Water Vapor Exchange Research Articles

Spatial variation of stomatal morphological traits in grassland plants of the Loess Plateau

The stomata control carbon and water vapor exchange between the leaves and atmosphere and may reflect plant adaptation to climate change to some extent. However, the spatial variation of stomatal traits on different sides of the leaves’ epidermis and their response to environmental changes remain unclear. Here, we measured stomatal density (SD), stomatal size (SS), and stomatal relative area (SRA) on both leaf surfaces of 181 plant species across an environmental gradient on the Loess Plateau. We calculated the proportion of stomatal traits between different epidermis surfaces of the leaves (abaxial and adaxial) and measured the total SD (SDtot), total SS (SStot), and total SRA (SRAtot) of the leaves as a whole. The SDtot, SStot, and SRAtot showed a log-normal distribution, differing across grassland types and plant functional groups, with a range of 22.57–890.09 stomata mm−2, 84.61–2022.81 μm2, and 0.51–43.71%, respectively. The SDtot, SStot, and SRAtot differed significantly between different grassland types. Meadow plants had larger SDtot, desert plants had larger SStot, and typical grassland plants had relatively larger SRAtot. The abaxial SD and SRA of the woody plants were significantly higher than those of herbaceous plants, but no significant differences were observed at the adaxial surface or for the overall leaf. A higher SDtot was correlated with a larger stomatal density ratio (R.SD) and smaller stomatal size ratio (R.SS). The R.SD, abaxial SD, abaxial SS, and adaxial SS were significantly correlated with environmental factors, but no such correlations were observed for the adaxial SD. These findings highlight the strong associations of stomatal traits with plant functional group and climate at a regional scale, representing the climatic adaptation strategies of stomatal traits across natural grassland communities.

Air traffic and contrail changes over Europe during COVID-19: a model study

Abstract. The strong reduction of air traffic during the COVID-19 pandemic provides a unique test case for the relationship between air traffic density, contrails, and their radiative forcing of climate change. Here, air traffic and contrail cirrus changes are quantified for a European domain for March to August 2020 and compared to the same period in 2019. Traffic data show a 72 % reduction in flight distance compared with 2019. This paper investigates the induced contrail changes in a model study. The contrail model results depend on various methodological details as discussed in parameter studies. In the reference case, the reduced traffic caused a reduction in contrail length. The reduction is slightly stronger than expected from the traffic change because the weather conditions in 2020 were less favorable for contrail formation than in 2019. Contrail coverage over Europe with an optical depth larger than 0.1 decreased from 4.6 % in 2019 to 1.4 % in 2020; the total cirrus cover amount changed by 28 % to 25 %. The reduced contrail coverage caused 70 % less longwave and 73 % less shortwave radiative forcing but, because of various nonlinearities, only 54 % less net forcing in this case. The methods include recently developed models for performance parameters and soot emissions. The overall propulsion efficiency of the aircraft is about 20 % smaller than estimated in earlier studies, resulting in 3 % fewer contrails. Considerable sensitivity to soot emissions is found, highlighting fuel and engine importance. The contrail model includes a new approximate method to account for water vapor exchange between contrails and background air and for radiative forcing changes due to contrail–contrail overlap. The water vapor exchange reduces available ice supersaturation in the atmosphere, which is critical for contrail formation. Contrail–contrail overlap changes the computed radiative forcing considerably. Comparisons to satellite observations are described and discussed in a parallel publication.

Investigating temperature and moisture profiles of seasonally frozen soil under different land covers using actively heated fiber Bragg grating sensors

In this study, the actively heated fiber Bragg grating (AH-FBG) technique is developed to measure temperature and moisture profiles of partially frozen soil. The working principle of the AH-FBG sensing system is introduced, which employs an aluminum oxide tube sensor (AOTS). The unfrozen water contents and ice contents are estimated by combining AH-FBG with the frequency-domain reflection (FDR) technique. The feasibility of AH-FBG for soil moisture measurement is evaluated by laboratory and field tests under four different land covers, i.e., bare soil, concrete slab, plastic mulch (PM), and grass. It is found that AH-FBG can be used to capture unfrozen water contents and ice contents with high accuracy. The relationships between the thermal conductivity measured by AOTSs and the total water contents of frozen and unfrozen soils can be well fitted by the Côté and Konrad model. Both PM and concrete slab have a warming effect on the ground soil, which makes their depths of soil freezing shallower than that of bare soil, while the grassy land has a deeper freezing depth due to the inherent plant root-to-soil structure. The field monitoring results indicate that the PM can effectively block water migration channels at the ground surface and prevent water vapor exchange between the atmosphere and the soil, resulting in a “pot effect” that commonly exists under an impermeable layer.

Super Sites for Advancing Understanding of the Oceanic and Atmospheric Boundary Layers

Abstract Air‐sea interactions are critical to large-scale weather and climate predictions because of the ocean's ability to absorb excess atmospheric heat and carbon and regulate exchanges of momentum, water vapor, and other greenhouse gases. These exchanges are controlled by molecular, turbulent, and wave-driven processes in the atmospheric and oceanic boundary layers. Improved understanding and representation of these processes in models are key for increasing Earth system prediction skill, particularly for subseasonal to decadal time scales. Our understanding and ability to model these processes within this coupled system is presently inadequate due in large part to a lack of data: contemporaneous long-term observations from the top of the marine atmospheric boundary layer (MABL) to the base of the oceanic mixing layer.We propose the concept of “Super Sites” to provide multi-year suites of measurements at specific locations to simultaneously characterize physical and biogeochemical processes within the coupled boundary layers at high spatial and temporal resolution. Measurements will be made from floating platforms, buoys, towers, and autonomous vehicles, utilizing both in-situ and remote sensors. The engineering challenges and level of coordination, integration, and interoperability required to develop these coupled ocean‐atmosphere Super Sites place them in an “Ocean Shot” class.

Ammonia, nitrous oxide, carbon dioxide, and water vapor fluxes after green manuring of faba bean under Mediterranean climate

Green manuring of legume crops has increased in interest in recent years because of environmental and economic costs of synthetic fertilizers and need of sustainable cropping systems. Climatic conditions can reduce the agronomic and environmental effectiveness of green manuring promoting reactive nitrogen losses, or fostering soil heterotrophic respiration. This study aims to quantify ammonia and nitrous oxide losses, as well as carbon dioxide exchanges following green manuring of faba bean (Vicia faba spp. minor L.) under semi-arid climate conditions, typical of the Mediterranean basin. Ammonia emissions were measured by means of an aerodynamic gradient approach equipped with a multi-channel wet-denuder system (ROSAA), alongside with a concentration-based inverse dispersion modelling assessment. Nitrous oxide losses were measured with an automated chamber system, whereas carbon dioxide and water vapor exchanges were monitored jointly by means of eddy covariance technique. Ammonia volatilization lasted for three weeks and was triggered by the increase of soil water content due to rainfall, occurring 5 days after green manuring. Total emissions were 0.31% of the nitrogen supplied by green manuring (230 kg N ha−1). Nitrous oxide emissions from soil were of low intensity during the measurement period and were directly corelated to the water filled pore space. The biogeochemical model CERES-EGC was validated with the measurements of nitrous oxide, then used to reproduce the dynamic until the sowing of the succeeding crop. Cumulated emissions of nitrous oxide were 1% of the total nitrogen supplied. Soil became a net source both of carbon dioxide and water vapor soon after green manuring, with an emission of 900 kg C-CO2 ha−1 by heterotrophic respiration (23% of added carbon). In terms of water balance, 19 mm of water evaporated from soil surface, originated from the addition of fresh biomass. Green manuring proved to be a significant agronomic strategy to improve soil fertility, limiting reactive nitrogen and carbon losses to the atmosphere.

Tropical Temperature Variability in the UTLS: New Insights from GPS Radio Occultation Observations

AbstractGlobal positioning system (GPS) radio occultation (RO) observations, first made of Earth’s atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of RO observations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer.

Differential responses to two heatwave intensities in a Mediterranean citrus orchard are identified by combining measurements of fluorescence and carbonyl sulfide (COS) and CO2 uptake

The impact of extreme climate episodes such as heatwaves on plants physiological functioning and survival may depend on the event intensity, which requires quantification. We unraveled the distinct impacts of intense (HW) and intermediate (INT) heatwave days on carbon uptake, and the underlying changes in the photosynthetic system, in a Mediterranean citrus orchard using leaf active (pulse amplitude modulation; PAM) and canopy level passive (sun-induced; SIF) fluorescence measurements, together with CO2 , water vapor, and carbonyl sulfide (COS) exchange measurements. Compared to normal (N) days, gross CO2 uptake fluxes (gross primary production, GPP) were significantly reduced during HW days, but only slightly decreased during INT days. By contrast, COS uptake flux and SIFA (at 760nm) decreased during both HW and INT days, which was reflected in leaf internal CO2 concentrations and in nonphotochemical quenching, respectively. Intense (HW) heatwave conditions also resulted in a substantial decrease in electron transport rates, measured using leaf-scale fluorescence, and an increase in the fractional energy consumption in photorespiration. Using the combined proxy approach, we demonstrate a differential ecosystem response to different heatwave intensities, which allows the trees to preserve carbon assimilation during INT days but not during HW days.

Carbon and Water Exchanges in a Mountain Meadow Ecosystem, Sierra Nevada, California

Ecosystem-atmosphere exchanges of carbon dioxide (CO2) and water vapor were investigated in a moist mountain meadow (Loney Meadow) at 1822 m MSL in the Sierra Nevada, California, USA. An eddy covariance (EC) tower was deployed for most of the snow-free period from May to September 2016. The meadow ecosystem progressed from a strong sink of CO2 in the peak of the growing season under saturated to wet soil conditions (−18.51 gC m−2 d−1) to a weak source (2.97 gC m−2 d−1) following a rapid decline in soil moisture as runoff decreased. The variability of Net Ecosystem Exchange (NEE) over diurnal, synoptic and seasonal timescales was dominated by Gross Primary Production (GPP) which ranged from 43 gC m−2 d−1 during the peak of the growing season to 19 gC m−2 d−1 during senescence. Ecosystem respiration was small in magnitude and variability compared to GPP. Approximations of annual NEE for the meadow ranged from −285 to −450 gC m−2 yr−1, which is high compared to grasslands, and more similar to mature wetland or forest ecosystems. At diurnal and synoptic scales, CO2 flux was driven most strongly by photosynthetically active radiation (PAR), while seasonally, the ecosystem was linked closely to changes in soil moisture. Light-use and water-use efficiencies of the meadow ecosystem were high compared with those found in most other ecosystems using comparable observations. These results suggest meadows have the potential to be large sinks of atmospheric CO2 and that their ability to do so is sensitive to water table height. This is important for understanding the future of carbon sequestration in mountain meadows in the context of changing hydroclimates and different land management decisions that impact meadow hydrology.

Homeostatic response of Aptian gymnosperms to changes in atmospheric CO2 concentrations

Abstract The sensitivity of plant carbon isotope fractionation (13Δleaf) to changes in atmospheric CO2 concentrations (Ca) is the subject of heavy debate, with some studies finding no sensitivity, while others show a strong dependency. We tested the hypothesis of photosynthetic homeostasis by using δ13C of n-alkanes, cuticles, and bulk organic matter of gymnosperm-rich rocks (Arundel Clay) from two sites deposited during the Aptian, a time that experienced signifi-cant Ca variations. Our results show no effect of Ca on 13Δleaf, and a relatively constant Ci/Ca (0.64 ± 0.04, 1σ; i—intercellular space), a value that is similar to that of modern gymnosperms. These results suggest that Aptian gymnosperms used homeostatic adjustments with rising Ca, probably involving increased carbon assimilation and/or stomatal closure, a response also found in modern gymnosperms. The similarity between Aptian and modern gymnosperms suggests that the processes responsible for regulating CO2 and water vapor exchange during photosynthesis have remained unaltered in gymnosperms for the past 128 m.y.

Leaf water δ18O reflects water vapour exchange and uptake by C3 and CAM epiphytic bromeliads in Panama.

The distributions of CAM and C3 epiphytic bromeliads across an altitudinal gradient in western Panama were identified from carbon isotope (δ13C) signals, and epiphyte water balance was investigated via oxygen isotopes (δ18O) across wet and dry seasons. There were significant seasonal differences in leaf water (δ18Olw), precipitation, stored 'tank' water and water vapour. Values of δ18Olw were evaporatively enriched at low altitude in the dry season for the C3 epiphytes, associated with low relative humidity (RH) during the day. Crassulacean acid metabolism (CAM) δ18Olw values were relatively depleted, consistent with water vapour uptake during gas exchange under high RH at night. At high altitude, cloudforest locations, C3 δ18Olw also reflected water vapour uptake by day. A mesocosm experiment with Tillandsia fasciculata (CAM) and Werauhia sanguinolenta (C3) was combined with simulations using a non-steady-state oxygen isotope leaf water model. For both C3 and CAM bromeliads, δ18Olw became progressively depleted under saturating water vapour by day and night, although evaporative enrichment was restored in the C3 W. sanguinolenta under low humidity by day. Source water in the overlapping leaf base 'tank' was also modified by evaporative δ18O exchanges. The results demonstrate how stable isotopes in leaf water provide insights for atmospheric water vapour exchanges for both C3 and CAM systems.

Carbon uptake and water vapor exchange in a pasture site in the Brazilian Cerrado

As Brazil becomes one of the largest agricultural and livestock producers, the expansion of cultivated pasture and crop areas in the Cerrado region is expected to cause significant changes in carbon and water vapor fluxes. While this change in land use promotes modifications in regional carbon uptake and water cycle, eddy covariance studies carried out in pasture areas in the Brazilian Cerrado remain scarce. The present study aimed to quantify the carbon uptake and water vapor exchange of these areas in the Brazilian Cerrado. The experimental site was installed in an area covered by Brachiaria sp. in Três Lagoas – MS, Brazil. Data were collected during two hydrological years (2016–2017 and 2017–2018). Pasture strongly acted as a carbon sink; its total accumulation reached −208.9 (±9.6) and −171.8 (±15.8) g C m−2 in the first and second hydrological years, respectively. Average evapotranspiration of 2.0 (±1.1) mm d−1 was observed in the two hydrological years. There was significant reduction in evapotranspiration at the pasture site during the driest months of the year (June and July), mainly in the second hydrological year, which recorded an average of 0.4 (±0.1) mm d−1. The lower recorded water availability in the soil led to the release of 86.7 g C m−2 (over 85 days) in the dry season of the second hydrological year. Ecosystem-level water use efficiency observed during the higher soil water deficit facilitated the survival of the grass. This information is the basis for further studies about the impact of climate change on carbon uptake and water vapor flux by cultivated and natural pasture; additionally, the present research will allow comparisons of carbon storage results to other ecosystems and to support studies of agrometeorological modeling.

Spatio - temporal variations of sea surface temperature in coastal waters of Khanh Hoa province (South Viet Nam) during the period of 2010-2019

Sea Surface Temperature (SST) is considered to be the most important parameter in oceanography. SST is not only an important physical parameter for studying the exchange of water vapor and heat between sea surface and atmosphere but also provides an useful index for oceanographic studies such as ocean circulation, water mass, ocean front, upwelling current, seawater mixing and ocean ecological environment. Empirical orthogonal function (EOF) and time series analysis was used to assess spatio-temporal variability of sea surface temperature in coastal waters of Khanh Hoa province (south Viet Nam) from Multi-scale Ultra-high Resolution Sea Surface Temperature (MUR SST) data for a period of 10 years (2010-2019). The first 4 EOF modes of SST explained 99.80% of the spatio-temporal variance in total variabilities. The first EOF mode explained 98.10% of the total variation of SST, representing the distribution and variability of SST in the summer with a decreasing trend from north to south. The second EOF mode explained 1,10% of the total variation of SST, representing the variation of the SST distribution in winter and spring with an increasing trend from shore to offshore. SST had a seasonal variation, it was highest in summer and lowest in winter. Trend analysis of SST time series in the past 10 years showed that SST increased 0,03oC/year in the period 1/2010-12/2019. The trend and the speed of increase/decrease of SST varied with each different period. The results of the study are useful documents to support the management and exploitation of marine ecosystems.

Simulation of Evapotranspiration Based on BEPS-TerrainLab V2.0 from 1990 to 2018 in the Dajiuhu Basin

Accurate estimations of evapotranspiration (ET) are essential for understanding land-atmosphere coupling and atmosphere-underlying surface energy and water vapor exchanges. Based on input data processing, this paper simulates the temporal and spatial variation of ET in the Dajiuhu Basin from 1990 to 2018 using the BEPS-TerrainLab V2.0 model. Compared with the ET measured by an eddy covariance (EC) tower, the model explained 80.1% of the ET variation. From 1990 to 2018, the average annual ET in the Dajiuhu Basin was 1262.7 mm/yr indicating a downward trend (−27.12 mm/yr). In 2005, a sudden change point was observed based on the Mann-Kendall (MK) test and 3-year moving t-test. Around 2005, the downward trend in ET slowed and the proportional trend of ET to precipitation changed from upward trend to downward trend. Regarding spatial distribution, the ET in the basin's central part was smaller than that in the basin's surrounding area, the ET of the southern slope was higher than that of the northern slope, and the decrease in the ET rate on the sunny side was lower than that on the shady side. ET decreased as the elevation increased, with the fastest decrease observed between 2184 and 2384 m. For different landcover types, the average ET exhibited the following order: deciduous forest > mixed forest > wetland > grass > agriculture land. Decreased solar radiation is the main reason for the decreased ET in the Dajiuhu Basin, followed by increased wind speed and relative humidity, which together contribute 83.9% to the ET trend. This paper provides a theoretical basis for the study of ET changes and the mechanism of ET and provides a decision-making reference for water resource management in the Dajiuhu Basin and even the South-to-North Water Transfer Project.

Automatic Stomatal Segmentation Based on Delaunay-Rayleigh Frequency Distance.

The CO2 and water vapor exchange between leaf and atmosphere are relevant for plant physiology. This process is done through the stomata. These structures are fundamental in the study of plants since their properties are linked to the evolutionary process of the plant, as well as its environmental and phytohormonal conditions. Stomatal detection is a complex task due to the noise and morphology of the microscopic images. Although in recent years segmentation algorithms have been developed that automate this process, they all use techniques that explore chromatic characteristics. This research explores a unique feature in plants, which corresponds to the stomatal spatial distribution within the leaf structure. Unlike segmentation techniques based on deep learning tools, we emphasize the search for an optimal threshold level, so that a high percentage of stomata can be detected, independent of the size and shape of the stomata. This last feature has not been reported in the literature, except for those results of geometric structure formation in the salt formation and other biological formations.

A long-term (2005–2016) dataset of hourly integrated land–atmosphere interaction observations on the Tibetan Plateau

Abstract. The Tibetan Plateau (TP) plays a critical role in influencing regional and global climate, via both thermal and dynamical mechanisms. Meanwhile, as the largest high-elevation part of the cryosphere outside the polar regions, with vast areas of mountain glaciers, permafrost and seasonally frozen ground, the TP is characterized as an area sensitive to global climate change. However, meteorological stations are biased and sparsely distributed over the TP, owing to the harsh environmental conditions, high elevations, complex topography and heterogeneous surfaces. Moreover, due to the weak representation of the stations, atmospheric conditions and the local land–atmosphere coupled system over the TP as well as its effects on surrounding regions are poorly quantified. This paper presents a long-term (2005–2016) in situ observational dataset of hourly land–atmosphere interaction observations from an integrated high-elevation and cold-region observation network, composed of six field stations on the TP. These in situ observations contain both meteorological and micrometeorological measurements including gradient meteorology, surface radiation, eddy covariance (EC), soil temperature and soil water content profiles. Meteorological data were monitored by automatic weather stations (AWSs) or planetary boundary layer (PBL) observation systems. Multilayer soil temperature and moisture were recorded to capture vertical hydrothermal variations and the soil freeze–thaw process. In addition, an EC system consisting of an ultrasonic anemometer and an infrared gas analyzer was installed at each station to capture the high-frequency vertical exchanges of energy, momentum, water vapor and carbon dioxide within the atmospheric boundary layer. The release of these continuous and long-term datasets with hourly resolution represents a leap forward in scientific data sharing across the TP, and it has been partially used in the past to assist in understanding key land surface processes. This dataset is described here comprehensively for facilitating a broader multidisciplinary community by enabling the evaluation and development of existing or new remote sensing algorithms as well as geophysical models for climate research and forecasting. The whole datasets are freely available at the Science Data Bank (https://doi.org/10.11922/sciencedb.00103; Ma et al., 2020) and additionally at the National Tibetan Plateau Data Center (https://doi.org/10.11888/Meteoro.tpdc.270910, Ma 2020).

A long-term (2005–2019) eddy covariance data set of CO<sub>2</sub> and H<sub>2</sub>O fluxes from the Tibetan alpine steppe

Abstract. The Tibetan alpine steppe ecosystem covers an area of roughly 800 000 km2 and contains up to 3.3 % soil organic carbon in the uppermost 30 cm, summing up to 1.93 Pg C for the Tibet Autonomous Region only (472 037 km2). With temperatures rising 2 to 3 times faster than the global average, these carbon stocks are at risk of loss due to enhanced soil respiration. The remote location and the harsh environmental conditions on the Tibetan Plateau (TP) make it challenging to derive accurate data on the ecosystem–atmosphere exchange of carbon dioxide (CO2) and water vapor (H2O). Here, we provide the first multiyear data set of CO2 and H2O fluxes from the central Tibetan alpine steppe ecosystem, measured in situ using the eddy covariance technique. The calculated fluxes were rigorously quality checked and carefully corrected for a drift in concentration measurements. The gas analyzer self-heating effect during cold conditions was evaluated using the standard correction procedure and newly revised formulations (Burba et al., 2008; Frank and Massman, 2020). A wind field analysis was conducted to identify influences of adjacent buildings on the turbulence regime and to exclude the disturbed fluxes from subsequent computations. The presented CO2 fluxes were additionally gap filled using a standardized approach. The very low net carbon uptake across the 15-year data set highlights the special vulnerability of the Tibetan alpine steppe ecosystem to become a source of CO2 due to global warming. The data are freely available at https://doi.org/10.5281/zenodo.3733202 (Nieberding et al., 2020a) and https://doi.org/10.11888/Meteoro.tpdc.270333 (Nieberding et al., 2020b) and may help us to better understand the role of the Tibetan alpine steppe in the global carbon–climate feedback.

Identification of Tropopause Height with Atmospheric Refractivity

AbstractInvestigation of the structure and variation of the tropopause is crucial for the development of an in-depth understanding of water-vapor exchange processes, the concentrations and nature of chemicals within the tropopause, and their role in climate change and the ecosphere. At present, the common methods used for the estimation of tropopause height are limited by their reliance on area, an overdependence on atmospheric temperature, and the use of many different data types, and thus lack strong generality. Therefore, this study used atmospheric refractivity data from multiple sources to determine the tropopause height. An objective covariance transform method was applied to identify transitions in a refractivity profile. The refractivity tropopause height was compared with the bending angle tropopause (BAT)/cold point tropopause (CPT)/lapse rate tropopause (LRT) height derived from radio occultation (RO) and radiosonde data and revealed a good agreement. An initial analysis of tropopause structure and seasonal changes derived from refractivity method afforded results that were consistent with existing research results, which proved the validity of the method. The refractivity method was also used to analyze various types of data downloaded from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Data Analysis and Archive Center (CDAAC), and show that this method is suitable for the analysis of RO data, radiosonde, reanalysis and analysis/forecast data. A series of experiments were used to verify the generality and utility of the refractivity covariance transform method to determine tropopause height, which will be useful for the analysis of long-term variations in tropopause height.

Variability of Diurnal Sea Surface Temperature during Short Term and High SST Event in the Western Equatorial Pacific as Revealed by Satellite Data

Near-surface diurnal warming is an important process in the climate system, driving exchanges of water vapor and heat between the ocean and the atmosphere. The occurrence of the hot event (HE) is associated with the high diurnal sea surface temperature amplitude (δSST), which is defined as the difference between daily maximum and minimum sea surface temperature (SST). However, previous studies still show some inconsistency for the area of HE occurrence and high δSST. The present study produces global δSST data based on the SST, sea surface wind data derived from microwave radiometers, and solar radiation data obtained from visible/infrared radiometers. The value of δSSTs are estimated and validated over tropical oceans and then used for investigating HE in the western equatorial Pacific. A three-way error analysis was conducted using in situ mooring buoy arrays and geostationary SST measurements by the Himawari-8 and Geostationary Operational Environmental Satellite (GOES). The standard deviation error of daily and 10-day validation is around 0.3 °C and 0.14–0.19 °C, respectively. Our case study in the western Pacific (from 110°E to 150°W) shows that the area of HE occurrence coincided well with the area of high δSST. Climatological analysis shows that the collocated area between high occurrence rate of HE and high δSST, which coincides with the western Pacific warm pool region in all seasons. Thus, this study provides more persuasive evidence of the relation between HE occurrence and high δSST.

Turbulent Flow in Plant Canopies: Historical Perspective and Overview

Studying the microclimate of plant canopies has long motivated scientists in various research fields such as agronomy, ecology or silviculture, and almost a century has passed since the first measurements of wind speed in a forest stand were published in the scientific literature. The behaviour of wind in canopies is an essential component of their microclimate, which largely conditions the rate of exchange of heat, water vapour, and other gases and particles of interest with the atmosphere. This review examines the evolution of our understanding of turbulent flow in plant canopies, focussing on the period that covers the last fifty years (1970–2020). We first describe how our knowledge and ideas have evolved since canopy flow became a topic of interest, and show how the 1970s was a pivotal decade in this field. Until then, canopy turbulence was considered to result from the superposition of standard surface-layer turbulence and small-scale turbulence generated in the wakes of plant elements. However, it was progressively found that the flow in plant canopies is dominated by large coherent structures, giving canopy turbulence unique characteristics. We thus describe the particular nature and structure of canopy flows, based on experimental observations accumulated over several decades. We show how canopy turbulence was reconsidered on the basis of a now widely-accepted analogy with a plane mixing layer, and we examine the significance of a key parameter, the “canopy-shear length scale”. Investigating the effects of canopy density and atmospheric stability, we then discuss the extent of the mixing-layer analogy and the limits of our current understanding of canopy turbulence. Finally, we review the modelling tools used in this field and show how their development has evolved to date to meet our needs. In conclusion, we present a historical summary of the evolution of this research field and suggest future directions.

The biophysical climate mitigation potential of boreal peatlands during the growing season

Peatlands and forests cover large areas of the boreal biome and are critical for global climate regulation. They also regulate regional climate through heat and water vapour exchange with the atmosphere. Understanding how land-atmosphere interactions in peatlands differ from forests may therefore be crucial for modelling boreal climate system dynamics and for assessing climate benefits of peatland conservation and restoration. To assess the biophysical impacts of peatlands and forests on peak growing season air temperature and humidity, we analysed surface energy fluxes and albedo from 35 peatlands and 37 evergreen needleleaf forests—the dominant boreal forest type—and simulated air temperature and vapour pressure deficit (VPD) over hypothetical homogeneous peatland and forest landscapes. We ran an evapotranspiration model using land surface parameters derived from energy flux observations and coupled an analytical solution for the surface energy balance to an atmospheric boundary layer (ABL) model. We found that peatlands, compared to forests, are characterized by higher growing season albedo, lower aerodynamic conductance, and higher surface conductance for an equivalent VPD. This combination of peatland surface properties results in a ∼20% decrease in afternoon ABL height, a cooling (from 1.7 to 2.5 °C) in afternoon air temperatures, and a decrease in afternoon VPD (from 0.4 to 0.7 kPa) for peatland landscapes compared to forest landscapes. These biophysical climate impacts of peatlands are most pronounced at lower latitudes (∼45°N) and decrease toward the northern limit of the boreal biome (∼70°N). Thus, boreal peatlands have the potential to mitigate the effect of regional climate warming during the growing season. The biophysical climate mitigation potential of peatlands needs to be accounted for when projecting the future climate of the boreal biome, when assessing the climate benefits of conserving pristine boreal peatlands, and when restoring peatlands that have experienced peatland drainage and mining.