is the dominant mode of pressurization under most conditions. The simulations also demonstrated that thermal transpiration and humidity-induced pressurization are not separate phenomena, but are dependent on the energy balance of the leaf, which in turn is affected by complex interactions between the leaf and environment.

Deshydratation de l'alun d'ammonium

The formation of the two kinds of partially dehydrated amorphous alum can be explained by the water vapour gradient which appears within the sample during dehydration and by existence of a critical water pressure P c at the reaction interface. The mechanisms of dehydration at low and high pressure are described.

Evolution of two oceanic extratropical cyclones as observed with the special sensor microwave/imager and the Geosat and ERS 1 altimeters

The evolution of two marine cyclones across the North Atlantic and the North Pacific Oceans is followed with a combination of sequential passive and active microwave data. The special sensor microwave/imager (SSM/I) provides estimates of atmospheric parameters: integrated water vapor, precipitation, and ocean surface wind speed. The Geosat and ERS 1 altimeter data are used to derive sea state parameters along the satellite tracks: the significant wave height and the ocean surface wind speed. The SSM/I integrated water vapor gradients are used to locate and follow the evolution of the atmospheric fronts. In regions of precipitation, often located behind the atmospheric fronts, the SSM/I surface wind speed cannot be derived, while the Geosat and ERS 1 altimeter wind speed yields estimates of wind speed gradients across the fronts. It is shown that the combination of SSM/I and satellite altimeters permits additional observations of surface fronts with higher resolution both in space and time than those given in the National Meteorological Center analyses.

Satellite-Model Coupled Analysis of Convective Potential in Florida with VAS Water Vapor and Surface Temperature Data

Abstract A system for time-continuous mesoscale weather analysis is applied to a study of convective cloud development in central Florida. The analysis system incorporates water vapor concentrations and surface temperatures retrieved from infrared VISSR (Visible–Infrared Spin Scan Radiometer) Atmospheric Sounder (VAS) satellite data, with coupling between the retrieval process and time integration of a mesoscale model. Analyses prepared with variations of this coupled system are compared with a control numerical analysis prepared with only conventional meteorological observations and are validated against surface and upper-air data collected for the Convection and Precipitation/Electrification experiment. The coupled analyses assimilate six sets of VAS data over an 8-h period on 19 July 1991 and depict water vapor gradients at far greater horizontal resolution than is available from conventional observations and with an overall accuracy better than the control analysis. The coupled system's ability to ass...

Water Vapor Diffusivity in Vitreous and Mealy Wheat Endosperm

The water vapor diffusivities of vitreous and mealy wheat endosperms were measured using diffusion cells containing wax-scaled cylinders of endosperm. The diffusion cells were incubated in a chamber that was maintained at a constant temperature and relative humidity. A water vapor gradient was created by filling each diffusion cell with desiccant. A steady-state condition was achieved within 24 h. The mean diffusivities for the vitreous endosperm samples of cvs Arizona and Len were 2·0 × 10-8 cm2/s. The diffusivities for mealy samples of cvs Logan and Titan were 3·5 and 9·1 × 10-8 cm2/2, respectively. The liquid water diffusivities of whole caryopses were similar for five vitreous and five mealy wheat samples tested. No change was detected in the diffusion mechanism for any of the samples in the temperature range tested. The energy of activation (E) was significantly (P < 0·05) higher for endosperm for cv. Len, a vitreous sample, than for cv. Titan, a mealy sample.

Evaluation of water vapor distribution in general circulation models using satellite observations

This paper presents a comparison of the water vapor distribution obtained from two general circulation models, the European Centre for Medium‐Range Weather Forecasts (ECMWF) model and the National Center for Atmospheric Reseach (NCAR) Community Climate Model (CCM), with satellite observations of total precipitable water (TPW) from SSM/I and upper tropospheric relative humidity (UTH) from GOES. Overall, both models are successful in capturing the primary features of the observed water vapor distribution and its seasonal variation. For the ECMWF model, however, a systematic moist bias in TPW is noted over well‐known stratocumulus regions in the eastern subtropical oceans. Comparison with radiosonde profiles suggests that this problem is attributable to difficulties in modeling the shallowness of the boundary layer and large vertical water vapor gradients which characterize these regions. In comparison, the CCM is more successful in capturing the low values of TPW in the stratocumulus regions, although it tends to exhibit a dry bias over the eastern half of the subtropical oceans and a corresponding moist bias in the western half. The CCM also significantly overestimates the daily variability of the moisture fields in convective regions, suggesting a problem in simulating the temporal nature of moisture transport by deep convection. Comparison of the monthly mean UTH distribution indicates generally larger discrepancies than were noted for TPW owing to the greater influence of large‐scale dynamical processes in determining the distribution of UTH. In particular, the ECMWF model exhibits a distinct dry bias along the ITCZ and a moist bias over the subtropical descending branches of the Hadley cell, suggesting an underprediction in the strength of the Hadley circulation. The CCM, on the other hand, demonstrates greater discrepancies in UTH than are observed for the ECMWF model, but none that are as clearly correlated with well‐known features of the large‐scale circulation.

Photosynthesis and water relations of the floodplain tree, boxelder (Acer negundo L.).

During summer, gas exchange and water relations were measured in mature boxelder (Acer negundo L.) trees growing on a floodplain in central Indiana, USA. A shallow (< 1.25-m deep) water table and repeated flooding kept the soil water potential above -0.5 MPa at all times. Net photosynthesis and stomatal conductance were influenced primarily by light and, to a lesser extent, by leaf temperature, but showed no relationships with leaf-to-air water vapor gradient or leaf water potential. Throughout the summer, there was no midday stomatal closure on any measurement day, and leaf water potential at dawn and minimum daily leaf water potential remained above -0.4 and -1.4 MPa, respectively. Nevertheless, there was a seasonal decline in leaf osmotic potentials at saturation and turgor-loss point. Seasonal changes in maximum daily net photosynthesis and stomatal conductance, minimum daily leaf water potential and soil-to-leaf hydraulic conductance were not related to seasonal changes in soil water potential, air or soil temperature, or water table depth. Seasonal responses of net photosynthesis to intercellular CO(2) indicated that net photosynthesis was controlled primarily by nonstomatal factors. High soil water and a shallow water table may have kept soil-to-leaf hydraulic conductance large (5-9 mmol m(-1) s(-1) MPa(-1)) throughout the summer, permitting the trees to keep their stomata open, yet maintain leaf turgor and high net photosynthesis during the hot, low-humidity afternoons. This could also account for the dominance of nonstomatal influences on net photosynthesis.

Impact of the Geosat altimeter wet tropospheric range correction in the Greenland‐Iceland‐Norwegian Sea

Atmospheric water vapor data derived from the special sensor microwave imager (SSM/I) are used to make time‐coincident wet tropospheric range corrections to Geosat altimeter data in the Greenland‐Iceland‐Norwegian (GIN) Sea. The SSM/I sensor is onboard a polar orbiting Defense Meteorological Satellite Program platform and provides water vapor data at 25‐km spatial resolution and an accuracy of 0.24 g cm−2. Water vapor height corrections were calculated and analyzed along all Geosat tracks crossing the area of interest in July and September 1987 and January and March 1988. Results indicate that the horizontal spatial variations in the altimeter height corrections can sometimes equal or exceed the magnitude (10–20 cm/100 km) of true GIN Sea sea surface height gradients, especially in the summer and autumn seasons. SSM/I and Geosat data must be closely matched in time, due to the potential rapid movement of atmospheric systems through this region (35–40 km h−1). The water vapor data must be timely enough to adequately describe atmospheric conditions at the time of the altimeter pass, or significantly false signals may be introduced into the sea surface height data. Substantial differences in water vapor gradients were observed in SSM/I data sets 12 hours apart. Because of this, it is suggested that future satellite altimeter platforms include a nadir‐viewing radiometer to provide time‐coincident atmospheric water vapor data.

Path length variations caused by atmospheric water vapor and their effects on the measurement of mesoscale ocean circulation features by a radar altimeter

Spaceborne radar altimeters make high‐precision measurements of the range between the satellite and the ocean surface. Atmospheric water vapor will lengthen the range as measured by an altimeter. It is theoretically possible for horizontal changes in atmospheric water vapor to be misinterpreted as changes in sea surface height (SSH) or to mask SSH changes that are present. Scanning multichannel microwave radiometer (SMMR) data from the entire 3‐month lifetime of the Seasat satellite are used to estimate water vapor path length corrections and to assess the probability that water vapor will significantly degrade the interpretation of Geosat altimeter data. It is shown that if one considers Geosat‐observed mesoscale circulation features of 10 cm or greater, particularly in the context of a regional circulation model, water vapor is not generally a problem. For mesoscale circulation features 5 cm or smaller, potential contamination by water vapor gradients becomes significant. Cumulative distributions describing water vapor statistics for a variety of spatial scales and in specific geographical regions, as well as the entire globe, are presented. These distributions are compared with ones derived using Fleet Numerical Oceanography Center (FNOC) predictions. It is demonstrated that FNOC‐derived water vapor path lengths, although providing a good mean correction, will not serve to compensate for small horizontal scale changes in water vapor.

Identification of Atmospheric Fronts over the Ocean with Microwave Measurements of Water Vapor and Rain

This paper describes some basic research techniques and algorithms developed to diagnose fronts in cyclonic storms over the ocean with data from satellite-borne microwave radiometers. Methods are developed for flagging strong gradients in integrated atmospheric water vapor and the presence of rain by using data from the SSMR on board the polar orbiting Seasat and Nimbus-7 satellites. Examination of 65 frontal systems showed that the water vapor gradient flag correctly identified 86 percent of the fronts, while the precipitation flagged 91 percent. The two types of flags emphasize different portions of the cyclone and are therefore complementary. Ultimately, these techniques are intended for operational use with data from the Special Sensor Microwave Imager which was launched in June 1987 on a satellite in the Defense Meteorological Satellite Program (DMSP).

Gas exchange of Agropyron desertorum: diurnal patterns and responses to water vapor gradient and temperature.

The response of leaf gas exchange to environmental variables were measured at different levels of drought stress for Agropyron desertorum, a naturalized perennial bunchgrass of the semiarid shrub steppes of western North America. Leaf conductance (stomatal plus boundary layer) was more sensitive to changes in water vapor gradient than to changes in leaf temperature. Assimilation was sensitive to both temperature and vapor gradient, and also appeared to be affected by conductance and high transpiration rates. The magnitudes of both assimilation and conductance decreased with increased drought conditions. Diurnal patterns of gas exchange were measured during 3 growing seasons. For a typical spring day with moderate leaf temperature and vapor gradient, diurnal patterns were similar for plants at different levels of soil water availability. Assimilation was relatively constant during most of the day, but conductance decreased during the afternoon. Total daily carbon gain was decreased to a lesser extent than daily water loss as soil water was depleted. Consequently, the ratio of daily carbon gain to daily water loss, i.e. daily water use efficiency, increased with decreased soil water content for diurnals under spring conditions. Diurnal patterns of assimilation for a typical summer day with high leaf temperature and vapor gradient differend from those for a spring day. An afternoon decrease in assimilation was typical during a summer day. Daily carbon gain, water use, and water use efficiency for summer diurnals decreased only under severe drought conditions. Almost complete recovery of assimilation and conductance occurred if leaf microclimate was ameliorated during the afternoon of either spring or summer diurnals. Thus, conditions responsible for a midday depression in assimilation during a single day did not have persistent effects on leaf gas exchange. Daily carbon gain of a typical summer day was restricted by leaf microclimate during the afternoon, but daily water use efficiency was not relatively increased by the amelioration of leaf microclimate.

Deposition of ultrafine aerosols in a plane channel from a laminar vapor-gas flow of nonuniform concentration

The deposition of a high-dispersity aerosol in a water-vapor concentration gradient field in a thermal diffusion chamber with a control element [1] has been experimentally investigated.

Rate of evaporation of tear fluid in the human compared with a physical model

Evaporation of tears occurs by diffusion, free (thermal), and forced convection. Under resting conditions only diffusion and free convection play a role. Therefore, a device should be applied with only these two parameters working. Creating a stationary known pressure gradient of water vapor over the eye, the evaporation rate was found to be 24.8 +/- 6.8 mg cm-2 h-1 in 21 test persons (no spectacle or contact lens wearers, no known general or eye disease, average age 25.6 years). The same test arrangement over a water surface of 1.7 cm2 gave a value of 43 mg cm-2 h-1 and simplified calculation, applying Fick's law of diffusion, 54 mg cm-2 h-1. These results suggest that there is a water vapor pressure of less than half the saturation water vapor pressure of less than half the saturation water vapor pressure corresponding to 32 degree C over the surface of the eye. Calculation yields a value of 230 mg per eye per 16 hours, corresponding to 32 degrees C (50% air humidity) over the surface of the eye and 22 degrees C (50% air humidity) room temperature. Assuming a tear secretion of 0.6 g per eye per 16 hours (Schirmer), about 40% of this amount evaporates under normal conditions.

Use of an Edible Film to Maintain Water Vapor Gradients in Foods

ABSTRACTAn edible, biiayer film consisting of a layer of stearic‐palmitic acid and a layer of hydroxypropyl methylcellulose was situated between two food components of markedly different water activities to determine the film's ability to retard equalization of water activity. Tomato paste or salted tomato paste was used as the high‐moisture food and ground crackers were used as the low‐moisture food. Compared to a filter paper control, the bilayer film substantially slowed transfer of water from the salted tomato paste to the crackers during 14 days at 25°C and 21 days at 5°C. During 70 days at ‐20°C, the film essentially stopped the transfer of water from tomato paste to the crackers.