Summary
With climate change, heat waves are becoming increasingly frequent, intense and broader in spatial extent. However, while the lethal effects of heat waves on humans are well documented, the ...impacts on flora are less well understood, perhaps except for crops. We summarize recent findings related to heat wave impacts including: sublethal and lethal effects at leaf and plant scales, secondary ecosystem effects, and more complex impacts such as increased heat wave frequency across all seasons, and interactions with other disturbances. We propose generalizable practical trials to quantify the critical bounding conditions of vulnerability to heat waves. Collectively, plant vulnerabilities to heat waves appear to be underappreciated and understudied, particularly with respect to understanding heat wave driven plant die‐off and ecosystem tipping points.
ABSTRACT
This study investigates the interannual variation of heat wave frequency (HWF) in South Korea during the past 42 years (1973–2014) and examines its connection with large‐scale atmospheric ...circulation changes. Korean heat waves tend to develop most frequently in late summer during July and August. The leading Empirical Orthogonal Function accounting for 50% of the total variance shows a mono‐signed pattern over South Korea, suggesting that the dominant mechanisms responsible for the heat wave are linked in a spatial scale much larger than the nation. It also exhibits a regional variation with more occurrences in the southeastern inland area. The regression of the leading principal component (PC) time series of HWF with large‐scale atmospheric circulation identifies a north–south dipole pattern between the South China Sea and Northeast Asia. When this large‐scale circulation mode facilitates deep convection in South China Sea, it tends to weaken moisture transport from the South China Sea to Northeast Asia. Enhanced deep convection in the South China Sea triggers a source of Rossby wave train along southerly wind that generates positive geopotential height anomalies around Korea. The anomalous high pressure pattern is accompanied by large‐scale subsidence in Korea, thereby providing a favourable condition for extreme hot and dry days in Korea. This study highlights that there is a decadal change of the relationship between Korean heat waves and large‐scale atmospheric circulation patterns. The tropical forcing tends to be weakened in the recent decade, with more influences from the Arctic variability from the mid‐1990s.
Heat waves generally hit the Sichuan Basin (SCB) in summer, especially in July and August. This study investigates the interannual variability of mid‐summer heat wave frequency (HWF) over the SCB. ...The leading mode of the SCB mid‐summer HWF, which explains 48% of the total variance, exhibits an uniform spatial pattern and an increasing trend. This leading pattern mainly dominated by two factors: the mid‐to‐upper‐level height anomalies over western Europe and tropical convection anomalies. On one hand, the mid‐level height anomalies over western Europe could affect the HWF over most parts of the SCB by inducing an eastward‐propagating atmospheric teleconnection wave train to northeastern Asia, which causes a northwestward shifted western North Pacific subtropical high, leading to significant subsidence anomalies over the SCB, hence induces less cloudiness and below‐normal precipitation in‐situ. Consequently, there are increased surface solar radiation and decreased soil moisture over the region, favouring the occurrence of HWs there. On the other hand, the enhanced Philippine Sea convection and suppressed tropical eastern Indian Ocean convection excite an anomalous lower‐tropospheric cyclone dominant over the western North Pacific and southern China, which could weaken the transport of water vapour from the tropic oceans to the northeastern SCB and then contributes to decreased precipitation, favouring a higher HWF.
This study investigates the interannual variability of mid‐summer heat wave frequency (HWF) over the Sichuan Basin (SCB). The leading mode of the SCB mid‐summer HWF, which explains 48% of the total variance, exhibits an uniform spatial pattern and an increasing trend. This leading pattern mainly dominated by two factors: the mid‐to‐upper‐level height anomalies over the western Europe and the tropical convections.
Digital phase-locked loop (DPLL) frequency synthesizers have become popular for wireless applications in the sub-10-GHz range. However, mm-wave synthesizers still rely on analog PLLs, predominantly ...of the sub-harmonic, integerN-type. This paper describes the design and implementation of a 50-66-GHz phase-domain DPLL that uses a fundamental frequency capacitively degenerated digitally controlled oscillator (DCO) with 40-kHz frequency step. Following frequency division with a modulus of only 4, a two-step 8-bit time-todigital converter (TDC) digitizes the phase of the 12.5-16.5-GHz divider output with 450-fs resolution. Digital calibration based on the statistical element selection technique augmented by mean adaptation is used to mitigate TDC nonlinearity that results from random mismatches. Additional digital calibration techniques are introduced to mitigate DCO non-linearity and phase mismatches in the digital phase extraction sub-system, and to ensure robust operation of the inductor-less 4× frequency divider over process, voltage and temperature (PVT) variations. A 65-nm CMOS prototype of the DPLL occupies 0.45 mm 2 excluding pads and consumes 46 mA from a 1-V supply. The PLL achieves best (worst) case rms jitter of 220 (302) fs, best (worst) phase noise of -83/-94.5/-122 (-79/-88/ -116) dBc/Hz at 0.1/1/10 MHz offset, and -52.2(-48.3) dBc fractional spur.
On the Measurement of Heat Waves Perkins, S. E.; Alexander, L. V.
Journal of climate,
07/2013, Letnik:
26, Številka:
13
Journal Article
Recenzirano
Odprti dostop
Despite their adverse impacts, definitions and measurements of heat waves are ambiguous and inconsistent, generally being endemic to only the group affected, or the respective study reporting the ...analysis. The present study addresses this issue by employing a set of three heat wave definitions, derived from surveying heat-related indices in the climate science literature. The definitions include three or more consecutive days above one of the following: the 90th percentile for maximum temperature, the 90th percentile for minimum temperature, and positive extreme heat factor (EHF) conditions. Additionally, each index is studied using a multiaspect framework measuring heat wave number, duration, participating days, and the peak and mean magnitudes. Observed climatologies and trends computed by Sen’s Kendall slope estimator are presented for the Australian continent for two time periods (1951–2008 and 1971–2008). Trends in all aspects and definitions are smaller in magnitude but more significant for 1951–2008 than for 1971–2008. Considerable similarities exist in trends of the yearly number of days participating in a heat wave and yearly heat wave frequency, suggesting that the number of available heat wave days drives the number of events. Larger trends in the hottest part of a heat wave suggest that heat wave intensity is increasing faster than the mean magnitude. Although the direct results of this study cannot be inferred for other regions, the methodology has been designed as such that it is widely applicable. Furthermore, it includes a range of definitions that may be useful for a wide range of systems impacted by heat waves.
Heat waves have pronounced impacts on human health, ecosystems, and society. Heat waves have become more frequent and intense globally and are likely to intensify further in a warming climate. Across ...the United States there is a warming trend in average surface temperatures, but concordant increase in heat wave severity appears absent. Limitations in heat waves studies may be responsible for limited detection of a heat wave warming signal. We track daily spatiotemporal evolution of heat waves using geometric concepts and clustering algorithms to investigate how heat manifests on the land surface. We develop a spatial metric combining heat wave frequency, magnitude, duration, and areal extent. We find mixed trends in some individual heat wave characteristics across the United States during 1981–2018. However, exploration of the spatiotemporal evolution of combined heat wave characteristics shows considerable increases during this period and indicates a substantial increase in heat wave hazard across the United States.
Plain Language Summary
Heat waves are detrimental to human health and the environment and are projected to intensify in the future. However, observed upward trends in heat wave severity have not previously been detected in some regions of the globe, including the United States. This study considers heat waves as spatiotemporal events allowing for detection of significant increases in severity and coverage across the United States. Our study outlines a novel approach that combines geometric concepts with a clustering algorithm to investigate how heat waves evolve. We track heat waves daily across the ground surface and capture the combination of frequency, magnitude, duration, and areal extent using a new index that provides a more complete picture of heat waves. Our results indicate significant increases in heat waves across the United States during the last four decades.
Key Points
A new spatial metric tracks daily heat wave evolution finding a significant upward trend in heat wave severity and coverage 1981–2018
The observed upward trend is not captured by examining individual heat wave characteristics such as frequency, magnitude, or duration
Findings suggest a substantial increase in the heat wave burden across the United States during the last four decades
During the winter, the cold wave activity over the mid-high latitudes has profound impacts on agriculture, economic and human wellbeing. Such extreme weather events have been connected with the East ...Asia winter monsoon system and significantly influence the climate over the Eurasian continent. However, the multidecadal variabilities and regional interconnections of the cold wave activity across the Northern Hemisphere are lesser-known. In this study, we investigate the multidecadal variations in the cold wave frequency (CWF) and find an inverse relationship between Greenland and central Eurasia. Observational and modeling evidence suggests that the Atlantic Multidecadal Oscillation (AMO) is likely to be the driving force of the multidecadal seesaw in CWF, while the effects of the Arctic sea ice are very limited. The increased sea surface temperature (SST) in association with the AMO warms the subpolar troposphere and weakens the predominant westerlies over mid-high latitudes, resulting in positive geopotential height anomalies over the subpolar region. This further weakens the Icelandic Low and strengthens the Siberian High, which directly induces the warming (cooling) over Greenland (central Eurasia). There is a strong coherence between the mean state of surface air temperature and temperature extremes. The AMO-induced warming/cooling in Greenland/central Eurasia corresponds well with less/more frequent cold wave activities. Our results provide new insight into the multidecadal variability of cold wave activities and suggest that the CWF in the Northern Hemisphere may be interlinked.
Increased heat-wave frequency across the United States has led to the need for improved predictability of heat-wave events. A detailed understanding of land–atmosphere interactions and the ...relationship between soil moisture and temperature extremes could provide useful information for prediction. This study identifies, for many locations, a threshold of soil moisture below which there is an increase in the sensitivity of atmospheric temperature to declining soil moisture. This shift to a hypersensitive regime causes the atmosphere to be more susceptible to atmospherically driven heat-wave conditions. The soil moisture breakpoint where the regime shift occurs is estimated using segmented regression applied to observations and reanalysis data. It is shown that as the soil gets drier, there is a concomitant change in the rate of decrease in latent heat flux and increase in sensible heat flux leading to a strong positive feedback of increased air temperature near the surface, which further dries out the soil. Central, southwestern, and southeastern parts of the United States seem to have regions of clear regime shifts, while the eastern part of the United States generally does not get dry enough to reveal significant breakpoints. Sensible heat flux is seen to be a primary driver of this increased temperature sensitivity aided by the drop in latent heat flux. An investigation of flux tower sites verifies the breakpoint–flux relationships found in reanalysis data. Accurate estimation of these breakpoints can contribute to improved heat-wave prediction.
The prediction of dynamic mooring line tension using the responses of floating structures is of great significance for the safety monitoring of the station-keeping. Due to the mooring line tension ...usually has the typical low-frequency and wave-frequency characteristics, this paper proposes a method of Low-frequency adds wave-frequency responses (LAWR) to predict the mooring line tension. Firstly, the optimal Long-short term memory (LSTM) neural network structure is determined through conducting the parameter sensitivity analysis on the performance of the LSTM model. Secondly, the low-frequency and wave-frequency tension are predicted using the corresponding low-frequency and wave-frequency responses as the input data, respectively. Then, the total mooring line tension is predicted by superposing the low-frequency tension and wave-frequency tension. Finally, the feasibility of the LAWR method is verified by considering the different data relevance between the training sets and validating sets, including the sea state condition contains different current direction, wave height and spectral peak frequency. The method proposed in this paper could further improve the prediction accuracy of mooring line tension more than 30%, which has great engineering significance.
•A method of Low-frequency adds wave-frequency responses (LAWR) to predict the mooring line tension.•The proposed LAWR method could improve about 30% prediction accuracy.•The influence factors are analyzed to obtain the optimal neural network structure.
PDF
