Large-scale Precipitation Events Research Articles

Role of cyclone activity in summer precipitation over the northern margin of Eurasia

In recent decades, the atmospheric moisture capacity has increased globally in concert with global warming, with a particularly notable warming trend in Arctic regions. However, due to limited observational data, the variation and causes of polar precipitation, especially large-scale precipitation events associated with Arctic cyclones, remain unclear. In this paper, GPM satellite data are compared with ERA5 reanalysis data to explore the characteristics of summer precipitation at the northern margin of the Eurasian region (NMER) and the influence of cyclone activity on precipitation. It is revealed that high precipitation values in the Arctic region, as indicated by the GPM and ERA5 data, are mainly concentrated at the NMER. However, the GPM data show an overall larger precipitation amount, while the station observations more closely agree with the ERA5 precipitation changes at the NMER. The cyclone identification results indicate that summer cyclones at the NMER are mainly distributed in the Barents, Kara and Laptev Seas, and the precipitation contribution rate of ERA5-derived cyclones is 37.35%, which is significantly higher than that of GPM-derived cyclones (29.47%). Furthermore, high cyclone activity results in more intense precipitation, with the top 5% of the strongest cyclones contributing 60% (GPM) and 40% (ERA5) to the total cyclonic precipitation.

Stable Isotopes Reveal the Effect of Canopy and Litter Layer Interception on Water Recharge in a Subtropical Manmade Forest of Southwest China

In areas completely covered by vegetation, the rainwater that passes through the canopy layer and litter layer is the source supplying surface runoff and seeping into the soil layer. To reveal the influence mechanisms of the canopy and litter layer on water supply in forest areas, this study conducted event-scale water sampling and hydrogen and oxygen isotopic comparison for the rainfall, canopy throughfall and litter layer throughfall in a manmade forest. The results show that canopy interception will lead to a more concentrated distribution and lower isotopic variability. The d-excess and the intercept and slope of the δD-δ18O regression line of the canopy throughfall are slightly higher than those of rainfall, reflecting that the evaporation fractionation caused by canopy interception is weak, and the selection process may play a leading role. Compared with the canopy throughfall, the distribution of the δ18O, δD and d-excess in the litter layer throughfall is more dispersed, and the slope and intercept of the δD-δ18O regression line are higher, indicating the strong influence of non-equilibrium fractionation on the water input caused by litter layer interception. The isotopic differences between the litter layer throughfall and canopy throughfall indicate that the water components of small-scale precipitation events (precipitation amount lower than 5 mm) and the early stage of large-scale precipitation events (precipitation amount higher than 25 mm) may be dissipated by litter layer interception, while that of the latter stage of large-size events is slightly affected. The findings of this study will be helpful to deeply understand the mechanisms of the water cycle in the forest ecosystem under the background of climate change and provide insights for the sustainable utilization of water resources.

Spatial and temporal variability in rainfall erosivity under Alpine climate - a Slovenian case study using optical disdrometer data

Rainfall erosivity is one of the most important parameters that influence soil erosion rates. It is characterized by a large spatial and temporal variability. For example, in Slovenia, which covers around 20,000 km2, the annual rainfall erosivity ranges from less than 1,000 MJ mm ha1 h1 to more than 10,000 MJ mm ha1 h1. Drop size distribution (DSD) data are needed to investigate rainfall erosivity characteristics. More than 2 years of DSD measurements using optical disdrometers located at six stations in Slovenia were used to investigate the spatial and temporal variability in rainfall erosivity in Slovenia. Experimental results have indicated that elevation is a poor predictor of rainfall erosivity and that erosivity is more strongly correlated to the mean annual precipitation. Approximately 90% of the total kinetic energy (KE) was accounted for in about 35% of 1 min disdrometer data. The highest 1 min intensities (I) and consequently also KE values were measured in summer followed by autumn and spring. The local KE-I equation yielded an acceptable fit to the measured data in case of all six stations. The relatively large percentage of 1 min rainfall intensities above 5 mm/h can at least partially explain some very high annual rainfall erosivity values (i.e., near or above 10,000 MJ mm ha1 h 1). Convective and large-scale precipitation events also result in various rainfall erosivity characteristics. The station microlocation and wind impacts in case of some stations yielded relatively large differences between the data measured using the optical disdrometer and the pluviograph. Preliminary conclusions have been gathered, but further measurements are needed to get even better insight into spatial and temporal variability in rainfall erosivity under Alpine climate in Slovenia.

Spatial and Temporal Variability in Rainfall Erosivity Under Alpine Climate: A Slovenian Case Study Using Optical Disdrometer Data

Rainfall erosivity is one of the most important parameters that influence soil erosion rates. It is characterized by a large spatial and temporal variability. For example, in Slovenia, which covers around 20,000 km2, the annual rainfall erosivity ranges from less than 1,000 MJ mm ha−1 h−1 to more than 10,000 MJ mm ha−1 h−1. Drop size distribution (DSD) data are needed to investigate rainfall erosivity characteristics. More than 2 years of DSD measurements using optical disdrometers located at six stations in Slovenia were used to investigate the spatial and temporal variability in rainfall erosivity in Slovenia. Experimental results have indicated that elevation is a poor predictor of rainfall erosivity and that erosivity is more strongly correlated to the mean annual precipitation. Approximately 90% of the total kinetic energy (KE) was accounted for in about 35% of 1 min disdrometer data. The highest 1 min intensities (I) and consequently also KE values were measured in summer followed by autumn and spring. The local KE-I equation yielded an acceptable fit to the measured data in case of all six stations. The relatively large percentage of 1 min rainfall intensities above 5 mm/h can at least partially explain some very high annual rainfall erosivity values (i.e., near or above 10,000 MJ mm ha−1 h−1). Convective and large-scale precipitation events also result in various rainfall erosivity characteristics. The station microlocation and wind impacts in case of some stations yielded relatively large differences between the data measured using the optical disdrometer and the pluviograph. Preliminary conclusions have been gathered, but further measurements are needed to get even better insight into spatial and temporal variability in rainfall erosivity under Alpine climate in Slovenia.

Extremeness of meteorological variables as an indicator of extreme precipitation events

Extreme large-scale precipitation events in central Europe are determined by the synoptic pattern, which is typically characterized by cyclones moving from the south into this region. Herein, we demonstrate that this synoptic pattern is accompanied by large-scale anomalies of dynamic and thermodynamic variables that occur in specific regions before or during the precipitation event. The extremeness of individual variables was evaluated by ERA-40 data relative to the corresponding climatology by the obtained distribution function values. Precipitation events leading to 14 significant summer floods in the period between 1958 and 2002 were used as reference events. The temporal correspondence of the anomalies among reference events was employed by selecting 26 synoptic predictors of extreme precipitation in the Czech Republic. A predictor evaluates the extremeness of a variable in a limited area on a specific day relatively to the start of the precipitation event (day D). While the predictors cover a five-day period from D − 2 to D + 2, most are relevant to the days with precipitation (from D to D + 2). The predictors cannot be considered individually because only opportune coincidences make the synoptic situations dangerous. In the present study, a combined EM index was used by evaluating the predictors as a whole, without considering their relationships. Days with minimum EM values correspond to significant flood events in the Czech Republic. Our results confirm that the synoptic pattern can be properly characterized by the extremeness of selected dynamic and thermodynamic variables, and that the anomalies are distinctly correlated with the extremity of large-scale precipitation and flood events in the Czech Republic. This approach is applicable to other regions where precipitation is significantly related to large-scale synoptic patterns, and could be employed also by operational detection of upcoming extreme precipitation events.

Severe storm in Bavaria, the Czech Republic and Poland on 12–13 July 1984: A statistic- and model-based analysis

This paper is devoted to a case study of one of the best-known historical severe storms in Europe. Organised convective systems with the characteristic behaviour of a supercell storm evolved in a meso-α scale convective complex, and the locally damaging weather affected a strip of land from Switzerland to Poland. The development of the storm resulted from a specific combination of processes at various spatial and temporal scales. This study focuses on an analysis of the synoptic and sub-synoptic meteorological conditions before and during the event. The re-analyses ERA-40 are used to investigate anomalies of dynamic and thermodynamic quantities at the synoptic scale. It is demonstrated that some quantities may reach abnormal values not only during large-scale precipitation events, but also during a case of severe convection. The non-hydrostatic limited-area NWP model COSMO is employed to re-forecast the event and to investigate the specific features of the convection environment with a horizontal resolution of 2.8 km. The skill of the re-forecast of precipitation is evaluated using verification methods based on selected categorical skill scores and area-related root mean square error values. Simulated precipitation and radar data show good agreement with observations in terms of daily totals and the spatio-temporal evolution of the storm. The outputs from the diagnostic Model for the Objective Analysis of Gust Fronts indicate that the simulated convection environment does not differ significantly from the real conditions. In addition, the analysis reveals vertical shear and stability conditions favourable for the intensification of convective processes by propagating gust fronts in the areas where new convection formed.

Observed precipitation in the Paran�-Plata hydrological basin: long-term trends, extreme conditions and ENSO teleconnections

The Parana-Plata basin is the second largest hydrological basin in South America and is of great importance for the countries of the region (Argentina, Bolivia, Brazil, Paraguay and Uruguay). The present study focuses on the long-term trends in basin-scale precipitation with special emphasis on the role of distribution changes in extreme large-scale precipitation events and on the characteristics and evolution of ENSO teleconnections over the last 50 years. First, we defined a Parana-Plata basin total precipitation index (PTPI) as the precipitations spatially averaged over the hydrological basin. On interannual time scales, such an index is mainly representative of anomalous monsoon precipitations in the northern part of the basin and large convective precipitation anomalies in the center of the basin (Paraguay-southern Brazil-Uruguay-northern Argentina) typical of the canonical ENSO teleconnection pattern. Our major findings clearly highlight a positive trend in yearly averaged PTPI mainly from the late 1960s to the early 1980s with a strong dependence from month-to-month. The largest precipitation increase is observed from November to May in southern Brazil and Argentina. A close examination of PTPI distributions during the two halves of the period 1950–2001 shows that the changes in the mean state from 1950–1975 to 1976–2001 result from significant changes in each calendar month mean state and in the tails of the PTPI anomaly distributions in May with lesser and weaker large-scale dry events and stronger large-scale wet events. Further studies will be needed to assess whether the observed trend in large-scale extreme precipitation conditions can be related to natural climate variability or anthropogenic activities and whether it is associated to changes in local/regional extreme events. The stronger wet conditions in different months seem to be associated to changes in ENSO characteristics (amplitude, propagation, spatial structure, ...) since the 1982–1983 El Nino. Indeed, spatial ENSO teleconnections (stronger in November and April–May) have greatly evolved from 1950–1975 to 1976–2001. Moreover, we demonstrate that there is a strong modulation and displacement of the teleconnection patterns from one event to another, impeding the definition of robust statistical relationship between ENSO and precipitation in the Parana-Plata basin (except maybe over a very limited area near the common border between Paraguay, Argentina and Brazil). Finally, the non-antisymmetrical patterns of precipitation between El Nino and La Nina conditions and the non-linear relationship between precipitation and either Nino3.4 or Nino1+2 sea surface temperature indices show that linear statistical forecast systems are actually of very limited use for impact predictions on society on a local or regional scale.