Extreme Precipitation Episodes Research Articles

Comparison of kinetic energy conversion characteristics of two extreme precipitation episodes during persistent unusually heavy rainfall on 17–23 July 2021 in Henan, China

A persistent heavy rainfall event, contributed by two extreme episodes (P1 and P2), occurred in Henan province of China from 17 to 23 July 2021. An active mesoscale vortex was present in the lower troposphere, along with a shallow trough moving eastward in the upper troposphere. The budgets of rotational kinetic energy (KR) and divergent kinetic energy (KD) at distinct stages are investigated. The lower-level KR anomaly, related to the mesoscale vortex growth therein, was responsible for P1. However, the upper-troposphere KR was dominant and strong during the P2 stage, associated with an eastward-developing upper-level trough. Prior to both P1 and P2, active inner interaction between rotational and divergent winds presented, manifested as a rapid increase of KVRVD signals. KVRVD and precipitation had similar pattern distribution and consistent evolution tendency. Therefore, KVRVD might be a useful indicator for the occurrence and intensification of rainfall. Expect for two components of winds interaction, the external source feeding kinetic energy growth is also explored. For P1, APE (available potential energy)- > KD- > KR was the key path towards rotational kinetic energy, providing energy for mesoscale vortex growth in lower troposphere, which was favorable for the extreme episode of P1. Nevertheless, for P2 stage, APE acted as a direct energy source towards KR in upper troposphere, with higher efficiency (via APE- > KR) and thus stronger KR. This kind of difference in conversion path depended on the vertically-coupled degree between the lower-level mesoscale vortex and the upper-level trough, in addition to the rotational and divergent wind speed. Their tilting/upright configuration produced strong baroclinic/barotropic process, accelerating energy conversion of APE- > KD/APE- > KR via GD/GR (i.e., generating KD/KR by divergent/rotational winds crossing geopotential height contours, usually as a generator resulting from barotropic/baroclinic process). Therefore, kinetic energy budget may be a valuable tool for identifying key weather systems development and forecasting extreme rainfall episodes.

The influence of an atmospheric river on a heavy precipitation event over the western Alps

On 2–3 October 2020, a heavy precipitation event severely affected northern Italy and in particular the western Alps, with rainfall amount exceeding 600 mm over 24 h. This event was associated with an upper-level trough over the western Mediterranean basin, a large-scale configuration typical of heavy precipitation phenomena on the southern side of the Alps, since it induces a northward transport of large amounts of moisture impinging on the orography. The present study shows that a relevant amount of moisture moved towards the Mediterranean basin in the form of an atmospheric river (AR), a long and narrow filament-shaped structure crossing the whole Atlantic Ocean, characterized in the present case by a maximum Integrated Vapour Transport exceeding 1000 kg m−1 s−1. Therefore, in addition to the local contribution from the Mediterranean Sea, a relevant amount of moisture moved from the Tropics towards the Mediterranean, feeding the precipitation systems.The presence of an AR represented a distinguishing aspect of the event, superimposed on the well-known dynamic-thermodynamic mechanisms of heavy precipitation over the Alps. High-resolution numerical simulations and diagnostic tools have been exploited to investigate in detail how the transport of water vapour associated with the AR has influenced the dynamics and favoured the severity of the heavy precipitation processes.The results disclose the role of the AR and add further details to the theoretical framework of heavy precipitation mechanisms in the Alpine area, improving our understanding of the complex interaction between large-scale flows and mesoscale dynamics during extreme precipitation episodes. Due to the relatively fast evolution of the synoptic disturbance, the typical mesoscale mechanisms would have led only to an ordinary intense rainfall event. The contribution of the AR turned the event into a devastating flood.

Parameters influencing run-off on vegetated urban soils: A case study in Marseilles, France

In urban areas, episodes of extreme precipitation may generate run-off floods, but urban soils are seldom considered in spatial planning documents. Our aim was to assess the infiltration of vegetated urban soils and to identify the soil parameters affecting infiltration in order to explore the potential of these soils for run-off mitigation under a Mediterranean climate with extreme precipitation episodes. We compared vegetated soils in the “urban” zone of the Marseilles’ 16th district (France) either lying on former clay quarries or tile factories (T) or not (NT) and with different land-uses. We used a simplified method to measure field saturated hydraulic conductivity (Kfs), measured penetration resistance in the first 0.15 m (Qd) and analysed physico-chemical properties in the first horizon. Levels of run-off production were estimated according to Kfs and an adaptation of USDA Hydrologic Soil Groups. All soils were Technosols or Anthrosols regardless of their prior history. We found that land-use history influenced Kfs, with T soils and soils with current vegetated land-use younger than 13 years having lower Kfs than NT soils and soils with land-use older than 13 years. Soil organic matter content influenced Kfs positively when soils were young, while vegetation cover and penetration resistance (most of the soils had Qd > 2 MPa) had not a clear effect on Kfs. Overall, combining Kfs and soil depth, 14.3% of the T soils and 84.6% of the NT soils had low levels of run-off production. It is therefore recommended that 1) Technosols or Anthrosols should be characterised for their unique physico-chemical and physical properties but also for their land-use history, and 2) the infiltration of these soils should be considered in spatial planning documents.

Assessment of the Impacts of Climate Change on Climatic Zones over the Korean Peninsula

In assessing the impact of climate change, the use of a multimodel ensemble (MME) is required to quantify uncertainties between scenarios and produce downscaled outlines for the simulation of climate under the influence of different factors including topography. This study of climate change scenarios from 13 global climate models (GCMs) assesses the impacts of future climate change. Unlike South Korea, North Korea lacks studies using climate change scenarios of the Coupled Model Intercomparison Project Phase 5 (CMIP5) and only recently did the country start the projection of extreme precipitation episodes. As such, one of the main purposes of this study is to predict changes in the average climatic conditions of North Korea in the future. The result of comparing downscaled climate change scenarios with observation data for a reference period indicates the high applicability of the MME. Furthermore, this study classifies climatic zones by applying the Köppen–Geiger climatic zones classification to the MME, which is validated for future precipitation and temperature. The result suggests that the continental climate that covers the inland area for the reference climate is expected to shift into the temperate climate. Moreover, the coefficient of variation (CV) in the temperature ensemble is particularly low for the southern coast of the Korean Peninsula, and, accordingly, a high possibility of the shifting climatic zone of the coast is predicted.

Extreme Precipitation Events in Summer in the Iberian Peninsula and Its Relationship With Atmospheric Rivers

This study identifies and characterizes the importance of the Atmospheric Rivers in the extreme precipitation episodes that strike the Iberian Peninsula and Portugal during the extended summer months (April to September) between 1950 and 2007. The extreme precipitation days are ranked taking into account a daily gridded precipitation database for the Iberian Peninsula at a 0.2o resolution. The ranking is based on the magnitude of the extreme precipitation days considering not only on the area affected above the 95th climatological percentile but also by the precipitation intensity within the anomalous area. The Atmospheric Rivers detection scheme is used for the North Atlantic Ocean basin that allows the identification of the persistent Atmospheric Rivers that impact the Iberian Peninsula for the extended summer months. It is shown, that there is a relationship between the Atmospheric Rivers and the extreme precipitation days in Portugal especially during the transition months of April, May and September. On the contrary when analysing the entire Iberia Peninsula the impact of ARs is considerably reduced. Moreover, the impacts of the Atmospheric Rivers is considerably higher for the top ranked events in Portugal but decreases when considering less intense extreme precipitation days.

Extreme precipitation in the Netherlands: An event attribution case study

Attributing the change in likelihood of extreme weather events, particularly those occurring at small spatiotemporal scales, to anthropogenic forcing is a key challenge in climate science. While a warmer world is associated with an increase in atmospheric moisture on a global scale, the impact on the magnitude of extreme precipitation episodes has substantial regional variability. Analysis of individual cases is important in understanding the extent of these changes on spatial scales relevant to stakeholders. Here, we present a probabilistic attribution analysis of the extreme precipitation that fell in large parts of the Netherlands on 28 July 2014. Using a step-by-step approach, we aim to identify changes in intensity and likelihood of such an event as a result of anthropogenic global warming while highlighting the challenges in performing robust event attribution on high-impact precipitation events that occur at small scales. A method based on extreme value theory is applied to observational data in addition to global and regional climate model ensembles that pass a robust model evaluation process. Results based on observations suggest a strong and significant increase in the intensity and frequency of a 2014-type event as a result of anthropogenic climate change but trends in the model ensembles used are considerably smaller. Our results are communicated alongside considerable uncertainty, highlighting the difficulty in attributing events of this nature. Application of our approach to convection-resolving models may produce a more robust attribution.

Ranking of multi‐day extreme precipitation events over the Iberian Peninsula

ABSTRACTAn objective method for ranking extreme precipitation episodes in different time scales over the Iberian Peninsula (IP) is presented. It relies on the extensive use of the high‐resolution (0.2°) gridded daily precipitation database available for the IP, spanning from 1950 to 2008. The sum of the normalized anomalies is computed over different time scales (2–10 days) to allow ranking the different anomalous precipitation multi‐day periods. The magnitude of a precipitation episode is given taking into account the area (in percentage) that has precipitation anomalies above two standard deviations (std) and the mean values of these anomalies over this area. With these criteria, we are able to evaluate the spatial extent of the precipitation episodes as well as their spatially integrated intensity. Different precipitation rankings are built for the entire IP, for Portugal and independently for six of the major river basins in the IP. The different rankings correctly detect and categorize the most extreme precipitation episodes which occurred on the various domains. For each domain, in general, results show that few events dominate the top ten of a particular ranking; specific extreme events at shorter time scales (e.g. 2–3 days) may be absent from the top rankings at longer time scales (e.g. 10 days). When comparing the different domains, the top ten events are generally different from each other, thus highlighting the spatial variability of extreme precipitation in the IP. Moreover the methodology used to build the rankings imply that some events are considered extreme due to particularly high precipitation totals, while other episodes are more dependent on extensive areas affected by less extreme precipitation values. The different rankings revealed to be an useful tool for future studies in identifying the meteorological impacts of extreme precipitation episodes at the regional scale covering relatively large areas.

Impacts of exceptional and extreme inter-annual climatic events on the net ecosystem carbon dioxide exchange of a Sitka spruce forest

Climatic parameters such as temperature and precipitation are key drivers of the global carbon cycle and both inter-annual climatic variability and extreme climatic events can influence carbon sequestration and greenhouse gas mitigation within terrestrial ecosystems. The impacts of the timing, intensity, frequency and duration of both exceptional and extreme inter-annual climatic events, characterised by major differences in rainfall and temperature, on a Sitka spruce forest in the Irish midlands were assessed using eddy covariance-based measurements of net ecosystem carbon dioxide exchange (NEE). Annual precipitation in 2009 (1156mm) represented an extremely wet year characterised by extreme precipitation episodes in July (155mm) and November (220mm), however the differences in precipitation were not uniformly distributed throughout the year with above average rainfall occurring during the growing season (April–August). The mean annual air temperature in 2010 (8.2°C) was 1.7°C lower than the 30 year mean and represented an extremely cold year, and extreme low temperature events, characterised by a number of extended sub-zero temperature periods, were recorded in December of 2009 and 2010 where the mean monthly temperatures were 0.7°C and −0.7°C, respectively. These climatic differences resulted in a 1.07tCha−1yr−1 difference between the annual estimates of NEE in 2009 (−8.14±1.94tCha−1yr−1) and 2010 (−9.21±0.99tCha−1yr−1), respectively. The measured NEE in 2009 and 2010 represented a 5.6% decrease and a 6.9% increase relative to the long-term mean measured at this site (2002–2008; −8.62±1.39tCha−1yr−1). The components of NEE, gross primary productivity (GPP) and ecosystem respiration (Reco), also showed differences between years. In 2009, GPP was ∼19% lower when compared to 2010, due to a reduction in net stand carbon assimilation at higher irradiances during the growing season. This was most likely due to the development of low oxygen conditions in the rooting zone during periods of higher soil water availability, which may have limited photosynthetic carbon assimilation through a reduction in stomatal conductance. The extended sub-zero temperatures experienced during the winter of 2010 had a greater impact on GPP, relative to Reco, resulting in a net loss of carbon during these periods. Variations in GPP were, however, positively correlated with Reco in both years. NEE was correlated with temperature in all years, with a slope (negative) of 0.2 to 0.3gCm−2d−1 per degree increase in air temperature. At temperatures below ∼0°C the forest was a carbon source, whilst there was a progressive increase in sink capacity as temperatures increased up to ∼20°C. The observed decrease in NEE was dependent on both the duration of exposure and the extent to which the temperature was reduced below zero. This information indicates that while temperature remains the key driver of NEE in this forest ecosystem, the observed reduction in NEE during periods of high water availability in 2009 suggests that under future climatic scenarios the magnitude and timing of extreme precipitation events may also have a significant impact on annual carbon gain.

Scaling and clustering effects of extreme precipitation distributions

Summary One of the impacts of climate change and human activities on the hydrological cycle is the change in the precipitation structure. Closely related to the precipitation structure are two characteristics: the volume ( m ) of wet periods (WPs) and the time interval between WPs or waiting time ( t ). Using daily precipitation data for a period of 1960–2005 from 590 rain gauge stations in China, these two characteristics are analyzed, involving scaling and clustering of precipitation episodes. Our findings indicate that m and t follow similar probability distribution curves, implying that precipitation processes are controlled by similar underlying thermo-dynamics. Analysis of conditional probability distributions shows a significant dependence of m and t on their previous values of similar volumes, and the dependence tends to be stronger when m is larger or t is longer. It indicates that a higher probability can be expected when high-intensity precipitation is followed by precipitation episodes with similar precipitation intensity and longer waiting time between WPs is followed by the waiting time of similar duration. This result indicates the clustering of extreme precipitation episodes and severe droughts or floods are apt to occur in groups.

Climatological aspects of extreme precipitation in Europe, related to mid-latitude cyclonic systems

Climatic aspects of extreme European precipitation are studied. Daily pluviometric data from 280 stations across Europe, covering the period from 1958 to 2000, are used. First, the criteria for extreme precipitation cases and episodes are communicated using threshold and spatial definitions. The cases and episodes meeting these criteria are grouped according to their area of appearance. Most of them are located in three major areas: Greece, the Alps, and the Iberian Peninsula. The existence of trends in the annual and seasonal time series of these extreme events is examined. Decreasing trends are found in most of the cases, for Greece, the Iberian Peninsula, and Europe, as a whole. The Alps present a different behavior, with no trend at all in the southern part, and a possible increasing trend in the northern part. Finally, the positive impact of altitude in the frequency of occurrence of extreme precipitation episodes in Europe is discussed.

MODELLING EXTREME PRECIPITATION IN HAZARDOUS MOUNTAINOUS AREAS. CONTRIBUTION TO LANDSCAPE PLANNING AND ENVIRONMENTAL MANAGEMENT

Abstract Extreme precipitation episodes are very common in Mediterranean area and can lead to serious and catastrophic environmental hazards. They have special incidence during autumn months, September, October and November (SON) with important impacts on society, leading frequently to significant economic losses and mortality. These events have special impact in mountainous areas where steep slopes enhance the effects of extreme precipitation. In mountainous areas rain gauge stations are sparse and normally in lower amount. Due to these reasons it is very important to map with higher accuracy the distribution of extreme precipitation. Also, in mountainous environments precipitation patterns can change in small distances that make the prediction more difficult, but also more important. A better prediction of areas with higher values of extreme precipitation will contribute to a better land use planning and avoid the effects of flash floods, land‐slides and soil erosion recognized as environmental problems...

On the roles of the northeast cold surge, the Borneo vortex, the Madden‐Julian Oscillation, and the Indian Ocean Dipole during the extreme 2006/2007 flood in southern Peninsular Malaysia

The mid‐December 2006 to late January 2007 flood in southern Peninsular Malaysia was the worst flood in a century and was caused by three extreme precipitation episodes. These extreme precipitation events were mainly associated with strong northeasterly winds over the South China Sea. In all cases, the northeasterlies penetrated anomalously far south and followed almost a straight trajectory. The elevated terrain over Sumatra and southern Peninsular Malaysia caused low‐level convergence. The strong easterly winds near Java associated with the Rossby wave‐type response to Madden‐Julian Oscillation (MJO) inhibited the counter‐clockwise turning of the northeasterlies and the formation of the Borneo vortex, which, in turn, enhanced the low‐level convergence over the region. The abrupt termination of the Indian Ocean Dipole (IOD) in December 2006 played a secondary role as warmer equatorial Indian Ocean helped in the MJO formation.

The role of dynamic and diabatic processes in the generation of cut-off lows over Northwest Africa

The present observational study addresses the role of dynamic and diabatic processes leading to the generation of four deep upper-level troughs/cut-offs, involved in two extreme precipitation episodes over West Africa during the cool season. The elongated potential vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the central North Atlantic. Strong deformation along the eastern side of the ridge leads to a thinning of the PV streamers. In some situations the tips of the streamers break up and form distinct and long-lived stationary cut-offs near West Africa, in particular if the presence of another PV ridge downstream allows a complete isolation from the midlatitude westerlies. In other situations a prior anticyclonic wave-breaking event over Europe leads to an advection of high-PV towards the Iberian Peninsula that merges with the streamer and impedes a complete cut-off. The observations presented here suggest that the rapid amplification of the PV ridges over the North Atlantic and thus the subsequent streamer formation are related to upstream latent heating through non-conservative diabatic reduction of upper-level PV and through the strong divergent outflow near the tropopause that support large negative isentropic PV advection. The intense latent heat release is promoted by cyclo- and frontogenesis, and the transport of warm, moist air by a low-level jet ahead of the surface cold front (often called a warm conveyor belt; WCB). Diabatic PV tendencies are highest where the WCB rises over the surface warm front to the northeast or east of the cyclone centre. In most cases the distinct heating occurs in connection with a strong upper-level jet and a rapid deepening of the involved surface cyclone. More quantitative dynamical and statistical studies of the suggested relation are needed to better understand the relative contributions of single factors to the large and synoptic scale evolution that leads to PV streamers/cut-offs near West Africa.

Climatological aspects of the extreme European rainfall of August 2002 and a trajectory method for estimating the associated evaporative source regions

Abstract. During the first half of August 2002, a sequence of extreme precipitation episodes affected many regions of central and southern Europe, culminating in one of the most severe flooding events ever experienced along sections of the river Elbe and its tributaries. In this paper, the synoptic meteorological situation during the primary flooding event, 11-13 August 2002, and its recent background is illustrated and discussed. Then, backward trajectory modelling of water vapour transport is employed to determine the sources and transport pathways of the moisture which rained out during the event. The Lagrangian trajectory model FLEXTRA is used together with high resolution operational meteorological analyses from the ECMWF to track a very large number of trajectories, initialized in a dense three-dimensional grid array over the extreme rainfall region. Specific humidity changes along each trajectory are mapped out to yield source-receptor relationships between evaporation and subsequent precipitation for the event. Regions of significant surface evaporation of moisture which later rained out were determined to be parts of the Aegean and Ligurian Seas during the initial stages of the event, while strong evaporation from eastern European land surfaces and from the Black Sea became dominant later on. The method also provides precipitation estimates based solely on specific humidity changes along Lagrangian airmass trajectories, which can be compared to ECMWF model forecast precipitation estimates.