Significance Carbon uptake by terrestrial ecosystems mitigates the impact of anthropogenic fossil fuel emissions on atmospheric CO2 concentrations, but the strength of this carbon sink is highly sensitive to large-scale extreme climate events. In 2012, the United States experienced the most severe drought since the Dust Bowl period, along with the warmest spring on record. Here, we quantify the impact of this climate anomaly on the carbon cycle. Our results show that warming-induced earlier vegetation activity increased spring carbon uptake, and thus compensated for reduced carbon uptake during the summer drought in 2012. This compensation, however, came at the cost of soil moisture depletion from increased spring evapotranspiration that likely enhanced summer heating through land-atmosphere coupling. The global terrestrial carbon sink offsets one-third of the world’s fossil fuel emissions, but the strength of this sink is highly sensitive to large-scale extreme events. In 2012, the contiguous United States experienced exceptionally warm temperatures and the most severe drought since the Dust Bowl era of the 1930s, resulting in substantial economic damage. It is crucial to understand the dynamics of such events because warmer temperatures and a higher prevalence of drought are projected in a changing climate. Here, we combine an extensive network of direct ecosystem flux measurements with satellite remote sensing and atmospheric inverse modeling to quantify the impact of the warmer spring and summer drought on biosphere-atmosphere carbon and water exchange in 2012. We consistently find that earlier vegetation activity increased spring carbon uptake and compensated for the reduced uptake during the summer drought, which mitigated the impact on net annual carbon uptake. The early phenological development in the Eastern Temperate Forests played a major role for the continental-scale carbon balance in 2012. The warm spring also depleted soil water resources earlier, and thus exacerbated water limitations during summer. Our results show that the detrimental effects of severe summer drought on ecosystem carbon storage can be mitigated by warming-induced increases in spring carbon uptake. However, the results also suggest that the positive carbon cycle effect of warm spring enhances water limitations and can increase summer heating through biosphere–atmosphere feedbacks.

Warm Spring Reduced Carbon Cycle Impact of the 2012 US Summer Drought

. Compound dry and hot conditions frequently cause large impacts on
ecosystems and societies worldwide. A suite of indices is available for the assessment of droughts and heatwaves, yet there is no index available for
incorporating the joint variability of dry and hot conditions at the sub-monthly scale. Here we introduce a daily-scale index, called the standardized
compound drought and heat index (SCDHI), to assess compound dry-hot
conditions. The SCDHI is based on a daily drought index (the standardized
antecedent precipitation evapotranspiration index – SAPEI), the daily-scale standardized temperature index (STI), and a joint probability distribution method. The new index is verified against real-world compound dry and hot
events and associated observed vegetation impacts in China. The SCDHI can
not only capture compound dry and hot events at both monthly and sub-monthly
scales, but is also a good indicator for associated vegetation impacts. Using the SCDHI, we quantify the frequency, severity, duration, and intensity of compound dry-hot events during the historical period and assess the
ability of climate models to reproduce these characteristics in China. We
find that compound events whose severity is at least light and which last
longer than 2 weeks generally persisted for 20–35 d in China. Southern China suffered from compound events most frequently, and the most severe
compound events were mainly detected in this region. Climate models
generally overestimate the frequency, duration, severity, and intensity of compound events in China, especially for western regions, which can be
attributed to a too strong dependence between the SAPEI and STI in those
models. The SCDHI provides a new tool to quantify sub-monthly
characteristics of compound dry and hot events and to monitor their
initiation, development, and decay. This is important information for
decision-makers and stakeholders to release early and timely warnings.

/pdf/a-standardized-index-for-assessing-sub-monthly-compound-dry-4g5b0ay8pm.pdf

A standardized index for assessing sub-monthly compound dry and hot conditions with application in China

The emergence of flash drought has attracted widespread attention due to its rapid onset. However, little is known about the recent evolution of flash droughts in terms of the speed of onset and the causes of such a rapid onset phase of flash droughts. Here, we present a comprehensive assessment of the onset development of flash droughts and the underlying mechanisms on a global scale. We find that 33.64-46.18% of flash droughts with 5-day onset of drying, and there is a significant increasing trend in the proportion of flash droughts with the 1-pentad onset time globally during the period 2000-2020. Flash droughts do not appear to be occurring more frequently in most global regions, just coming on faster. In addition, atmospheric aridity is likely to create a flash drought-prone environment, and the joint influence of soil moisture depletion and atmospheric aridity further accelerates the rapid onset of flash droughts. 

/pdf/accelerating-flash-droughts-induced-by-the-joint-influence-5u1y3dm8.pdf

Accelerating flash droughts induced by the joint influence of soil moisture depletion and atmospheric aridity

Note on Series of Positive Terms

Flash droughts are characterized by a period of rapid intensification over sub-seasonal time scales that culminates in the rapid emergence of new or worsening drought impacts. This study presents a new flash drought intensity index (FDII) that accounts for both the unusually rapid rate of drought intensification and its resultant severity. The FDII framework advances our ability to characterize flash drought because it provides a more complete measure of flash drought intensity than existing classification methods that only consider the rate of intensification. The FDII is computed using two terms measuring the maximum rate of intensification (FD_INT) and average drought severity (DRO_SEV). A climatological analysis using soil moisture data from the Noah land surface model from 1979–2017 revealed large regional and interannual variability in the spatial extent and intensity of soil moisture flash drought across the US. Overall, DRO_SEV is slightly larger over the western and central US where droughts tend to last longer and FD_INT is ~75% larger across the eastern US where soil moisture variability is greater. Comparison of the FD_INT and DRO_SEV terms showed that they are strongly correlated (r = 0.82 to 0.90) at regional scales, which indicates that the subsequent drought severity is closely related to the magnitude of the rapid intensification preceding it. Analysis of the 2012 US flash drought showed that the FDII depiction of severe drought conditions aligned more closely with regions containing poor crop conditions and large yield losses than that captured by the intensification rate component (FD_INT) alone.

Development of a Flash Drought Intensity Index

. The term “flash drought” is frequently invoked to describe droughts that
develop rapidly over a relatively short timescale. Despite extensive and
growing research on flash drought processes, predictability, and trends,
there is still no standard quantitative definition that encompasses all
flash drought characteristics and pathways. Instead, diverse definitions
have been proposed, supporting wide-ranging studies of flash drought but
creating the potential for confusion as to what the term means and how to
characterize it. Use of different definitions might also lead to different
conclusions regarding flash drought frequency, predictability, and trends
under climate change. In this study, we compared five previously published
definitions, a newly proposed definition, and an operational satellite-based
drought monitoring product to clarify conceptual differences and to
investigate the sensitivity of flash drought inventories and trends to the
choice of definition. Our analyses indicate that the newly introduced Soil
Moisture Volatility Index definition effectively captures flash drought
onset in both humid and semi-arid regions. Analyses also showed that
estimates of flash drought frequency, spatial distribution, and seasonality
vary across the contiguous United States depending upon which definition is used.
Definitions differ in their representation of some of the largest and most
widely studied flash droughts of recent years. Trend analysis indicates that
definitions that include air temperature show significant increases in flash
droughts over the past 40 years, but few trends are evident for
definitions based on other surface conditions or fluxes. These results
indicate that “flash drought” is a composite term that includes several
types of events and that clarity in definition is critical when monitoring,
forecasting, or projecting the drought phenomenon.

/pdf/flash-drought-onset-over-the-contiguous-united-states-2q0yng85h1.pdf

Flash drought onset over the contiguous United States: sensitivity of inventories and trends to quantitative definitions

In this paper, we prove some new dynamic inequalities from which some known dynamic inequalities on time scales, some integral and discrete inequalities due to Hardy, Copson, Chow, Levinson, Pachpatte Yang and Hwang will be deduced as special cases. Also, some new corresponding integral and discrete inequalities will be formulated. The results will be proved by employing the chain rule, integration by parts formula, Hölder’s inequality and Jensen’s inequality on time scales. Mathematics subject classification (2010): 26A15, 26D10, 26D15, 39A13, 34A40, 34N05.

/pdf/some-new-generalized-forms-of-hardy-s-type-inequality-on-6tybdroqu6.pdf

Some new generalized forms of Hardy's type inequality on time scales

Abstract In this paper, we prove some new dynamic inequalities on time scales which as special cases contain several generalizations of integral and discrete inequalities due to Hardy, Copson, Leindler, Bennett, Pachpatte and Pečarić and Hanjš.

Inequalities of Hardy type and generalizations on time scales

In this paper, we prove some new inequalities of Levinson-type on time scales. Also we will prove some new extensions of these inequalities via convexity.

Levinson type inequalitis and their extensions via convexity on time scales

In June of 2021 the Southwest United States experienced a record‐breaking heatwave. This heatwave came at a time when the region was in severe drought. As drought alters the surface energy budget in ways that affect lower atmosphere temperature and circulations, it is possible that the combined drought‐heat event was a cascading climate hazard, in which preexisting drought exacerbated the heatwave. We apply satellite observation and numerical experiments with the Weather Research and Forecasting (WRF) model to test for land‐atmosphere feedbacks during the heatwave consistent with drought influence. We find a modest positive drought‐heat effect, as WRF simulations that include the drought have marginally higher air temperatures than those that exclude the initial drought conditions, with more substantial effects in wetter, forested areas. Evidence of drought‐heat‐drought‐coupled feedbacks was similarly modest in our simulations, as accounting for drought preconditioning led to a small reduction in simulated precipitation in the region.

Mahmoud Osman

Papers

Flash drought onset over the contiguous United States: sensitivity of inventories and trends to quantitative definitions

Some new generalized forms of Hardy's type inequality on time scales

Inequalities of Hardy type and generalizations on time scales

Levinson type inequalitis and their extensions via convexity on time scales

Cascading Drought‐Heat Dynamics During the 2021 Southwest United States Heatwave