Surface-air Temperature Difference Research Articles

Contributions of Soil Moisture and Vegetation on Surface-Air Temperature Difference during the Rapid Warming Period

The difference (DIF) between land surface temperature (Ts) and near surface air temperature (Ta) is the key indicator of the energy budget of the land surface, which has a more complex process than the individual Ts or Ta. However, the spatiotemporal variations and influencing factors of DIF remain incomplete. The contribution of vegetation and soil moisture (SM) as key driving factors to DIF is not yet clear. Here, we analyzed the spatiotemporal variation patterns of DIF in China from 2011 to 2023 using in situ Ts and Ta data. A convergent cross-mapping method was employed to explore the causal relationship between SM, NDVI and DIF, and subsequently calculated the contribution of NDVI and SM variations to DIF under different climatic backgrounds. The results indicate that during the study period, DIF values were all above 0 °C and showed a significant increasing trend with a national mean slope of 0.02 °C/a. In general, vegetation and SM have a driving effect on DIF, with vegetation contributing more to DIF (0.11) than SM (0.08) under different surface properties. The background values of SM and temperature have a significant effect on the spatial and temporal distribution of DIF, as well as the correlation of vegetation and soil moisture to DIF. The study outcomes contribute to a better understanding of the coupling relationship between the land surface and atmosphere, which are also crucial for addressing climate change and ecological environmental management.

Spatio-Temporal Patterns of Warm-Season Ground Surface Temperature—Surface Air Temperature Difference over China Mainland

Examining large-scale characteristics of the difference between ground surface temperature (GST) and surface air temperature (SAT) and its long-term trend will help understand land surface energy exchange and the effect of land-atmosphere interaction on climate change and variability. Based on a homogenized monthly dataset of GST and SAT from 1961 to 2018, this study analyzes the spatial distribution and long-term trend of the difference between ground surface temperature and surface air temperature (GST–SAT) in the warm season (April to October) over China mainland. The results show that the warm-season mean GST–SAT in the Qinghai-Tibet Plateau and the northwestern deserts have the largest GST–SAT. On average, the GST–SAT in China is the greatest in summer, with the maximum monthly value occurring in July. During 1961–2018, the warm-season mean GST–SAT undergoes a significant increasing trend (0.04 °C/10yr, p < 0.01), with the largest increase seen in mid-late spring (April and May), and the smallest increase in August. Spatially, the GST–SAT increases significantly in the northern region, decreases slightly in the southern region, and remains unchanged in the Qinghai-Tibet Plateau. The warm-season mean GST–SAT is significantly positively correlated with altitude and sunshine duration (R = 0.50, 0.40; p < 0.05), and significantly negatively correlated with relative humidity and precipitation (R = 0.48, −0.42; p < 0.05), in the country on a whole in the analysis period.

Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations

Abstract. Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg−1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface. Based on observations and results of a mixed-layer model it is shown that the maxima of turbulent fluxes are, however, smaller than the jump of the net terrestrial radiation flux across the upper part of a cloud due to the (i) shallowness of the mixed layer and (ii) the presence of a downward entrainment heat flux. The mixed-layer model also shows that the buoyancy production of TKE is substantially smaller in stratocumulus over the Arctic sea ice compared to subtropics due to a smaller surface moisture flux and smaller decrease in specific humidity (or even humidity inversions) right above the cloud top. In a case of strong wind, wind shear shapes the ABL turbulent structure, especially over rough sea ice, despite the presence of a strong cloud-top cooling. In the presence of mid-level clouds, cloud-top radiative cooling and thus also TKE in the lowermost cloud layer are strongly reduced, and the ABL turbulent structure becomes governed by stability, i.e., by the surface–air temperature difference and wind speed. A comparison of slightly unstable and weakly stable cases shows a strong reduction of TKE due to increased stability even though the absolute value of wind speed was similar. In summary, the presented study documents vertical profiles of the ABL turbulence with a high resolution in a wide range of conditions. It can serve as a basis for turbulence closure evaluation and process studies in Arctic clouds.

Regional variability of the mean and extreme characteristics of the present meteorological regime of the eastern part of the Baltic Sea catchment

The features of the regional heterogeneity of the modern meteorological regime are assessed, as a refinement of existing estimates of large-scale climate changes – for the eastern part of the Baltic Sea catchment on the example of the Leningrad Region and nearby territories of neighbouring subjects of the Russian Federation (northern part of the region), as well as the Zapadnaya Dvina catchment within the Republic of Belarus (southern part of the region). Significant differences in surface air temperature and snow water equivalent (hereinafter referred to as snow storage) were revealed for the winter period, with similar precipitation, more intense interannual and long-term changes in the southern part of the region. The common feature for the region is the presence of a significant correlation of the long-term January – March atmospheric circulation indices variability only with the variability of surface air temperature, as well as minor differences in the number of anomalous years with similar anomaly amplitudes. Surface air temperature has the greatest contribution to the long-term variability of snow storage everywhere, which is most pronounced in the south of the region. The turning points in the long-term variability of surface air temperature coincide in the north and south of the region, the turning points in the course of total precipitation do not coincide. The number of anomalies (exceeding the standard deviation) in the long-term series of characteristics in the northern and southern parts of the region differ little (9–12 cases in the positive and negative ranges of values) in the absence of coincidences and the similarity of the amplitude of the anomalies. The range of values of extreme threshold values (extreme percentiles) of surface air temperature in the north of the region is lower than the range of values in the south, the variability of small percentiles exceeds the variability of large ones; the rate of long-term increase in average temperatures is accompanied by a significant increase in small percentiles in the north and large percentiles in the south of the region. The values of extreme threshold values of precipitation and their standard deviation vary little across the territory; positive trends in maximums and negative trends in minimum thresholds are small, consistent with a slight increase in mean total precipitation. According to the spatial distribution of average values of snow storage, the values of their extremely small and large threshold values in the north of the region are higher than in the south; in the north of the region, the values of percentiles in the interval 1985–2002 stand out as the lowest.

Impacts of greenhouse gases and anthropogenic aerosols changes on surface air temperature in East Asia under different post-pandemic period emission scenarios

In order to know how surface air temperature (SAT) changes in East Asia under different emission scenarios after the COVID-19 outbreak, we investigated the impacts of greenhouse gases (GHGs) and anthropogenic aerosols changes on SAT in East Asia by using the aerosol-climate coupled model BCC-AGCM 2.0_CUACE/Aero, combining with the post-pandemic emission scenarios proposed by Covid multi-Earth system model intercomparison project (CovidMIP scenarios for short, including fossil-fueled recovery, moderate green stimulus, strong green stimulus, hereinafter as FFF, MGG, SGG, respectively). We assessed the impacts of changes in GHGs and anthropogenic aerosols together and separately on SAT in East Asia and its typical subregions during 2020–2050. The results show that by mid-21st-century, SAT in East Asia will increase by 0.81 ± 0.083 °C under Baseline (same as SSP2-4.5 scenario), i.e., SAT difference between 2045–2050 and 2020–2025), and there will be more intense warming in all the three scenarios in East Asia, in which the largest SAT difference (SAT-d) compared to Baseline is 0.33 ± 0.11 °C under SGG and the smallest SAT-d is 0.07 ± 0.14 °C under FFF. To further explore the mechanism of these SAT-d, we analyzed the trend of surface longwave and shortwave net radiation flux driven by GHGs and anthropogenic aerosols there. It is found that in early period (2020–2035), the role of aerosol changes is bigger than that of GHG changes in dominating SAT-d, particularly sulfate, whose reduction will become the main contributor to SAT-d by affecting the net solar flux at surface. In later period (2036–2050), because of GHGs’ longer atmospheric lifetime than aerosols, the role of decreasing GHGs concentrations will determine the drop in SAT-d through affecting the net longwave flux at surface.

Surface air temperature differences of intra- and inter-local climate zones across diverse timescales and climates

The local climate zone (LCZ) scheme has been widely used in urban heat island studies in the past decade. However, the general principle of LCZ surface air temperature (SAT) differences and their climatic differences on different timescales remain largely unclear. Here we investigated both the inter- and intra-LCZ differences at four timescales across nine Chinese megacities characterized with significantly different climates, based on three-year hourly SAT measurements collected at 898 stations from 2017 to 2019. The results showed that (1) at the annual mean scale, the ΔTinter (i.e., SAT differences between various LCZ types and LCZ D): and ΔTintra (i.e., standard deviation of SAT measurements belonging to the same LCZ) were larger during the night than the day for most LCZ types; and the ΔTinter and ΔTintra for warm temperate cities were usually greater than those of subtropical cities. (2) At the seasonal scale, the ΔTinter and ΔTintra of most LCZs often peaked at winter night. The largest seasonal variation of ΔTinter was observed in mid-temperate cities. (3) At the daily scale, the day-to-day difference of ΔTinter was smaller during the day than the night and mainly ranged from −0.5 °C to 0.5 °C. By contrast, the day-to-day difference of ΔTintra was relatively large both during the day and night. (4) At the hourly scale, both the ΔTinter and ΔTintra variations throughout a diurnal cycle followed a characteristic U-shape for most LCZs. We consider these findings are helpful for better understanding the inter- and intra-LCZ thermal differences across various timescales and climates.

The global spatiotemporal heterogeneity of land surface-air temperature difference and its influencing factors

The water and energy in the land surface and lower atmosphere have a strong coupling relationship. Apart from the land surface temperature (Ts) and air temperature (Ta), the land surface-air temperature difference (Ts-Ta) is also an essential parameter reflecting the coupling process. However, the global spatiotemporal variations and influencing factors of Ts-Ta remain not well explored. Here, ERA5-land reanalysis data, GIMMS NDVI data, and elevation data were used to analyze the global spatiotemporal heterogeneity and influencing factors of Ts-Ta. It was found that annual mean Ts-Ta exhibited a decreasing trend from the equator to polar areas. And the annual Ts-Ta increased at 0.009 °C/10a from 1981 to 2020. The variations of global net radiation mainly determined the spatiotemporal heterogeneity of global Ts-Ta. The different properties of the land surface and near-surface atmosphere were the main factors affecting the Ts-Ta, including soil moisture, vegetation, snow cover, and the water vapor content in the atmosphere. In addition, Ts and Ta also affected each other. These findings are conducive to a better understanding of the land-atmosphere coupling, and it is of great significance to take better measures to adapt the global climate change.

Downwelling longwave radiation and sensible heat flux observations are critical for surface temperature and emissivity estimation from flux tower data

Land surface temperature (LST) is a preeminent state variable that controls the energy and water exchange between the Earth’s surface and the atmosphere. At the landscape-scale, LST is derived from thermal infrared radiance measured using space-borne radiometers. In contrast, plot-scale LST estimation at flux tower sites is commonly based on the inversion of upwelling longwave radiation captured by tower-mounted radiometers, whereas the role of the downwelling longwave radiation component is often ignored. We found that neglecting the reflected downwelling longwave radiation leads not only to substantial bias in plot-scale LST estimation, but also have important implications for the estimation of surface emissivity on which LST is co-dependent. The present study proposes a novel method for simultaneous estimation of LST and emissivity at the plot-scale and addresses in detail the consequences of omitting down-welling longwave radiation as frequently done in the literature. Our analysis uses ten eddy covariance sites with different land cover types and found that the LST values obtained using both upwelling and downwelling longwave radiation components are 0.5–1.5 K lower than estimates using only upwelling longwave radiation. Furthermore, the proposed method helps identify inconsistencies between plot-scale radiometric and aerodynamic measurements, likely due to footprint mismatch between measurement approaches. We also found that such inconsistencies can be removed by slight corrections to the upwelling longwave component and subsequent energy balance closure, resulting in realistic estimates of surface emissivity and consistent relationships between energy fluxes and surface-air temperature differences. The correspondence between plot-scale LST and landscape-scale LST depends on site-specific characteristics, such as canopy density, sensor locations and viewing angles. Here we also quantify the uncertainty in plot-scale LST estimates due to uncertainty in tower-based measurements using the different methods. The results of this work have significant implications for the combined use of aerodynamic and radiometric measurements to understand the interactions and feedbacks between LST and surface-atmosphere exchange processes.

Constraining Arctic Climate Projections of Wintertime Warming With Surface Turbulent Flux Observations and Representation of Surface-Atmosphere Coupling

The drivers of rapid Arctic climate change—record sea ice loss, warming SSTs, and a lengthening of the sea ice melt season—compel us to understand how this complex system operates and use this knowledge to enhance Arctic predictability. Changing energy flows sparked by sea ice decline, spotlight atmosphere-surface coupling processes as central to Arctic system function and its climate change response. Despite this, the representation of surface turbulent flux parameterizations in models has not kept pace with our understanding. The large uncertainty in Arctic climate change projections, the central role of atmosphere-surface coupling, and the large discrepancy in model representation of surface turbulent fluxes indicates that these processes may serve as useful observational constraints on projected Arctic climate change. This possibility requires an evaluation of surface turbulent fluxes and their sensitivity to controlling factors (surface-air temperature and moisture differences, sea ice, and winds) within contemporary climate models (here Coupled Model Intercomparison Project 6). The influence of individual controlling factors and their interactions is diagnosed using a multi-linear regression approach. This evaluation is done for four sea ice loss regimes, determined from observational sea ice loss trends, to control for the confounding effects of natural variability between models and observations. The comparisons between satellite- and model-derived surface turbulent fluxes illustrate that while models capture the general sensitivity of surface turbulent fluxes to declining sea ice and to surface-air gradients of temperature and moisture, substantial mean state biases exist. Specifically, the central Arctic is too weak of a heat sink to the winter atmosphere compared to observations, with implications to the simulated atmospheric circulation variability and thermodynamic profiles. Models were found to be about 50% more efficient at turning an air-sea temperature gradient anomaly into a sensible heat flux anomaly relative to observations. Further, the influence of sea ice concentration on the sensible heat flux is underestimated in models compared to observations. The opposite is found for the latent heat flux variability in models; where the latent heat flux is too sensitive to a sea ice concentration anomaly. Lastly, the results suggest that present-day trends in sea ice retreat regions may serve as suitable observational constraints of projected Arctic warming.

Influence patterns of soil moisture change on surface-air temperature difference under different climatic background

The surface-air temperature difference (Ts-Ta) is the main contributor to the sensible heat flux, and also an important indicator for land degradation. However, as the main influencing factor, the effect of soil moisture (SM) on Ts-Ta at the global scale has not been well articulated. Here, based on the ERA5-land reanalysis data from 1981 to 2019, the impacts of SM on Ts-Ta were studied. It was found that Ts-Ta over 54% of the global land increased, and SM across 70.7% of the world land decreased. In the increased SM areas, the increased soil evaporation weakened the increasing trend of Ts resulting in smaller Ts-Ta. In the decreased SM areas, the latent heat flux increased with soil evaporation and Ts-Ta decreased when SM was relatively high, and the larger sensible heat flux due to decreased soil evaporation aggravated Ts-Ta when SM was relatively low. The effect of SM on Ts-Ta presented nonlinear relationship due to the different background value of SM and temperature. The variation of SM at low SM or low temperature areas had an amplification effect on Ts-Ta. These findings will provide new insights into the different regional characteristics of global changing climate and the improvement of land degradation assessment indicators.

Systematic assessment of the diabatic processes that modify low-level potential vorticity in extratropical cyclones

Abstract. Diabatic processes significantly affect the development and structure of extratropical cyclones. Previous studies quantified the dynamical relevance of selected diabatic processes by studying their influence on potential vorticity (PV) in individual cyclones. However, a more general assessment of the relevance of all PV-modifying processes in a larger ensemble of cyclones is currently missing. Based on a series of twelve 35 d model simulations using the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts, this study systematically quantifies the diabatic modification of positive and negative low-level PV anomalies along the cold front, warm front, and in the center of 288 rapidly intensifying extratropical cyclones. Diabatic PV modification is assessed by accumulating PV tendencies associated with each parametrized process along 15 h backward trajectories. The primary processes that modify PV typically remain temporally consistent during cyclone intensification. However, a pronounced case-to-case variability is found when comparing the most important processes across individual cyclones. Along the cold front, PV is primarily generated by condensation in half of the investigated cyclones in the cold season (October to March). For most of the remaining cyclones, convection or long-wave radiative cooling is the most important process. Similar results are found in the warm season (April to September); however, the fraction of cyclones with PV generation by convection as the most important process is reduced. Negative PV west of the cold front is primarily produced by turbulent mixing of momentum, long-wave radiative heating, or turbulent mixing of temperature. The positive PV anomaly at the warm front is most often primarily generated by condensation in the cold season and by turbulent mixing of momentum in the warm season. Convection is the most important process only in a few cyclones. Negative PV along the warm front is primarily produced by long-wave radiative heating, turbulent mixing of temperature, or melting of snow in the cold season. Turbulent mixing of temperature becomes the primary process in the warm season, followed by melting of snow and turbulent mixing of momentum. The positive PV anomaly in the cyclone center is primarily produced by condensation in most cyclones, with only few cases primarily associated with turbulent mixing or convection. A composite analysis further reveals that cyclones primarily associated with PV generation by convection exhibit a negative air–surface temperature difference in the warm sector, which promotes a heat flux directed into the atmosphere. These cyclones generally occur over warm ocean currents in the cold season. On the other hand, cyclones that occur in a significantly colder environment are often associated with a positive air–surface temperature difference in the warm sector, leading to PV generation by long-wave radiative cooling. Finally, long-wave radiative heating due to a negative air–surface temperature difference in the cold sector produces negative PV along the cold and warm front, in particular in the cold season.

Comprehensive evaluation of surface air temperature reanalysis over China against urbanization-bias-adjusted observations

The reanalysis datasets (CRA40) have now been developed in China Meteorological Administration. We aim to compare the differences in surface air temperature (SAT) between observational that has been adjusted for urbanization bias and reanalysis data (NCEPV1, NCEPV2, ERA5, CFSR, MERRA, JRA55, 20CRV3 and CRA40) over mainland China during 1961–2015. The main results are presented as follows. The correlation and standard deviation between the reanalysis data and observations exhibit highly consistent interannual variability and dispersion, with the interannual SAT variability in JRA55 being closest to the observations for 1961–2015 and that of ERA5 for 1979–2015; the dispersions of 20CRV3 is most consistent with the observations for 1961–2015 and that of NCEPV1 for 1979–2015. Although annual mean SAT of the reanalysis is generally 0–2.0 °C lower than the observations, the bias in the SAT climatology of 20CRV3 is the least for 1961–2015 in all reanalysis datasets and that of CRA40 is the least for 1979–2015. The trends of NCEPV1 is closer to the observations than other reanalysis for 1961–2015 and that of 20CRV3 for 1979–2015. The biases in terms of interannual variability, dispersion, climatology, and linear trend are increase with altitude. Overall, in terms of the similarity of multiple measures to the urbanization bias-adjusted observations, JRA55 and CRA40 show the best performances for the periods 1961–2015 and 1979–2015 respectively in reproducing various aspects of climatological and climate change features in mainland China.

Thermal analysis of marine structural steel EH36 subject to non-spreading cryogenic spills. Part II: finite element analysis

ABSTRACT In Part I of this paper, six liquid nitrogen (LN2) pool boiling tests were carried out to locally cool down six different EH36 steel plates to cryogenic temperatures. Leindenfrost and Critical Heat Flux (CHF) points of LN2 boiling curve were estimated. These estimations together with the recorded temperature histories are used in this part for the development of a heat flux curve for EH36 steel-LN2 pool boiling through Finite Element Thermal Analysis (FETA) of the problem investigated experimentally in Part I. Thermal conductivity and heat capacity of EH36 are defined by two unprecedented temperature functions based upon experimental studies in cryogenic temperatures. A user subroutine written in FORTRAN defines the air convection coefficient as a function of surface-air temperature difference, which changes with time and location. Results of this study contribute to the Accidental Limit State (ALS) design of marine and offshore structures for liquified gas spill scenarios.

On the u☆-U Relationship in the Stable Atmospheric Boundary Layer over Arctic Sea Ice

A relationship between the friction velocity u☆ and mean wind speed U in a stable atmospheric boundary layer (ABL) over Arctic sea ice was considered. To that aim, the observations collected during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment were used. The observations showed the so-called “hockey-stick” shape of the u☆−U relationship, which consists of a slow increase of u☆ with increasing wind speed for U<Utr and a more rapid almost linear increase of u☆ for U>Utr, where Utr is the wind speed of transition between the two regimes. Such a relationship is most pronounced at the highest observational levels, namely at 9 and 14 m, and is also sharper when the air-surface temperature difference exceeds its average values for stable conditions. It is shown that the Monin–Obukhov similarity theory (MOST) reproduces the observed u☆−U relationship rather well. This suggests that at least for the SHEBA dataset, there is no contradiction between MOST and the “hockey-stick” shape of the u☆−U relationship. However, the SHEBA data, as well as the single-column simulations show that for cases with strong stability, u☆ significantly decreases with height due to the shallowness of the ABL. It was shown that when u☆ was assumed independent of height, the value of the normalized drag coefficient, i.e., of the so-called stability correction function for momentum, calculated using observations at a certain level, can be significantly underestimated. To overcome this, the decrease of u☆ with height was taken into account in the framework of MOST using local scaling instead of the scaling with surface fluxes. Using such an extended MOST brought the estimates of the normalized drag coefficient closer to the Businger–Dyer relation.

Radiatively driven NH3 release from agricultural field during wintertime slack season

A chemical ionization mass spectrometer (CIMS) was deployed to measure ambient ammonia (NH3) with a time-resolution of ~5 min. Unexpectedly high levels of NH3, peaking at 15.5 ppbv and averaging at 2.1 ± 1.9 ppbv, were observed near an agricultural field in Nanjing, China, during wintertime, a slack season for farming. Re-partitioning from aerosols can only explain a small portion of this NH3 due to the considerably low air temperature, whereas the major source was emission from soil, as indicated by significant positive correlations of the NH3 concentration with the ground surface temperature, solar radiation intensity, and particularly the surface-air temperature difference. The dissolved/adsorbed NH3 and NH4NO3 in soil likely originated from conversion of nitrogen-containing organic matter by microorganisms through ammonification and nitrification. The upward transfer of NH3 released from the soil was facilitated by convective mixing driven by solar radiation, which produced the required high soil-air temperature gradient. This winter release of NH3 from the soil is not captured by commonly used long-term, passive-sampler based methods, giving rise to an incorrect assumption that during winter, a slack season for farming, soil emits no NH3 and so this source has remained unresolved by typical emission inventory models. Our results indicate that contrary to the common assumption, agricultural background emissions are a non-negligible source of NH3 even in wintertime, which may exert a significant impact on regional air pollution formations. Hence, a refined NH3 emission inventory is critically needed to account for the haze events in wintertime northern China.

Vertical structure of the cool island in a large urban park

Greening of cities is one of the mitigation strategies for urban hot environments in summer, because green parks in cities have a lower temperature than the surrounding built environment in summer, called cool islands phenomena. This study took the micro-meteorological measurements in an urban park (Shinjyuku Gyoen, Tokyo Japan) and analyzes the vertical structure of a cool island at night. Vertical profiles of air temperature were measured simultaneously in the park and the nearby town area by the tethered balloons. In the park area, a cold air pool was present in the first 50 m from the ground, as opposed to neutral stratification in the town area. The cold air pool grew vertically at night. Meanwhile, surface air temperature difference between the park and the town increased up to 2 °C.The ground-based instruments recorded the park breeze which is cold airflow to town area for 21 nights over two summer months. In most cases, the park breeze had at least 16 m depth and traveled for more than 122 m from the park edge. It is hypothesized that the park breeze is the gravity current. The observations made in our study can aid evaluating worth of urban green.

Diurnal to seasonal ventilation in Brazilian caves

Limited studies, particularly in the low-latitudes, exist that monitor cave ventilation, an important process dictating cave geochemical properties. To investigate cave ventilation, we present multi-year monitoring results, including observations of cave-air CO2, cave-air temperature, and cave-air relative humidity, from three Brazilian caves: Paraíso (4.07°S, 55.45°W), Tamboril (16.19°S, 46.59°W), and Jaraguá (21.08°S, 56.58°W). Cave ventilation regimes on numerous temporal scales are established. Highest concentrations of cave-air CO2 are recorded during austral summers from a multi-year monitoring effort at Jaraguá Cave. Conversely, lowest cave-air CO2 concentration are recorded during austral winters. The variability in cave-air CO2 is attributed to seasonally varying temperature contrasts between surface- and cave-air, which results in buoyancy-driven cave-air CO2 exchange with surface-air CO2. Speleothem growth as calcite precipitation is expected to be biased towards the austral winter season when cave-air CO2 is lowest. This understanding of cave ventilation affects the geochemical interpretation of speleothems as recorders of past climate change from caves in the low-latitudes. Next, continuous CO2 measurements near the entrance of Tamboril Cave suggest diurnal ventilation attributed to diurnal cave- and surface-air temperature differences. CO2 monitored from the deeper part of Paraíso and Jaraguá Cave do not exhibit this diurnal cycle. Further, virtual temperature calculations suggest that buoyancy contrasts between cave- and surface-air drive daily to weekly cave ventilation in all three caves. The established cave ventilation regimes provide an additional framework to interpret speleothem based terrestrial environmental change spanning the low- and mid-latitudes. By investigating spatially disparate caves across Brazil, we find buoyancy driven cave ventilation to be a unifying mechanism. Our results suggest that exchange between cave- and surface-air occurs seasonally in caves across the low- and mid-latitudes. These results suggest buoyancy driven ventilation is important not only at mid-latitudes but also in the tropics where speleothems are typically suited to investigate paleomonsoon variability. Therefore, prior knowledge of cave ventilation in caves that contain speleothems suitable for paleoclimate reconstructions is critical to robustly infer subsequent geochemical trends across different timescales (seasonal - centennial).

A climatology of surface–air temperature difference over the Tibetan Plateau: Results from multi‐source reanalyses

AbstractThe Tibetan Plateau (TP), known as earth's “Third Pole,” influences regional and even global weather and climate systems through its mechanical and thermal‐dynamical forcing. Near‐surface (2 m) air temperature (Ta) and surface (skin) temperature (Ts) are two crucial parameters of land–atmosphere interactions and climate change. Their difference (ΔT = Ts − Ta) determines the heating source over the TP that drives the Asian summer monsoon. This study focuses on climatology, inter‐annual variability, and long‐term trend of ΔT over the TP in the last four decades (1979–2018), based on four latest reanalysis datasets including ERA‐Interim, ERA5, MERRA2, and JRA55, along with observational data. We show that ΔT‐based different datasets display fairly different climatology in terms of seasonality, spatial distribution, and long‐term trend. ΔT exhibits a clear seasonality with negative value in winter and positive ones in summer despite different strengths and timings presented by the reanalyses. Along with global warming, all reanalyses except JRA55 exhibit obvious downwards trends of ΔT in a spatially non‐uniform way. The median ΔT among the four reanalyses features uniform decreases in all seasons, with the most distinct area on the northern TP, as well as the largest and least decreases in autumn and spring, respectively. Further analysis shows that the differences in ΔT are most likely associated with discrepancies in radiation forcing, snow cover, wind speed, and boundary layer height within the reanalyses. The present findings highlight the difficulty for the state‐of‐the‐art reanalyses to represent the climate change over the TP and point to possible factors behind the deficiencies identified.

Air cooling by tree transpiration: A case study of Olea europaea, Citrus sinensis and Pinus pinea in Mediterranean town

Transpiration and surface temperature of Olea, Citrus and Pinus trees in an urban garden in southern Italy were studied from May until August, the hottest period of the year in the investigated area. Transpiration was measured by sap flow heat dissipation method at 10-min scale, while the surface-air temperature difference was estimated by energy balance approach. The radial trend of sap flow density at tree scale was modelled as a function of the sapwood area, while the upscaling to garden level was performed using the quantile method integrated by trunk diameter classification. The results at hourly and daily time scales showed that Olea had the highest transpiration followed by Citrus and Pinus trees. The cooling effect resulted dependent on the species showing Citrus as the most efficient species among the investigated ones. The study might contribute to a better understanding of cooling effect due to tree transpiration under rainfed conditions.

Effect of Wind Speed and Leads on Clear-Sky Cooling over Arctic Sea Ice during Polar Night

Abstract A simple analytical model of the atmospheric boundary layer (ABL) coupled to sea ice is presented. It describes clear-sky cooling over sea ice during polar night in the presence of leads. The model solutions show that the sea ice concentration and wind speed have a strong impact on the thermal regime over sea ice. Leads cause both a warming of the ABL and an increase of stability over sea ice. The model describes a sharp ABL transition from a weakly stable coupled state to a strongly stable decoupled state when wind speed is decreasing. The threshold value of the transition wind speed is a function of sea ice concentration. The decoupled state is characterized by a large air–surface temperature difference over sea ice, which is further increased by leads. In the coupled regime, air and surface temperatures increase almost linearly with wind speed due to warming by leads and also slower cooling of the ABL. The cooling time scale shows a nonmonotonic dependency on wind speed, being lowest for the threshold value of wind speed and increasing for weak and strong winds. Theoretical solutions agree well with results of a more realistic single-column model and with observations performed at the three Russian “North Pole” drifting stations (NP-35, -37, and -39) and at the Surface Heat Budget of the Arctic Ocean ice camp. Both modeling results and observations show a strong implicit dependency of the net longwave radiative flux at the surface on wind speed.