Nighttime Heat Research Articles

Mechanisms for regional compound hot extremes in the mid‐lower reaches of the Yangtze River

AbstractHot temperature extremes, particularly during summer, impose severe adverse effects on human health and natural systems. Although considerable attention has been paid to individual hot days and nights, little is known about mechanisms leading to compound hot extremes that combine daytime and nighttime heat. This important type of hot extreme has occurred frequently in the populated mid‐lower reaches of the Yangtze River during the past three to four decades. Through composite analysis, dynamic–thermodynamic processes leading to the occurrence of compound hot extremes are investigated. Results show that an anomalous anticyclone, exhibiting a quasi‐barotropic structure and persisting through day and night, is a prerequisite for the occurrence of compound hot extremes in the mid‐lower reaches of the Yangtze River. The anomalous anticyclone reduces cloud cover and increases solar radiation, causing extreme daytime high temperature. Simultaneously, a descending branch within the column of the anomalous anticyclone enhances local adiabatic heating. This descending branch and resultant adiabatic heat are sustained and further strengthened by anomalous descending motions as part of vertical cells associated with positive precipitation anomalies to the north and south of the anticyclone. During compound hot extremes, intensified southerlies transport additional water vapour toward the study area, helping to establish a stable atmospheric stratification in the low troposphere. This low‐level stable moist layer plays a particularly important role to offset nighttime radiative cooling and, therefore, elevates nighttime low temperatures to form compound hot extremes. Consequently, compound hot extremes in the study area are hot‐humid in nature.

Development of a Building Energy Simulation Model for Control of Multi-Span Greenhouse Microclimate

In this study, we propose a building energy simulation model of a multi-span greenhouse using a transient system simulation program to simulate greenhouse microenvironments. The proposed model allows daily and seasonal control of screens, roof vents, and heating setpoints according to crop needs. The proposed model was used to investigate the effect of different thermal screens, natural ventilation, and heating setpoint controls on annual and maximum heating loads of a greenhouse. The experiments and winter season weather conditions of greenhouses in Taean Gun (latitude 36.88° N, longitude 126.24° E, elevation 45 m) Chungcheongnam-do, South Korea was used for validation of our model. Nash–Sutcliffe efficiency coefficients of 0.87 and 0.71 showed good correlation between the computed and experimental results; thus, the proposed model is appropriate for performing greenhouse thermal simulations. The results showed that the heating loads of the triple-layered screen were 70% and 40% lower than that of the single-screen and double-screen greenhouses, respectively. Moreover, the maximum heating loads without a screen and for single-, double-, and the triple-layered screens were 0.65, 0.46, 0.41, and 0.34 MJ m−2, respectively. The analysis of different screens showed that Ph-77 (shading screen) combined with Ph-super (thermal screen) had the least heating requirements. The heating setpoint analysis predicted that using the designed day- and nighttime heating control setpoints can result in 3%, 15%, 14%, 15%, and 40% less heating load than when using the fixed value temperature control for November, December, January, February, and March, respectively.

Thermal performance evaluation and enhancement of a parabolic trough collector

In this article, the effect of concentration technology on the thermal performance of a solar water heating system is investigated. A three-dimensional numerical simulation of a parabolic trough collector (PTC) is carried out using the commercial software ANSYS. The performance of a line-focus concentration system is compared to that of an integrated collector storage solar water heater. The PTC provides more useful heat but has higher nighttime thermal losses. It is proposed to reduce nighttime energy loss by insulating the PTC storage tank. The effects of the insulation coverage strategy and medium are discussed. Numerical results show that the thermal insulation material significantly influences heat transfer. Indeed, addition of glass wool reduces heat collection during the day and improves heat preservation at night, while adding an air-filled glass tube increases heat transfer during the day and reduces nighttime thermal losses. The effect of tank insulation coverage is also investigated. This solar system performs best when the upper half of the tank is insulated. The maximum water temperature is 360.5 K, the overall thermal loss coefficient is 9.38 W/m2 K, and the nighttime thermal loss coefficient is 6.2 W/m K. The optimum PTC configuration is one in which the upper half of the tank is insulated by a 0.01 m-thick layer of air that is held in place by glass. This optimized system provides 29% more useful heat, a lower overall heat loss coefficient of 14.9%, and a lower nighttime heat loss coefficient of 62% than the non-insulated-tank-based PTC.

Temperature Measurements on a Solar and Low Enthalpy Geothermal Open-Air Asphalt Surface Platform in a Cold Climate Region

Solar heat, already captured by vast asphalt fields in urban areas, is potentially a huge energy resource. The vertical soil temperature profile, i.e., low enthalpy geothermal energy, reveals how efficiently the irradiation is absorbed or radiated back to the atmosphere. Measured solar irradiation, heat flux on the asphalt surface and temperature distribution over a range of depths describe the thermal energy from an asphalt surface down to 10 m depth. In this study, those variables were studied by long-term measurements in an open-air platform in Finland. To compensate the nighttime heat loss, the accumulated heat on the surface should be harvested during the sunny daytime periods. A cumulative heat flux over one year from asphalt to the ground was 70% of the cumulative solar irradiance measured during the same period. However, due to the nighttime heat losses, the net heat flux during 5 day period was only 18% of the irradiance in spring, and was negative during autumn, when the soil was cooling. These preliminary results indicate that certain adaptive heat transfer and storage mechanisms are needed to minimize the loss and turn the asphalt layer into an efficient solar heat collector connected with a seasonal storage system.

Anthropogenically-driven increases in the risks of summertime compound hot extremes

Compared to individual hot days/nights, compound hot extremes that combine daytime and nighttime heat are more impactful. However, past and future changes in compound hot extremes as well as their underlying drivers and societal impacts remain poorly understood. Here we show that during 1960–2012, significant increases in Northern Hemisphere average frequency (~1.03 days decade−1) and intensity (~0.28 °C decade−1) of summertime compound hot extremes arise primarily from summer-mean warming. The forcing of rising greenhouse gases (GHGs) is robustly detected and largely accounts for observed trends. Observationally-constrained projections suggest an approximate eightfold increase in hemispheric-average frequency and a threefold growth in intensity of summertime compound hot extremes by 2100 (relative to 2012), given uncurbed GHG emissions. Accordingly, end-of-century population exposure to compound hot extremes is projected to be four to eight times the 2010s level, dependent on demographic and climate scenarios.

Comparative study on the annual performance between loop thermosyphon solar water heating system and conventional solar water heating system

Loop thermosyphon (LT) is usually introduced to overcome the freezing and corrosion problems associated with the conventional solar water heating (SWH) system. Compared with the conventional SWH system, the LT-SWH system possesses a lower nighttime heat loss because of the thermal diode property of loop thermpsyphon but bigger daytime heat loss because of the secondary heat exchange. However, the effect of above interaction to the system performance is rarely reported based on long-term running. In this study, based on the typical meteorological year data of Fuzhou city, annual performances of above two systems, including the effective number of supplying days, effective heat gain and nighttime heat loss, are comparatively analyzed under two different operational modes. Variations of above mentioned variables with the increment in the set temperature are discussed. The results indicate that, under the discontinuous heating mode, the effective numbers of supplying days of SWH system and LT-SWH system are 139 and 153, respectively. While the numbers of days are respectively 168 and 173 under the continuous heating mode. The SWH system possesses an expected bigger nighttime heat loss ratio with an average annual value of 15.07% corresponding to 6.15% for the LT-SWH system. Particularly, for the LT-SWH system, the different relative magnitudes of heat loss coefficients functioning at different times leads to a smaller temperature drop at night and also a smaller temperature rise at the subsequent day. It generates an unanticipated results that corresponds to the same month from November to April, the two systems have the approximate effective heat gain. The set temperature significantly influences the relative magnitudes of annual effective number of supplying days and annual effective heat gain, the superiority of LT-SWH system gradually diminishes and even reverses with the increment in the set temperature. The bigger daytime heat loss dominating the dominance is responsible for that transition. Combining with a longer static payback period, it is conditional to substitute the conventional SWH system with the LT-SWH system, especially when the water temperature on demand is high.

Anthropogenic Influences on the Persistent Night-Time Heat Wave in Summer 2018 over Northeast China

Persistent night-time heat waves like the event of summer 2018 in Northeast China are extremely rare (about a one-in-a-500-year event) in the natural world, but now have become about a one-in-60-year event with anthropogenic warming.

Experimental investigation of thermal performance of a heating system combining solar air collector with hollow ventilated interior wall

A novel heating system combining solar air collector with hollow ventilated interior wall (SAC-HVIW) has the ability to supply thermal energy for the nighttime heating and improve the nighttime thermal comfort in residential buildings on Tibetan Plateau. In this paper, a test chamber is constructed to investigate the heating performance of this system. Nine cases with different ventilation rates and supply air temperatures in the channel are measured under steady-state conditions. Results show that the heating capacity of the hollow ventilated interior wall increases with the logarithmic mean temperature difference (LMTD). U-value of the interior wall changes smoothly with the ventilation rate and the LMTD of the system, but the effectiveness of the interior wall almost keeps constant for all cases with the mean value of 0.85. The radiant heat transfer coefficient of the hollow ventilated interior wall keeps the constant of 5.43 W/(m2•K), whereas the convective heat transfer coefficient increases smoothly with the ventilation rate and the inlet air temperature. Radiant heat transfer is dominant with the weight of more than 60%. Experimental data are used to evaluate the heating performance of HVIW, and it is also beneficial to the design of this system.

Urban heat island behaviors in dryland regions

Urban heat island (UHI) characteristics and mitigation strategies for dryland cities differ from those for wetter urban regions. Whereas the latter typically see daytime surface UHIs, the rapid heating and cooling of deserts surrounding arid cities often produces daytime ‘urban cool islands’ and nighttime UHIs. Degrees of aridness, extent of vegetation, elevation, latitude, humidity, topography, and typical building types are likely to influence dryland UHI dynamics. This study analyzes variations in thermal effects at multiple scales for 10 dryland urban regions representing varied geographies worldwide with an aim to establish a broader understanding of the spectrum of UHI patterns in dryland cities. We used GIS to assemble daytime and nighttime satellite imagery, determined land surface temperature and vegetation at a 30-meter scale, and analyzed typical neighborhood-scale examples of six land cover types in each region. The 10 regions showed large variation in thermal effects. We found a strong daytime surface UHI in only one. Nighttime heat islands were more pronounced. However, all regions showed strong small-scale variation in temperature, averaging a 12.3 °C difference between mean top-quintile and bottom-quintile surface temperatures. Samples of urban forest landscapes cooled daytime temperatures an average of 5.6 °C compared to metro averages. Irrigated lawn and multistory building land cover samples also had a substantial cooling effect. Xeriscaped landscapes amplified daytime heating. Our results indicate that UHIs for dryland cities are unlikely to be reduced by xeriscape strategies, but that shade-maximizing urban forestry and built form hold promise to reduce heat islands.

Response of Urban Heat Stress to Heat Waves in Athens (1960–2017)

The increasing frequency, intensity and duration of heat waves seem to follow the observed global warming in recent decades. Vulnerability to heat waves is expected to increase in urban environments mainly due to population density and the effect of the urban heat island that make cities hotter than surrounding non-urban areas. The present study focuses on a vulnerable area of the eastern Mediterranean, already characterized as a ‘hot spot’ with respect to heat-related risk and investigates the change in heat stress levels during heat wave compared to non-heat wave conditions as well as the way that heat stress levels respond to heat waves in urban, compared to non-urban, environments. The adoption of a metric accounting for both the intensity and duration of the hot event yielded a total of 46 heat wave episodes over a nearly 60-year period, but with very rare occurrence until the late 1990s and a profound increased frequency thereafter. The results reveal a difference of at least one thermal stress category between heat wave and non-heat wave periods, which is apparent across the entire range of the thermal stress distribution. The analysis demonstrates a robust intensification of nighttime heat stress conditions in urban, compared to non-urban, sites during severe heat waves. Nevertheless, severe heat waves almost equalize heat stress conditions between urban and non-urban sites during midday.

Daytime and nighttime heat wave characteristics based on multiple indices over the China–Pakistan economic corridor

The sharp rise in temperature has increased the frequency, intensity, duration, and timing of heat waves (HWs) over different regions of the world. Due to climate change, the China–Pakistan Economic Corridor (CPEC) is one of the highly vulnerable regions to HWs and needs comprehensive research studies to investigate the HW phenomenon in the region. This study analyzed the spatial and temporal changes in the daytime and nighttime HW characteristics based on multiple indices over the CPEC region. We used daily maximum and minimum temperatures (hereafter Tmax and Tmin) of 48 meteorological stations for the time period of 1980–2016. The non-parametric modified Mann–Kendall, Theil–Sen’s test, least square method, student t-test, and chi-square goodness-of-fit test techniques were used to analyze the long-term spatiotemporal changes in the daytime and nighttime HW characteristics. The results of the study show that the number of annual daytime/nighttime HW events, annual sum of participating daytime/nighttime HW days, the average length of annual daytime/nighttime HW events, duration of the longest annual daytime/nighttime HW event, the average magnitude of all annual daytime/nighttime HW events, amplitude of the hottest annual daytime/nighttime HW event, and the ending date of annual last daytime/nighttime HW event exhibited significant increasing trends at the rate of 0.78/1.43 events decade−1, 10/11.82, 2/1.74, 2.16/1.52 days decade−1, 0.40/0.59, 0.24/0.73 °C decade−1, and 12.29/10 days decade−1, respectively. Despite all, the onset date of the annual first daytime/nighttime HW event has shown a significant decreasing trend of − 5.71/− 5 days decade−1. The obvious positive trend of HW behaviors indicates that the country has experienced more frequent, stronger, more intense, and longer HWs during the study period. The spatial pattern of the trend indicates that the southern, central and eastern parts of Pakistan exhibited prominent and consistent HW activities, while the northwestern mountainous regions showed high spatial variability with some stations exhibited decreasing trends in HW indices. The findings of this study will be a base for the projection and mitigation of HWs in the region. Based on the study findings, we recommend that the mechanism of HW and its natural and anthropogenic drivers should be thoroughly investigated over the study region.

Integrating field-based heat tents and cyber-physical system technology to phenotype high night-time temperature impact on winter wheat

BackgroundMany agronomic traits have been bred into modern wheat varieties, but wheat (Triticum aestivum L.) continues to be vulnerable to heat stress, with high night-time temperature (HNT) stress shown to have large negative impact on yield and quality. Global mean temperature during the day is consistently warming with the minimum night temperature increasing at a much quicker pace. Currently, there is no system or method that allows crop scientists to impose HNT stress at key developmental stages on wheat or crops in general under field conditions, involving diverse genotypes and maintaining a dynamic temperature differential within the tents compared to the outside.ResultsThrough implementation of a side roll up and a top ventilation system, heaters, and a custom cyber-physical system using a Raspberry Pi, the heat tents were able to consistently maintain an elevated temperature through the night to differentiate heat stress impact on different genotypes. When the tents were placed in their day-time setting they were able to maintain ambient day-time temperature without having to be removed and replaced on the plots. Data averaged from multiple sensors over three consecutive weeks resulted in a consistent but small temperature difference of 0.25 °C within the tents, indicating even distribution of heat. While targeting a temperature differential of 4 °C, the tents were able to maintain an average differential of 3.2 °C consistently throughout the night-time heat stress period, compared to the outside ambient conditions. The impact of HNT stress was confirmed through a statistically significant yield reduction in eleven of the twelve genotypes tested. The average yield under HNT stress was reduced by 20.3% compared to the controls, with the highest reduction being 41.4% and a lowest reduction of 6.9%. Recommendations for fine-tuning the system are provided.ConclusionThis methodology is easily accessible and can be widely utilized due to its flexibility and ease of construction. This system can be modified and improved based on some of the recommendations and has the potential to be used across other crops or plants as it is not reliant on access to any hardwired utilities. The method tested will help the crop community to quantify the impact of HNT stress, identify novel donors that induce tolerance to HNT and help the breeders develop crop varieties that are resilient to changing climate.

Projected near-term changes in three types of heat waves over China under RCP4.5

The changes in three aspects of frequency, intensity and duration of the compound, daytime and nighttime heat waves (HWs) over China during extended summer (May–September) in a future period of the mid-21st century (FP; 2045–2055) under RCP4.5 scenario relative to present day (PD; 1994–2011) are investigated by two models, MetUM-GOML1 and MetUM-GOML2, which comprise the atmospheric components of two state-of-the-art climate models coupled to a multi-level mixed-layer ocean model. The results show that in the mid-21st century all three types of HWs in China will occur more frequently with strengthened intensity and elongated duration relative to the PD. The compound HWs will change most dramatically, with the frequency in the FP being 4–5 times that in the PD, and the intensity and duration doubling those in the PD. The changes in daytime and nighttime HWs are also remarkable, with the changes of nighttime HWs larger than those of daytime HWs. The future changes of the three types of HWs in China in two models are similar in terms of spatial patterns and area-averaged quantities, indicating these projected changes of HWs over the China under RCP4.5 scenario are robust. Further analyses suggest that projected future changes in HWs over China are determined mainly by the increase in seasonal mean surface air temperatures with change in temperature variability playing a minor role. The seasonal mean temperature increase is due to the increase in surface downward longwave radiation and surface shortwave radiation. The increase in downward longwave radiation results from the enhanced greenhouse effect and increased water vapour in the atmosphere. The increase in surface shortwave radiation is the result of the decreased aerosol emissions, via direct aerosol–radiation interaction and indirect aerosol–cloud interaction over southeastern and northeastern China, and the reduced cloud cover related to a decrease in relative humidity.

Shifting planting date of Boro rice as a climate change adaptation strategy to reduce water use

Suitable adaptation strategies for dry season Boro rice cultivation under future climate change scenarios are important for future food security in Bangladesh. This study assessed the effect of shifting trans−/planting date of dry season Boro rice as an adaptation strategy, with focus on water requirements under future climate scenarios. Potential crop water requirement, effective rainfall and irrigation requirement to satisfy crop evapotranspiration of Boro rice were estimated using CropWat 8.0 for early, normal and late planting dates for 2050s and 2080s. Future climate scenarios were constructed using five global circulation model (GCM) outputs for RCP 4.5 and 8.5 by statistical downscaling and bias correction. Number of days exceeding the threshold temperatures (maximum of 35 °C and minimum of 25 °C) was counted for critical period of Boro rice to understand compatibility of the changed planting dates. Results indicate that late planting can substantially reduce irrigation demand by increasing rainfall availability during Boro growth duration, but the option is very limited due to both day- and night-time heat stress. An early planting, on the other hand, accounts for high water demand but ensures suitable temperature during the critical growth stages of the crop. The normal planting dates show the possibility of day-time heat stress. So, late planting of temperature-tolerant cultivars or early planting of high-yielding varieties would be recommended based on local water availability. However, adjustment of the planting date is currently limited because high temperature-tolerant cultivars are not available in the study region.

Climate Change and Nighttime Heat Stress: Tales of Two Cities in the US Midwest

This study explores nighttime heat stress in two Midwestern regions in the United States, encompassing the cities of Minneapolis and Milwaukee. Daily minimum temperature data were obtained from the MACAv2-METDATA dataset at a 4-km resolution. The dataset was downloaded both for the historical (1950-2005) and Representative Concentration Pathways (RCP) 4.5 (2006-2099) simulations from 11 global climate models. MODIS land cover data at a 5´×5´ resolution were used to delineate urban and non-urban areas. Heat stress was indicated by the occurrence of hot nights in two criterions. First, the number of days with daily minimum temperatures above 300K (27°C) was counted to calculate decadal frequency. Second, the 95th percentile of daily minimum temperatures in the historical period was used as a threshold to calculate the duration of hot nights. The study finds that (1) hot nights (> 300K) are practically non-existent in the historical simulation but are likely to occur typically 2-3 times per decade with the RCP4.5 simulations; (2) the frequency of such events in the future can exceed 25 per decade in urban areas whereas it can be just about 1 per decade in non-urban areas depending on models; and (3) hot nights (> 95th percentile threshold) are likely to last longer in the future simulations. Overall, heat stress is projected to increase both in frequency and duration, and the urban heat island effect in terms of heat stress is projected to intensify in the future.

Theoretical and Experimental Contributions on the Use of Smart Composite Materials in the Construction of Civil Buildings with Low Energy Consumption

The paper presents the theoretical and experimental studies undertaken for the realization of an intelligent composite material with phase shift that has optimal characteristics in the thermal energy storage process and an experimental method for integrating the material with phase change in a possible efficient system to be used in the construction of a dwelling. It analyzes the main factors in designing such systems (the temperature limits between which the system must operate, the melting/solidification temperature of the Phase Change Material (PCM), the latent heat of the PCM, the degree of thermal loading, the bed configuration of PCM capsules and a PCM-RB01 material is set. A micro-encapsulation method was chosen and a “solar wall” is made where the incident solar radiation is absorbed by the PCM embedded in the wall, so the stored heat is used for heating and ventilation of a home. Experimental research has shown that developed PCM allows a maximum room temperature reduction of about 4 °C during the day and can reduce the night-time heating load. Also, despite the lower thermal energy absorption capacity, the developed PCM-RB01 material provides a superior physical stability compared to the classical types of integration.

Relationship between thermal comfort indices and internal temperature of grazing lactating Holstein × Gyr cows in western Amazonia

ABSTRACT The relationship between the indices of thermal comfort and internal temperature in lactating cows was evaluated inRondônia, western Amazonia in a 2 × 2 crossover trial carried out with eight Girolando cows, ½ Holstein × ½ Gyr (n=4) and – Holstein × ¼ Gyr (n=4), grazing palisade grass supplemented with or without soybean oil concentrates. Data logger thermometers adapted to intravaginal devices (hormone free) were used to measure internal temperatures (IT) every 10 min over 48 h. Concomitantly, environmental parameters (air temperature, relative humidity, wind speed, and solar radiation) were collected for the estimation of the temperature-humidity index (THI), equivalent temperature index (ETI), and predicted respiratory rate (PRR). Supplementation with soybean oil did not affect the internal temperature of the cows. IT varied significantly between the genetic groups, with the highest mean observed at night in the – Holstein × ¼ Gyr cows, compared with that in the ½ Holstein × ½ Gyr cows (39.54 °C vs 39.06 °C, respectively). THI, ETI, and PRR were significantly and positively correlated with the IT of the Girolando cows; however, the PRR was not found to be adequate in indicating the night-time heat stress in cows.

Cyber-Physical System for Energy-Efficient Stadium Operation

The environmental impacts of medium to large-scale buildings receive substantial attention in research, industry, and media. This article studies the energy savings potential of a commercial soccer stadium during day-to-day operation. Buildings of this kind are characterized by special purpose system installations like grass heating systems and by event-driven usage patterns. This work presents a methodology to holistically analyze the stadium’s characteristics and integrate its existing instrumentation into a Cyber-Physical System, enabling to deploy different control strategies flexibly. In total, seven different strategies for controlling the studied stadium’s grass heating system are developed and tested in operation. Experiments in winter season 2014/2015 validated the strategies’ impacts within the real operational setup of the Commerzbank Arena , Frankfurt, Germany. With 95% confidence, these experiments saved up to 66% of median daily weather-normalized energy consumption. Extrapolated to an average heating season, this corresponds to savings of 775MWh and 148t of CO 2 emissions. In winter 2015/2016 an additional predictive nighttime heating experiment targeted lower temperatures, which increased the savings to up to 85%, equivalent to 1GWh (197t CO 2 ) in an average winter. Beyond achieving significant energy savings, the different control strategies also met the target temperature levels to the satisfaction of the stadium’s operational staff. While the case study constitutes a significant part, the discussions dedicated to the transferability of this work to other stadiums and other building types show that the concepts and the approach are of general nature. Furthermore, this work demonstrates the first successful application of Deep Belief Networks to regress and predict the thermal evolution of building systems.

An Analysis of the Prevalence of Heat Waves in the United States between 1948 and 2015

AbstractUnusually hot weather is a major concern to public health as well as other systems (e.g., ecological, economical, energy). This study utilized spatially continuous and homogenized observational surface climate data to examine changes in the regularity of heat waves in the continental United States. This included the examination of heat waves according only to daytime temperatures, nighttime temperatures, and both daytime and nighttime temperatures. Results confirmed a strong increase in the prevalence of heat waves between the mid-1970s and the dataset end (2015), and that increase was preceded by a mild decrease since the dataset beginning (1948). Results were unclear whether the prevalence of nighttime or simultaneous daytime–nighttime heat waves increased the most, but it was clear that increases were largest in the summer. The largest gains occurred in the West and Southwest, and a “warming hole” was most conspicuous in the northern Great plains. The changes in heat wave prevalence were similar to changes in the mean temperatures, and more so in the daytime heat waves. Daytime and nighttime heat waves coincided with one another more frequently in recent years than they did in the 1970s. Some parts of the United States (West Coast) were more likely than other parts to experience daytime and nighttime heat waves simultaneously. While linear trends were not sensitive to the climate dataset, trend estimation method, or heat wave definition, they were mildly sensitive to the start and end dates and extremely sensitive to the climate base period method (fixed in time or directly preceding any given heat wave).

A landscape connectivity model to quantify contributions of heat sources and sinks in urban regions

The effect of landscape configuration on urban temperatures is always an important issue in landscape planning and mitigation of urban heat islands. However, landscape indices used in previous studies did not focus on thermal processes. We proposed a landscape source-sink distance (LSSD) index used to quantify the landscape connectivity and investigate its contribution to variations in land surface temperature (LST) in Beijing. Monthly LST was derived from MODIS remote sensing products in 2002 and 2012. Landscape composition and connectivity was calculated based on QuickBird and IKONOS images. The LSSD of each LST grid was calculated according to the accumulative shortest distance between green-impervious, water-impervious, and green-water types. The contributions of landscape composition and connectivity to variations in LST were assessed using a geographically weighted regression model. Heat sources and sinks were designated as having positive and negative effects on the LST, respectively. Results showed that (1) green spaces served as heat sinks both day and night. Water areas served as daytime heat sinks and nighttime heat sources; (2) the influence of green and water types on daytime LST varied in different months while their influence on nighttime LST was stable seasonally; and (3) a large distance between green and impervious land increased variations in day-night LST while a large distance for water-impervious connectivity might mitigate diurnal variations in LST. This study shows that landscape planners need to rationally use landscape connectivity among different landscape types and should focus on specific time and season for the effective mitigation of urban heating.