Variations In Precipitation Intensity Research Articles

Extreme Precipitation on Consecutive Days Occurs More Often in a Warming Climate

Abstract Extreme precipitation occurring on consecutive days may substantially increase the risk of related impacts, but changes in such events have not been studied at a global scale. Here we use a unique global dataset based on in situ observations and multimodel historical and future simulations to analyze the changes in the frequency of extreme precipitation on consecutive days (EPCD). We further disentangle the relative contributions of variations in precipitation intensity and temporal correlation of extreme precipitation to understand the processes that drive the changes in EPCD. Observations and climate model simulations show that the frequency of EPCD is increasing in most land regions, in particular, in North America, Europe, and the Northern Hemisphere high latitudes. These increases are primarily a consequence of increasing precipitation intensity, but changes in the temporal correlation of extreme precipitation regionally amplify or reduce the effects of intensity changes. Changes are larger in simulations with a stronger warming signal, suggesting that further increases in EPCD are expected for the future under continued climate warming.

Simple and approximate estimations of future precipitation return values

Abstract. We present estimates of future 20-year return values for 24 h precipitation based on multi-model ensembles of temperature projections and a crude method to quantify how warmer conditions may influence precipitation intensity. Our results suggest an increase by as much as 40–50 % projected for 2100 for a number of locations in Europe, assuming the high Representative Concentration Pathway (RCP) 8.5 emission scenario. The new strategy was based on combining physical understandings with the limited information available, and it utilised the covariance between the mean seasonal variations in precipitation intensity and the North Atlantic saturation vapour pressure. Rather than estimating the expected values and interannual variability, we tried to estimate an upper bound for the response in the precipitation intensity based on the assumption that the seasonal variations in the precipitation intensity are caused by the seasonal variations in temperature. Return values were subsequently derived from the estimated precipitation intensity through a simple and approximate scheme that combined the 1-year 24 h precipitation return values and downscaled annual wet-day mean precipitation for a 20-year event. The latter was based on the 95th percentile of a multi-model ensemble spread of downscaled climate model results. We found geographical variations in the shape of the seasonal cycle of the wet-day mean precipitation which suggest that different rain-producing mechanisms dominate in different regions. These differences indicate that the simple method used here to estimate the response of precipitation intensity to temperature was more appropriate for convective precipitation than for orographic rainfall.

Seasonal Differences in Precipitation Sensitivity to Soil Moisture in Bangladesh and Surrounding Regions

Abstract Precipitation sensitivity to soil moisture and its seasonal and diurnal changes are investigated in Bangladesh and surrounding regions using a regional climate model with a 5-km grid spacing. In the control experiment, soil moisture is calculated by a land surface scheme, and simulated accuracy of seasonal and diurnal variations in precipitation intensity and frequency is capable of assessing the soil moisture impact on precipitation. In sensitivity experiments with wetter land surfaces, daytime precipitation intensity decreases over the southern plains for both the premonsoon and mature monsoon seasons because of the weakening of surface heating and vertical mixing in the planetary boundary layer (PBL). Weakened vertical turbulent flux of moisture reduces condensation heating and upward motion in the mid- and upper troposphere, which suppresses development of convective precipitation. The simulated precipitation intensity response to soil moisture suggests that land surface wetness contributes to the seasonal contrast in observed precipitation intensity (i.e., stronger in the premonsoon than the mature monsoon seasons). Meanwhile, the precipitation frequency response to soil moisture varies with season and by region. Over the southern plains in the wet land surface experiments, daytime precipitation frequency decreases (increases) during the premonsoon (mature monsoon) season compared with the dry land surface experiments, as influenced by seasonal differences in relative humidity and the condensation process in the lower troposphere. Around the northern mountainous area, higher soil moisture increases precipitation frequency regardless of season because of additional water vapor supply from the ground and frequent orographic precipitation forced by the mountainous topography.

Precipitation Intensity Effects on Groundwater Recharge in the Southwestern United States

Episodic recharge as a result of infrequent, high intensity precipitation events comprises the bulk of groundwater recharge in arid environments. Climate change and shifts in precipitation intensity will affect groundwater continuity, thus altering groundwater recharge. This study aims to identify changes in the ratio of groundwater recharge and precipitation, the R:P ratio, in the arid southwestern United States to characterize observed changes in groundwater recharge attributed to variations in precipitation intensity. Our precipitation metric, precipitation intensity magnification, was used to investigate the relationship between the R:P ratio and precipitation intensity. Our analysis identified significant changes in the R:P ratio concurrent with decreases in precipitation intensity. The results illustrate the importance of precipitation intensity in relation to groundwater recharge in arid regions and provide further insights for groundwater management in nonrenewable groundwater systems and in a changing climate.

Relationships of bankfull channel width and discharge parameters for modern fluvial systems in the Japanese Islands

The relationships between the bankfull channel width and the mean, bankfull, and maximum discharges of Japanese rivers were examined using the hydrological and geomorphological data from 368 sites. The relationships between the bankfull channel width and the mean and bankfull discharges do not show any distinct regional variations. In contrast, the relationship between the maximum discharge and the bankfull channel width shows regional variations, and the lower and higher maximum discharges relative to bankfull channel widths are documented in the fluvial systems in Hokkaido and Southwest Japan, respectively. These variations are interpreted to reflect regional variations in precipitation intensity during the rainy season, and the magnitude and frequency of typhoon-related flooding. The relationship between the bankfull channel width and the bankfull discharge can be described by an empirical equation similar to that described from modern fluvial systems on the European and American continents. Consequently, this empirical equation may have wider applicability for the estimation of hydrological features of modern and ancient fluvial systems, not only in active margin settings influenced by mid-latitude temperate climates, but also in passive continental margins and continental interior basins.

Spatial analysis of variations in precipitation intensity in the USA

In this study, we used various spatial analytical methods to examine variations and trends in precipitation intensity in the conterminous USA. We found that three different measures of precipitation intensity were highly correlated; intensity increased in a spatially coherent fashion in the northeastern quarter of the USA and generally decreased in the center portion of the western USA. Evidence is presented that spatial and temporal patterns in the trends of precipitation intensity are related to the Atlantic multidecadal oscillation. Our results are generally in agreement with others who are reporting an upward trend in precipitation intensity during a period when the planet appears to have warmed.

Elements of Fractal Generalization of Dual‐Porosity Model for Solute Transport in Unsaturated Fractured Rocks

In this study, new elements were developed to generalize the dual‐porosity model for moisture infiltration and solute transport in unsaturated rocks, taking into account fractal aspects of the percolation process. Random advection was considered as a basic mechanism of solute transport in self‐similar fracture systems. In addition to spatial variations in the infiltration velocity field, temporal fluctuations were also taken into account. The rock matrix, which is a low‐permeability component of the heterogeneous geologic medium, acts as a trap for solute particles and moisture. Scaling relations were derived for the moisture infiltration flux, the velocity correlation length, the average velocity of infiltration, and the velocity correlation function. The effect of temporal variations in precipitation intensity on the infiltration processes was analyzed. It showed that the mode of solute transport is determined by the power exponent in the advection velocity correlation function and the dimensionality of the trapping system, both of which may change with time. Therefore, depending on time, various transport regimes may be realized: superdiffusion, subdiffusion, or classical diffusion. The complex structure of breakthrough curves arising from changes in the transport regimes was also examined. A renormalization of the solute source strength due to characteristic fluctuations of highly disordered media was established.

Radar Observations of Intense Orographic Precipitation Associated with Typhoon Xangsane (2000)

Abstract With measurements from two ground-based Doppler radars located in northern Taiwan, this study documents the detailed aspects of intense orographic precipitation associated with Typhoon Xangsane (2000) as it moved northward immediately off the eastern coast of Taiwan, bringing strong low-level northeasterly to north-northeasterly winds impinging on the mountainous northern coast. With relatively good, persistent coverage of radar echoes on both inland and upstream regions, this particular event provides a unique description of the orographic precipitation and its relationship with orographic geometry, strong upstream oncoming flow, and the precipitation inherently associated with typhoon circulations. In this case, the heaviest precipitation was observed to occur primarily over two coastal mountain barriers: Mount Da-Tun (DT) and the Nangang-Keelung Range (NKR). Barrier DT is an approximately 3D mountain barrier, and the NKR, adjacent to the southeast of DT, is a relatively lower, narrower 2D mountain range. In particular, the distinct distribution and intensity of precipitation over the two barriers were observed. Analyses of vertical cross sections passing through the major regions of heavy precipitation over DT and NKR indicate the region of low-level heavy precipitation tended to shift downstream as the low-level oncoming flow intensified, and the precipitation exhibited a deeper, wider extent and stronger intensity at stronger oncoming flow regimes. However, changes in the location of maximum precipitation over DT (NKR) were confined mainly to regions over windward slopes (near and downstream of the mountain crest). The degree of downstream shift of low-level heaviest precipitation with respect to different magnitudes of oncoming flow was relatively limited (∼8 km) over NKR, as compared with a larger downstream shift of ∼15–17 km over DT. This contrasting aspect can be understood as a consequence of a longer “lifting section” and relatively lower fall speed of hydrometeors over the windward slope of DT. In addition, the precipitation inherently associated with the typhoon circulations was found to be an important contributor to the observed variations in precipitation intensity over DT and NKR. Stronger background typhoon precipitation and a shorter downstream shift of precipitation (i.e., a quasi-stationary precipitation feature) over NKR may explain the fact of larger precipitation accumulation observed over this narrower, lower barrier.

Estimation of the Equivalent Radar Reflectivity Factor from Measured Snow Size Spectra

In this paper, a method for the estimation of radar reflectivity from measured snow particle size distributions is presented based on earlier works of Marshall and Gunn and of Smith. During two snowfalls, the method was applied to estimate the equivalent reflectivity factor from measured snow size distributions obtained by the Particle Size and Velocity (PARSIVEL) optical disdrometer. The results are compared with the data of conventional C-band Doppler radar. Here, two snowfalls are presented as case studies. In addition, a comparison during one rainfall is included, which shows good agreement between the two instruments. In the case of snow, the calculation of the equivalent reflectivity factor from the PARSIVEL data is based on a relation between the mass and the size of the snow particles. In this study, a mass–size relation for graupel-like snow was used for all snowfalls. Because this is a crude description of naturally occurring snow, which can be of any other type (e.g., dendrites), the differences with the radar-measured reflectivities here are strongly dependent on the snow particle type. Nevertheless, in one case, the snowfall was fairly homogeneous in time, space, and snow type, and so the agreement was reasonably good, with a relatively constant underestimation (3–5 dB) of the radar data by PARSIVEL and a low variance of the differences. This underestimation could be due to non-graupel-like particles or the tendency of PARSIVEL to underestimate the reflectivities, as outlined in the text. The other snowfall was convective, with strong spatial and temporal variations in precipitation intensity and snow type. The instrument differences in this case ranged from −6 to 16 dB because of changing snow types, but both instruments showed the same qualitative variations. The agreement can be improved by an advanced signal processing of PARSIVEL in which the snow type is determined automatically and a proper mass–size relation is used.

Trends in Canadian precipitation intensity

Past research has unveiled important variations in total precipitation, often related to large‐scale shifts in atmospheric circulation, and consistent with projected responses to enhanced greenhouse warming. More recently, however, it has been realized that important and influential changes in the variability of daily precipitation events have also occurred in the past, often unrelated to changes in total accumulation. This study aims to uncover variations in daily precipitation intensity over Canada and to compare the observed variations with those in total accumulation and two dominant modes of atmospheric variability, namely the North Atlantic Oscillation (NAO) and the Pacific/North America teleconnection pattern (PNA). Results are examined on both annual and seasonal bases, and with regions defined by similarities in monthly variability. Seasonally increasing trends in total precipitation that result from increases in all levels of event intensity during the 20th century are found in southern areas of Canada. During the latter half of the century increases are concentrated in heavy and intermediate events, with the largest changes occurring in Arctic areas. Variations in precipitation intensity can, however, be unrelated to variations in the total accumulation. Consistent with these differences, the precipitation responses to the NAO and PNA are often found to occur only at specific levels of event intensity. Precipitation responses to the NAO occur in northeastern regions in summer and winter with the intensity affected in both seasons. The PNA strongly influences precipitation in many regions of the country during autumn and winter. In particular, it strongly influences variations in southern British Columbia and the Prairies, affecting the intensity in only some areas. However, it only influences the frequency of heavier events in autumn and winter in Ontario and southern Quebec, where this response is actually more robust than the response in total accumulation. During these seasons a negative PNA generally leads to more extreme precipitation events.

Observed and model‐simulated diurnal cycles of precipitation over the contiguous United States

We analyzed diurnal variations in precipitation, surface pressure, and atmospheric static energy over the United States from observations and NCAR regional climate model (RegCM) simulations. Consistent with previous studies, the mean (1963–1993) pattern of the diurnal cycle of summer U.S. precipitation is characterized by late afternoon maxima over the Southeast and the Rocky Mountains and midnight maxima over the region east of the Rockies and the adjacent plains. Diurnal variations of precipitation is weaker in other seasons, with early to late morning maxima over most of the United States in winter. The diurnal cycle in precipitation frequency accounts for most of the diurnal variations, while the diurnal variations in precipitation intensity are small. The broad pattern of the diurnal cycle of summer precipitation is fairly stable, but the interannual variability in the diurnal cycle of winter precipitation is large. The diurnal cycle of July connective available potential energy (CAPE) is dominated by a solar driven march of a high‐CAPE (2–4 kJ kg−1 ) tongue moving from the Southeast into the Northwest, with maximum values in the late afternoon to early evening over most of the United States. The solar driven diurnal and semidiurnal cycles of surface pressure result in significant large‐scale convergence over most of the western United States during the day and over the region east of the Rockies at night. The diurnal cycle of low‐level large‐scale convergence suppresses daytime convection and favors nighttime moist convection over the region east of the Rockies and the adjacent plains. The nocturnal maximum in the region east of the Rockies is also enhanced by the eastward propagation of late afternoon thunderstorms generated over the Rockies. Over the Southeast and the Rockies, both the static instability and the surface convergence favor afternoon moist convection in summer, resulting in very strong late afternoon maxima of precipitation over these regions. RegCM simulations of 1993 summer precipitation with three different cumulus convection schemes (Grell, Kuo, CCM3) all had deficiencies in capturing the broad pattern of the diurnal cycle of precipitation over the United States. The model also overestimated precipitation frequency and underestimated precipitation intensity. The simulated diurnal cycles of surface pressure and CAPE were weak compared to observations. All the schemes produced too much cloudiness over the Southeast for July 1993 which reduced surface solar radiation and thus daytime peak warming at the surface. The model's criteria for onset of moist convection appear to be too weak, so moist convection in the model starts too early and occurs too often with all the three schemes.