Reply on RC2

Abstract. The surface energy balance is a key factor influencing the ground thermal regime. With ongoing climate change, it is crucial to understand the interactions of the individual heat fluxes at the surface and within the subsurface layers, as well as their relative impacts on the permafrost thermal regime. A unique set of high-altitude meteorological measurements was analysed to determine the energy balance at three mountain permafrost sites in the Swiss Alps (Murtèl–Corvatsch, Schilthorn and Stockhorn), where data have been collected since the late 1990s in the framework of the Swiss Permafrost Monitoring Network (PERMOS). All stations are equipped with sensors for four-component radiation, air temperature, humidity, and wind speed and direction, as well as ground temperatures and snow height. The three sites differ considerably in their surface and ground material composition, as well as their ground ice contents. The energy fluxes were calculated based on two decades of field measurements. While the determination of the radiation budget and the ground heat flux is comparatively straightforward (by the four-component radiation sensor and thermistor measurements within the boreholes), larger uncertainties exist for the determination of turbulent sensible and latent heat fluxes. Our results show that mean air temperature at Murtèl–Corvatsch (1997–2018, 2600 m a.s.l.) is −1.66 ∘C and has increased by about 0.8 ∘C during the measurement period. At the Schilthorn site (1999–2018, 2900 m a.s.l.) a mean air temperature of −2.60 ∘C with a mean increase of 1.0 ∘C was measured. The Stockhorn site (2003–2018, 3400 m a.s.l.) recorded lower air temperatures with a mean of −6.18 ∘C and an increase of 0.5 ∘C. Measured net radiation, as the most important energy input at the surface, shows substantial differences with mean values of 30.59 W m−2 for Murtèl–Corvatsch, 32.40 W m−2 for Schilthorn and 6.91 W m−2 for Stockhorn. The calculated turbulent fluxes show values of around 7 to 13 W m−2 using the Bowen ratio method and 3 to 15 W m−2 using the bulk method at all sites. Large differences are observed regarding the energy used for the melting of the snow cover: at Schilthorn a value of 8.46 W m−2, at Murtèl–Corvatsch 4.17 W m−2 and at Stockhorn 2.26 W m−2 are calculated, reflecting the differences in snow height at the three sites. In general, we found considerable differences in the energy fluxes at the different sites. These differences help to explain and interpret the causes of a warming atmosphere. We recognise a strong relation between the net radiation and the ground heat flux. Our results further demonstrate the importance of long-term monitoring to better understand the impacts of changes in the surface energy balance components on the permafrost thermal regime. The dataset presented can be used to improve permafrost modelling studies aiming at, for example, advancing knowledge about permafrost thaw processes. The data presented and described here are available for download at the following site: https://doi.org/10.13093/permos-meteo-2021-01 (Hoelzle et al., 2021).

<abstract> <b><sc>Abstract.</sc></b> Surface energy balance is critical to the understanding of crop evapotranspiration (ET) requirements and crop water stresses. The objective of this study was to evaluate the simulation of crop growth, water and surface energy balance components, canopy temperature, and water stress under fully irrigated and deficit-irrigated corn in eastern Colorado using a hybrid version of the Root Zone Water Quality Model (RZWQM) and the Simultaneous Heat and Water model (SHAW) (RZ-SHAW). The field experiment was conducted in 2010 under both full and deficit irrigation conditions with energy balance measured using the Bowen ratio method. The model simulated grain yield satisfactorily, with an error of less than 5%. Leaf area index, daily ET, soil water content, canopy temperature, and energy balance components, including net radiation (R_n), latent heat (LE), sensible heat (H), and ground heat flux (G), were simulated well, with coefficients of determination (R²) ≥ 0.64 and model efficiencies (ME) ≥ 0.57 for both full and deficit irrigation fields. The RZ-SHAW model accurately predicted the responses of crop growth and ET to water stress, and the simulated water stress was in good agreement with measured elevated canopy temperature with deficit irrigation after silking. However, the model performance was not acceptable in predicting plant height for water-stressed corn, and the simulated increase in canopy temperature under deficit irrigation was about 30% of the observed temperature increase. This study suggests that while the RZ-SHAW model can be used to evaluate the response of corn yield and water and energy balances to water stress, the simulation of the effects of water deficit on plant height and canopy temperature need further improvement.

The effect of a thin and light greening system with bamboo charcoal layer for water retention on heat fluxes, in particular latent heat flux (evapotranspiration rate), under no irrigation condition, on the rooftop of a building in Higashi-Hiroshima, West Japan, was investigated. In April 2019, lawn seeds (Zoysia tenuifolia) were sown which were germinated, reached a height of 70 mm by May when 100% of the vegetation area was covered. The air temperature and humidity at two different heights (0.3 m and 1.8 m) above greening soil surface, latent, and sensible heat fluxes were estimated. Bowen ratio was employed to collect the data on surface heat balance and soil water content during the summer season (June to September) in 2019 on the rooftop of a building in Higashi-Hiroshima, West Japan. The latent heat during daytime for a week without rainfall in each month was compared with the evapotranspiration rate. Owning to the vegetation development, the ground heat flux on greening soil surface decreased from -400 W/m2 to -200 W/m2 (flux from air to soil) during sunny daytime in July, and it was less than -100 W/m2 in August, although net radiation was maintained around 800 W/m2 over the season except in September. The monthly net radiation flux for an entire day (daytime and nighttime) ranged between 55 and 125 W/m2 (average: 95 W/m2) for the summer season of which 32-66% (average: 48%) was occupied by latent heat. Evapotranspiration from greening soil ranged between 1.24 and 1.82 mm/day, averaged at 1.51 mm/day throughout the season, which corresponded to about 26% of total rainfall over the season ( r 2 = 0.88 , p &lt; 0.01 ; S . E = 0.06 ) between the estimated and measured values. These observations suggested that the thin and bamboo coal light soil layer greening system, even without constant irrigation, could maintain the development of lawn grass and transformed more than half of net radiation to latent heat, i.e., evapotranspiration, insulating most ground heat in midsummer, which may be mostly due to bamboo charcoal sublayer.

&amp;lt;p&amp;gt;We compare Earth's surface energy balance components (monthly mean radiation and energy fluxes) from the ERA5-Land reanalysis with their measured equivalents from FLUXNET stations in the period 2001-2018. The comparison (FLUXNET station vs. the nearest grid point of ERA5-Land) is focused on Central European sites with an elevation under 1000 m above sea level during the summer half of the year (April - September) that roughly corresponds to the growing season in Central Europe. FLUXNET stations are further divided into four groups according to biomes (and surface types) that they represent: grass, crop, forest or wetland. We put special emphasis on the turbulent heat fluxes: sensible (H) and latent heat (LE) heat flux and their relative share in a form of evaporative fraction (EF, a ratio of latent heat flux, LE, to the sum of both turbulent fluxes, H + LE).&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;br&amp;gt;There's a perfect fit between monthly FLUXNET observations and ERA5-Land data in case of some radiation fluxes: incoming shortwave (SWin) or outgoing longwave (LWout) radiation. Incoming longwave radiation (LWin) is underestimated by ERA5-Land, while the score of reflected shortwave radiation (SWout) is mixed.&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;br&amp;gt;Latent heat flux (LE) is overestimated by the reanalysis, while sensible heat flux (H) is lower in the reanalysis than in FLUXNET observations. This leads to overestimation of the evaporative fraction (EF) by ERA5-Land in the growing season, with monthly values of EF being mostly between 0.7 - 0.9. On the other hand, FLUXNET's EF values range from 0.2 to 1.0. When focused on individual biomes, the best fit between ERA5-Land and FLUXNET is seen for grass. The shape of &amp;quot;half-annual&amp;quot; (Apr - Sep) cycle is also well captured for grass locations. In other major biomes (cropland or forest) we see not only disagreement in terms of absolute values of H and LE fluxes (or EF) but in the shape of &amp;quot;half-annual&amp;quot; cycle too. There's no significant decline of EF in summer for cropland locations in ERA5-Land. The magnitude of a steady EF increase in forest sites of FLUXNET is weaker in ERA5-Land.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;br&amp;gt;Acknowledgement: &amp;quot;SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797)&amp;amp;#8221;.&amp;lt;/p&amp;gt;

The Tibetan Plateau (TP) has become a focus of strong scientific interest due to its role in the global water cycle and its reaction to climate change. Regional flux estimates of sensible and latent heat are important variables for linking the energy and hydrological cycles at the TP’s surface. Within this framework, a 3-year dataset (2008–2010) of eddy covariance measured turbulent fluxes was compiled from four stations on the TP into a standardised workflow: corrections and quality tests were applied using an internationally comparable software package. Second, the energy balance closure (C EB) was determined and two different closure corrections applied. The four stations (Qomolangma, Linzhi, NamCo and Nagqu) represent different locations and typical land surface types on the TP (high altitude alpine steppe with sparse vegetation, a densely vegetated alpine meadow, and bare soil/gravel, respectively). We show that the C EB differs between each surface and undergoes seasonal changes. Typical differences in the turbulent energy fluxes occur between the stations at Qomolangma, Linzhi and NamCo, while Nagqu is quite similar to NamCo. Specific investigation of the pre-monsoon, the Tibetan Plateau summer monsoon, post-monsoon and winter periods within the annual cycle reinforces these findings. The energy flux of the four sites is clearly influenced by the Tibetan Plateau monsoon. In the pre-monsoon period, sensible heat flux is the major energy source delivering heat to the atmosphere, whereas latent heat flux is greater than sensible heat flux during the monsoon season. Other factors affecting surface energy flux are topography and location. Land cover type also affects surface energy flux. The energy balance residuum indicates a typically observed overall non-closure in winter, while closure (or ‘turbulent over-closure’) is achieved during the Tibetan Plateau summer monsoon at the Nagqu site. The latter seems to depend on ground heat flux, which is higher in the wet season, related not only to a larger radiation input but also to a thermal decoupling of dry soils. Heterogeneous landscape modelling using a MODIS product is introduced to explain energy non-closure.

Urbanization drives economic growth and improves living standards but also brings challenges, such as increased energy consumption, air pollution, and climate changes. A key issue is the Urban Heat Island effect, where urban areas experience higher temperatures, greater heat retention, and altered energy flux patterns compared to vegetation-rich rural regions.These changes not only exacerbate local heating but also disrupt natural ecosystems and water cycles.This study explores the urban heat island effect by analyzing differences in surface energy budgets between urban, grassland, and cropland sites in Nanjing, China. Data collected from May to September 2013 include variables such as air temperature, four-component radiation, and energy fluxes. The results reveal that urban areas for most of the time exhibit higher nighttime temperatures due to increased heat release by impervious surfaces. In contrast, rural sites, particularly in cropland, demonstrate effective cooling mechanisms through higher latent heat flux driven by evapotranspiration. Grassland temperatures fall between the other two site types, reflecting moderate cooling effects. In general, urban areas show a higher proportion of energy being converted into sensible and ground heat flux. In contrast, rural areas allocate more energy to latent heat flux. This study highlights the need to integrate vegetation into urban planning to reduce heat and improve thermal comfort.

Abstract. The rapid urbanization and population growth in Delhi have led to significant changes in the land and land cover, resulting in increased emissions and alterations in the urban energy balance. To understand these long-term trends and identify contributions factors, a study was conducted using Landsat series data and meteorological data from the ECMWF ERA-5 reanalysis. The study focused on estimating urban energy fluxes, including Net Radiation, Sensible heat flux, Latent heat flux and Ground heat flux with anthropogenic heat considered as the residual. The findings reveal a substantial increase in the anthropogenic heat flux, rising from 172 W/m2 in 1990 to 281 W/m2 in 2022. Seasonal variations were also observed, with the highest energy flux values occurring during the summer season, followed by post monsoon and winter season. Net radiation ranged from 650 to 700 W/m2, sensible heat flux ranged between 250–300 W/m2, latent heat flux ranged between 250–300 W/m2 and ground heat flux ranged 30–120 W/m2. Urban areas exhibited higher energy fluxes, emphasizing the importance of effective planning interventions to mitigate emissions in such areas. The study highlights the potential of Earth observation based approaches in estimating and balancing urban energy fluxes, while also emphasising the need to consider seasonal and spatial variations in the land use pattern when formulating strategies to mitigate emissions in the urban areas.

Modeling the effects of drip irrigation under plastic mulch on vapor and energy fluxes in oasis agroecosystems, Xinjiang, China

Abstract. It has previously been argued that current Soil Vegetation Atmosphere Transfer (SVAT) models are over-parameterised given the calibration data typically available. Using the Generalised Likelihood Uncertainty Estimation (GLUE) methodology, multiple feasible model parameter sets are here conditioned on latent heat fluxes and then additionally on the sensible and ground heat fluxes at a single site in Amazonia. The model conditioning schemes were then evaluated with a further data set collected at the same site according to their ability to reproduce the latent, sensible and ground heat fluxes. The results indicate that conditioning the model on only the latent heat flux component of the energy balance does not constrain satisfactorily the predictions of the other components of the energy balance. When conditioning on all heat flux objectives, significant additional constraint of the feasible parameter space is achieved with a consequent reduction in the predictive uncertainty. There are still, however, many parameter sets that adequately reproduce the calibration/validation data, leading to significant predictive uncertainty. Surface temperature measurements, whilst also subject to uncertainty, may be employed usefully in a multi-objective calibration of SWAT models.

The Simple Biosphere Model (SiB2) and the 2 × 2 km resolution National Land use/Land Cover database were used to investigate the effects of Land Use/Cover Change (LUCC) on land surface energy balance and climate in Jilin Province, northeast China, from 1990 to 2005. The spatial patterns of the components of surface energy balance (i.e., net radiation (R n), latent heat (LH), sensible heat (SH), and albedo (α)) and climate (i.e., canopy temperature (T c), diurnal temperature range (DTR)), as well as the roles of land cover type in variations of energy balance and climate, were investigated. The results showed that there were general similar trends in R n, LH, SH, and α in the LUCC process. The spatial patterns of T c and DTR also showed consistent relationships with LUCC processes. Leaf area index (LAI) and canopy conductance (g c) were found to be the key factors in controlling the spatial patterns of the components of surface energy balance and T c. Using linear correlation method, the gaps of the components of surface energy balance were well-explained by the differences of LAI and g c, and R n had a better correlation with T c and DTR, in the process of LUCC. The surface energy partitioning of R n into LH and SH could not only dampen or strengthen the temperature difference, but also change the relative size of albedo-based R n when the albedo gap was small, between land cover types.

Instantaneous and daily values of the surface energy balance over agricultural fields using remote sensing and a reference field in an arid environment

Abstract. The Bowen ratio surface energy balance method is a relatively simple method to determine the latent heat flux and the actual land surface evaporation. The Bowen ratio method is based on the measurement of air temperature and vapour pressure gradients. If these measurements are performed at only two heights, correctness of data becomes critical. In this paper we present the concept of a new measurement method to estimate the Bowen ratio based on vertical dry and wet bulb temperature profiles with high spatial resolution. A short field experiment with distributed temperature sensing (DTS) in a fibre optic cable with 13 measurement points in the vertical was undertaken. A dry and a wetted section of a fibre optic cable were suspended on a 6 m high tower installed over a sugar beet trial plot near Pietermaritzburg (South Africa). Using the DTS cable as a psychrometer, a near continuous observation of vapour pressure and air temperature at 0.20 m intervals was established. These data allowed the computation of the Bowen ratio with a high spatial and temporal precision. The daytime latent and sensible heat fluxes were estimated by combining the Bowen ratio values from the DTS-based system with independent measurements of net radiation and soil heat flux. The sensible heat flux, which is the relevant term to evaluate, derived from the DTS-based Bowen ratio (BR-DTS) was compared with that derived from co-located eddy covariance (R2 = 0.91), surface layer scintillometer (R2 = 0.81) and surface renewal (R2 = 0.86) systems. By using multiple measurement points instead of two, more confidence in the derived Bowen ratio values is obtained.

基于三江平原稻田2005~2007年5~10月涡度相关通量观测数据, 分析了该区稻田能量通量的日变化、季节变化和能量分配特征以及能量平衡状况。结果表明: 三江平原稻田净辐射和潜热通量日变化均表现为明显的单峰特征, 感热通量日变化在水稻发育进入成熟期后才较明显, 而土壤热通量在水稻整个发育期内日变化特征都不明显。稻田净辐射季节变化特征显著, 6月下旬至7月上旬达到最大值18~20 MJ·m^-2·d^-1。潜热通量季节变化与净辐射同步, 最大值为13~19 MJ·m^-2·d^-1。相比之下感热通量较小, 观测期间变化于-3.90~ 3.94 MJ·m^-2·d^-1, 且没有明显的季节变化。5~10月土壤热通量呈下降趋势, 变化于-2.67~3.62 MJ·m^-2·d^-1。三江平原地区稻田能量分配特征明显, 潜热通量占净辐射的比例(<i>LE</i>/<i>R</i>n) 5~10月平均值为0.67, 表明净辐射大部分以潜热通量形式所消耗, 但生长旺季<i>LE</i>/<i>R</i>n略大于生长季初期和末期。感热通量占净辐射的比例(<i>H</i>s/<i>R</i>n)的季节变化特征与<i>LE</i>/<i>R</i>n比值相反, 观测期间平均值为0.10。这导致波文比在水稻生长旺季较小而在初期和末期较大。5~10月土壤热通量占净辐射的比例(<i>G</i>/<i>R</i>n)呈逐渐下降趋势, 其月平均值由5月的0.14下降到10月的-0.08。线性回归法和能量平衡比率均表明三江平原稻田能量明显不闭合, 2005、2006年5~10月能量不闭合度分别为22%和16%, 而2007年能量"过闭合", 能量平衡比率平均值为1.07。

New satellite-derived latent and sensible heat fluxes are performed by using WindSat wind speed, WindSat sea surface temperature, the European Centre for Medium-range Weather Forecasting (ECMWF) air humidity, and ECMWF air temperature from 2004 to 2014. The 55 moored buoys are used to validate them by using the 30 min and 25 km collocation window. Furthermore, the objectively analyzed air-sea heat fluxes (OAFlux) products and the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis 2 (NCEP2) products are also used for global comparisons. The mean biases of sensible and latent heat fluxes between WindSat flux results and buoy flux data are–0.39 and–8.09 W/m2, respectively. In addition, the root-mean-square (RMS) errors of the sensible and latent heat fluxes between them are 5.53 and 24.69 W/m2, respectively. The RMS errors of sensible and latent heat fluxes are observed to gradually increase with an increasing buoy wind speed. The difference shows different characteristics with an increasing sea surface temperature, air humidity, and air temperature. The zonal average latent fluxes have some high regions which are mainly located in the trade wind zones where strong winds carry dry air in January, and the maximum value centers are found in the eastern waters of Japan and on the US east coast. Overall, the seasonal variability is pronounced in the Indian Ocean, the Pacific Ocean, and the Atlantic Ocean. The three sensible and latent heat fluxes have similar latitudinal dependencies; however, some differences are found in some local regions.

Active subspaces for sensitivity analysis and dimension reduction of an integrated hydrologic model