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Peter P. Sullivan and James C. McWilliams

Abstract

Imagery and numerical modeling show an abundance of submesoscale oceanic eddies in the upper ocean. Large-eddy simulation (LES) is used to elucidate eddy impacts on the atmospheric boundary layer (ABL) forced by winds, convection, and an eddy with varying radius; the maximum azimuthal eddy speed is 1 m s−1. Simulations span the unstable regime −1/L = [0, ∞], where L is the Monin–Obukhov (M–O) stability parameter. A linearized Ekman model and the LES couple ABL winds to an eddy through rough-wall M–O boundary conditions. The eddy currents cause a surface stress anomaly that induces Ekman pumping in a dipole horizontal pattern. The dipole is understood as a consequence of surface winds aligned or opposing surface currents. In free convection a vigorous updraft is found above the eddy center and persists over the ABL depth. Heterogeneity in surface temperature flux is responsible for the full ABL impact. With winds and convection, current stress coupling generates a dipole in surface temperature flux even with constant sea surface temperature. Wind, pressure, and temperature anomalies are sensitive to an eddy under light winds. The eddy impact on ABL secondary circulations is on the order of the convective velocity scale w* but grows with increasing current speed, decreasing wind, or increasing convection. Flow past an isolated eddy develops a coherent ABL “wake” and secondary circulations for at least five eddy radii downwind. Kinetic energy exchanges by wind work indicate an eddy-killing effect on the oceanic eddy current, but only a spatial rearrangement of the atmospheric wind work.

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Deon Terblanche, Amanda Lynch, Zihan Chen, and Scott Sinclair

Abstract

Patterns of freshwater availability—its variability and distribution—are already shifting as a function of global climate change and climate variability. High-resolution global gridded reanalysis products present an important tool to understand the already observed changes and thereby improve future scenarios as the climate evolves. A historical 100-yr-long district rainfall dataset and a unique set of highly detailed rainfall data from the highveld of South Africa spanning a 10-yr period provide an opportunity to independently evaluate the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis product. Evaluation is challenged by the episodic nature of significant rainfall events of southern Africa as well as differences in spatial and temporal resolution between model output and surface precipitation data. Here we present a convergent methodology spanning annual to event time scales and regional to gauge-level spatial scales to identify the characteristics of systematic biases in variability and amount of rain as well as timing of events. We find that ERA5 is consistently wetter than observed in ways that affect the timing of individual events while performing well on metrics associated with large-scale trends and seasonal variability. Errors are associated with both stratiform and convective rainfall types, but the timing of onset of convective rainfall is a challenge that is critical in this summer-rainfall-dominated region.

Significance Statement

High-resolution gridded datasets are invaluable tools for gaining improved understanding of historical rainfall variations under the influence of climate change. In addition, these datasets provide consistent information for purposes such as water resources management. Quantification of dataset biases provides important guidance for robust decision-making as well as for the development of future climate scenarios. However, rainfall is an especially challenging quantity to assess. With the increasing incidence of drought and flood, methods that independently validate this high-resolution gridded data are needed to ensure high-quality knowledge support. This study demonstrates an approach using convergent streams of evidence to assess the European Centre for Medium-Range Weather Forecasts gridded rainfall dataset with the purpose of better understanding the evolving characteristics of rainfall in southern Africa.

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Denis S. Willett, Brian White, Tom Augspurger, Jonathan Brannock, Jenny Dissen, Patrick Keown, Otis B. Brown, and Adrienne Simonson
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Xiaowei Hong, Riyu Lu, Shangfeng Chen, and Shuanglin Li

Abstract

The Silk Road pattern (SRP), which is the leading mode of upper-tropospheric meridional wind anomalies over midlatitude Eurasia, has been widely used to explain the impacts of the summer North Atlantic Oscillation (SNAO) on East Asian climate. However, the relationship between the SNAO and SRP has not been fully elaborated yet. This study classifies the SNAO into two categories according to whether it is closely associated with the SRP or not: the strongly linked category and weakly linked category, on the interannual time scale. The SNAO of the strongly linked category features a concentrated and significant southern pole over the northwestern Europe, and corresponding significant negative (positive) precipitation and upper-tropospheric wind convergence (divergence) anomalies over the northwestern Europe. The wind convergence (divergence) anomalies directly induce the positive (negative) planetary vortex stretching anomalies, which contribute overwhelmingly to positive (negative) Rossby wave source anomalies of the northwestern Europe. These Rossby wave source anomalies, acting as disturbances, further inspire circulation anomalies of surrounding regions, including meridional wind anomalies over the Caspian Sea, which are crucial for the SRP formation. As a result, the downstream SRP is triggered. All these essential features responsible for a strong SNAO–SRP linkage are weak for the weakly linked category. The SNAO–SRP correspondence on the interdecadal time scale is also discussed, and generally similar results are found. Results suggest the importance of shapes for the SNAO southern pole (including the location, the space extent, and the intensity) in determining whether the SNAO can closely link the SRP. Therefore, the shape of the SNAO southern pole should be involved in the discussion of the SNAO’s remote impacts.

Open access
Martin Rempel, Peter Schaumann, Reinhold Hess, Volker Schmidt, and Ulrich Blahak

Abstract

A wealth of forecasting models is available for operational weather forecasting. Their strengths often depend on the lead time considered, which generates the need for a seamless combination of different forecast methods. The combined and continuous products are made in order to retain or even enhance the forecast quality of the individual forecasts and to extend the lead time to potentially hazardous weather events. In this study, we further improve an artificial neural network–based combination model that was recently proposed in a previous paper. This model combines two initial precipitation ensemble forecasts and produces exceedance probabilities for a set of thresholds for hourly precipitation amounts. Both initial forecasts perform differently well for different lead times, whereas the combined forecast is calibrated and outperforms both initial forecasts with respect to various validation scores and for all considered lead times (from +1 to +6 h). Moreover, the robustness of the combination model is tested by applying it to a new dataset and by evaluating the spatial and temporal consistency of its forecasts. The changes proposed further improve the forecast quality and make it more useful for practical applications. Temporal consistency of the combined product is evaluated using a flip-flop index. It is shown that the combination provides a higher persistence with decreasing lead times compared to both input systems.

Free access
Yumeng Liu, Xianhong Meng, Lin Zhao, Zhaoguo Li, Hao Chen, Lunyu Shang, Shaoying Wang, Lele Shu, and Guangwei Li

Abstract

Under the intensification of global warming, the characteristics of the Three Rivers source region (TRSR; i.e., headwaters of the Yellow River, the Yangtze River, and the Lancang River) in China were diagnosed in the summer season from 1979 to 2015 using observations and reanalysis data. The diagnoses indicate that summer precipitation decreased from 1979 to 2002 [by 9.01 mm day−1 (10 yr)−1; p < 0.05 by Student’s t test] and increased significantly after 2002 [by 5.52 mm day−1 (10 yr)−1]. This abrupt change year (2002) was further confirmed by the cumulative anomaly method, the moving t-test method, and the Yamamoto method. By compositing the thermodynamics before and after the abrupt change year (2002), the results reveal that increased water vapor and more substantial lower-level convergence were present over the TRSR during 2003–15. This marked interdecadal variability in the TRSR summer precipitation responded to the interdecadal position and intensity of the large-scale forcing East Asian westerly jet (EAWJ), which is significantly modulated by the low-frequency variability associated with Southern Oscillation index. The connection between the interannual TRSR precipitation and the location and intensity of EAWJ was also explored. The position index of the EAWJ is negatively (with correlation coefficient R of −0.446; p < 0.05 by Student’s t test) correlated with the precipitation over the TRSR, implying that southward and northward years of EAWJ are respectively associated with intensifying and weakening the TRSR summer precipitation, whereas the intensity of EAWJ is insignificantly correlated with the TRSR summer precipitation.

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Xueheng Shi, Claudie Beaulieu, Rebecca Killick, and Robert Lund

Abstract

This paper presents a statistical analysis of structural changes in the Central England temperature series, one of the longest surface temperature records available. A changepoint analysis is performed to detect abrupt changes, which can be regarded as a preliminary step before further analysis is conducted to identify the causes of the changes (e.g., artificial, human-induced, or natural variability). Regression models with structural breaks, including mean and trend shifts, are fitted to the series and compared via two commonly used multiple changepoint penalized likelihood criteria that balance model fit quality (as measured by likelihood) against parsimony considerations. Our changepoint model fits, with independent and short-memory errors, are also compared with a different class of models termed long-memory models that have been previously used by other authors to describe persistence features in temperature series. In the end, the optimal model is judged to be one containing a changepoint in the late 1980s, with a transition to an intensified warming regime. This timing and warming conclusion is consistent across changepoint models compared in this analysis. The variability of the series is not found to be significantly changing, and shift features are judged to be more plausible than either short- or long-memory autocorrelations. The final proposed model is one including trend shifts (both intercept and slope parameters) with independent errors. The analysis serves as a walk-through tutorial of different changepoint techniques, illustrating what can be statistically inferred.

Open access
Jiuchang Wei, Qianwen Shao, Yang Liu, and Dora Marinova

Abstract

The link between climate change and human conflict has received substantial attention in academic research using different measures of “conflict”; however, it is yet to interpret interpersonal violence in terms of homicide. This study takes a global perspective to investigate how climate change, typically represented by temperature and precipitation, directly and indirectly affects national homicide rates across countries. From longitudinal archival data from 171 countries from 2000 to 2018, we detect a direct and positive relationship between higher temperatures and homicide, whereas an indirect pathway between wetter climate and homicide through the occurrence of more natural hazards has also been shown in our empirical results. The relationship between climate change and homicide can be moderated by the level of information and communication technologies (ICT). We conclude that the development of ICT contributes to building the countries’ resilience to climate change with better information and communication technologies to help alleviate the negative impacts of climate change on homicide.

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L. van Schalkwyk, R. C. Blamey, L. L. Dyson, and C. J. C. Reason

Abstract

A climatology of synoptic drylines on the subtropical southern African interior plateau (SAP) is developed using ERA5 reanalysis specific humidity and surface temperature gradients and an objective detection algorithm. Drylines are found to occur regularly during spring and summer (September–March), and almost daily during December of that period, but rarely in winter. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November with a mean of 10 drylines and then over the central (mean of 12) and western SAP (mean of 20) in December. During midsummer, drylines over the eastern SAP are negatively correlated with drylines in the west. Between 1980 and 2020, a significant correlation exists between ENSO and dryline days over the eastern (r = 0.44; p value = 0.004) and central (r = 0.41; p value = 0.008) SAP with fewer drylines (up to 10) occurring during years with increased surface moisture and more drylines (up to 45) occurring during years with decreased surface moisture. Drylines forming over the eastern parts of the SAP were more likely to move westward than drylines over the central and western parts. Onset times across the SAP show that drylines have a tendency to form during either the late morning to early afternoon (1100 and 1400 LST) or during the early evening hours (1700 and 2000 LST), suggesting that the surface heat trough (Kalahari heat low) and westward moisture transport mechanisms, such as the Limpopo low-level jet and ridging highs, are responsible for the formation of most drylines across the SAP.

Significance Statement

“Drylines” are used to describe boundaries separating regions of very dry air from those with much higher moisture content. The importance of these drylines is that they tend to act as a trigger for thunderstorms, which can produce severe weather. In this study, we build a long-term climatological description of drylines in subtropical southern Africa. We find that drylines are most frequent over eastern South Africa during the early summer, a time when storms with large hail and damaging winds are most likely to occur. Drylines are sensitive to moisture circulation patterns and respond differently during El Niño and La Niña years, with generally more drylines during El Niño over eastern South Africa and fewer during La Niña.

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Seung-Hee Ham, Seiji Kato, Fred G. Rose, Sunny Sun-Mack, Yan Chen, Walter F. Miller, and Ryan C. Scott

Abstract

Cloud vertical profile measurements from the CALIPSO and CloudSat active sensors are used to improve top-of-atmosphere (TOA) shortwave (SW) broadband (BB) irradiance computations. The active sensor measurements, which occasionally miss parts of the cloud columns because of the full attenuation of sensor signals, surface clutter, or insensitivity to a certain range of cloud particle sizes, are adjusted using column-integrated cloud optical depth derived from the passive MODIS sensor. Specifically, we consider two steps in generating cloud profiles from multiple sensors for irradiance computations. First, cloud extinction coefficient and cloud effective radius (CER) profiles are merged using available active and passive measurements. Second, the merged cloud extinction profiles are constrained by the MODIS visible scaled cloud optical depth, defined as a visible cloud optical depth multiplied by (1 − asymmetry parameter), to compensate for missing cloud parts by active sensors. It is shown that the multisensor-combined cloud profiles significantly reduce positive TOA SW BB biases, relative to those with MODIS-derived cloud properties only. The improvement is more pronounced for optically thick clouds, where MODIS ice CER is largely underestimated. Within the SW BB (0.18–4 μm), the 1.04–1.90-μm spectral region is mainly affected by the CER, where both the cloud absorption and solar incoming irradiance are considerable.

Significance Statement

The purpose of this study is to improve shortwave irradiance computations at the top of the atmosphere by using combined cloud properties from active and passive sensor measurements. Relative to the simulation results with passive sensor cloud measurements only, the combined cloud profiles provide more accurate shortwave simulation results. This is achieved by more realistic profiles of cloud extinction coefficient and cloud particle effective radius. The benefit is pronounced for optically thick clouds composed of large ice particles.

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