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Dr. Nirajan Dhakal
Spelman College

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Research Keywords & Expertise

0 Climate Change
0 Sustainability
0 Water Resource Engineering
0 Water - Food - Energy Nexus
0 hydrologic extremes

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Preprint content
Published: 03 March 2021
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Despite the importance of seasonality of extreme precipitation events to stormwater management, there are limited number of studies examining seasonality of daily and monthly precipitation extremes over the contiguous United States. In this study, a circular statistical method was used for spatio-temporal assessment of seasonality of daily and monthly precipitation extremes and their teleconnections with large-scale climate patterns over the contiguous United States. Historic precipitation time series over the period of 64 years (1951–2014) for 1108 sites was used for the analysis. Calendar dates for extreme precipitation were used to characterize seasonality within a circular statistics framework which includes indices reflecting the mean date and variability of occurrence of extreme events. The rainfall seasonality during negative and positive phases of the El Niño–Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation were also investigated. Results showed that extreme precipitation seasonality varied across the contiguous United States with distinct spatial pattern of seasonality (strong seasonality) in the western and mid-western regions and mixed spatial pattern in the eastern region. In addition, extreme precipitation seasonality during negative and positive phases of three climate indices revealed that large-scale climate variabilities have strong influence on the mean date of occurrence of extreme precipitation but generally weak influence on the strength of seasonality in the contiguous United States. Results from our study might be helpful for sustainable water resource management, flood risk mitigation, and prediction of future precipitation seasonality.

 

ACS Style

Nirajan Dhakal; Bhikhari Tharu; Ali Aljoda. Seasonality of precipitation extremes and their connections with large-scale climate patterns over the contiguous United States. 2021, 1 .

AMA Style

Nirajan Dhakal, Bhikhari Tharu, Ali Aljoda. Seasonality of precipitation extremes and their connections with large-scale climate patterns over the contiguous United States. . 2021; ():1.

Chicago/Turabian Style

Nirajan Dhakal; Bhikhari Tharu; Ali Aljoda. 2021. "Seasonality of precipitation extremes and their connections with large-scale climate patterns over the contiguous United States." , no. : 1.

Journal article
Published: 09 July 2020 in Water
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Climate change is likely to impact precipitation as well as snow accumulation and melt in the Northeastern and Upper Midwest United States, ultimately affecting the quantity and seasonal distribution of streamflow. The objective of this study is to analyze seasonality of long-term daily annual maximum streamflow (AMF) records and its changes for 158 sites in Northeastern and Upper Midwest Unites States. A comprehensive circular statistical approach comprising a kernel density method was used to assess the seasonality of AMF. Temporal changes were analyzed by separating the AMF records into two 30-year sub-periods (1951–1980 and 1981–2010). Results for temporal change in seasonality showed mixed pattern/trend across the stations. While for majority of stations, the distribution of AMF timing is strongly unimodal (concentrated around spring season) for the period 1951–1980, the seasonal modes have weakened during the period 1981–2010 for several stations along the coastal region with simultaneous emergence of multiple modes indicating changes of seasonality therein. The fresh statistical approach based on non-parametric circular density estimates reduces some of the limitations of previous studies to detect and model event timing distributions with multiple seasons and addresses issues of non-stationarity in the data records of extreme events.

ACS Style

Nirajan Dhakal; Richard N. Palmer. Changing River Flood Timing in the Northeastern and Upper Midwest United States: Weakening of Seasonality over Time? Water 2020, 12, 1951 .

AMA Style

Nirajan Dhakal, Richard N. Palmer. Changing River Flood Timing in the Northeastern and Upper Midwest United States: Weakening of Seasonality over Time? Water. 2020; 12 (7):1951.

Chicago/Turabian Style

Nirajan Dhakal; Richard N. Palmer. 2020. "Changing River Flood Timing in the Northeastern and Upper Midwest United States: Weakening of Seasonality over Time?" Water 12, no. 7: 1951.

Journal article
Published: 21 November 2019 in International Journal of Climatology
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In the eastern US, the empirical probability distribution of extreme daily precipitation comprises heavy rainfall events stemming from North Atlantic tropical cyclones (TCs). At many locations, these events influence estimates of extreme value statistics (e.g., 100‐year event), thus have important bearing on the sizing of flood protection infrastructure and, in general, flood risk management and preparedness. Consequently, a characterization of location‐specific and regional patterns in precipitation extremes and changes therein have salience for both scientific and engineering concerns. To this end, analysis of seasonal and annual maximum daily precipitation at 667 long‐term stations across the eastern US was pursued to ascertain recent changes in the extreme events over the 1950‐2011 period. Three key results from this study illuminate less understood facets of recent changes in precipitation extremes: (a) an overall increase in the fraction of seasonal and annual maximum precipitation events linked to TCs, (b) a dramatic increase in the correlation between Accumulated Cyclone Energy Index and the leading principal component of extreme precipitation, and (c) changes in the spatial patterns of regions with highest TC‐related risk for heavy precipitation.

ACS Style

Nirajan Dhakal; Shaleen Jain. . International Journal of Climatology 2019, 1 .

AMA Style

Nirajan Dhakal, Shaleen Jain. . International Journal of Climatology. 2019; ():1.

Chicago/Turabian Style

Nirajan Dhakal; Shaleen Jain. 2019. "." International Journal of Climatology , no. : 1.

Journal article
Published: 09 May 2019 in Geosciences
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Almost every year, north Atlantic tropical cyclones (TCs) are responsible for significant socioeconomic losses across the Mid-Atlantic USA. However, the extent to which TC activity contributes to the changes in the probability distributions of the extreme precipitation have not yet been comprehensively characterized for this region. In this study, a quantile regression method was used to investigate the trends of the lower (τ = 0.2) and upper (τ = 0.8) quantiles of annual and seasonal daily maximum precipitation series for the region using the station-based daily precipitation data for the period 1950–2011. Results show that the rates of changes in the upper quantile have greatly strengthened for the region. Analysis of the spatial pattern of the lower and upper quantile trends for TC and non-TC extreme precipitation series shows that trends have larger magnitudes in most of the sites for TC precipitation series as compared with the non-TC precipitation series for both the lower and upper quantiles. Additionally, the highest trends are observed in the upper quantile for TC time series indicating that TC precipitation is contributing more to the upper tails of the extreme precipitation distribution as compared to the non-TC precipitation. Results from this study have implications for the improved design and reassessment of flood-controlling infrastructure.

ACS Style

Nirajan Dhakal. Changing Impacts of North Atlantic Tropical Cyclones on Extreme Precipitation Distribution across the Mid-Atlantic United States. Geosciences 2019, 9, 207 .

AMA Style

Nirajan Dhakal. Changing Impacts of North Atlantic Tropical Cyclones on Extreme Precipitation Distribution across the Mid-Atlantic United States. Geosciences. 2019; 9 (5):207.

Chicago/Turabian Style

Nirajan Dhakal. 2019. "Changing Impacts of North Atlantic Tropical Cyclones on Extreme Precipitation Distribution across the Mid-Atlantic United States." Geosciences 9, no. 5: 207.

Research article
Published: 14 April 2018 in International Journal of Climatology
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Changes in extreme precipitation are associated with changes in their probability distributions and the characteristics of quantiles derived from fitted distributions. In this study, the linear quantile regression method is employed to analyse spatio‐temporal trends of extreme precipitation and to study the impact of North Atlantic tropical cyclones in the distribution of extreme precipitation for the southeastern United States. Daily annual maximum precipitation over the period of 64 years (1950–2013) for 107 sites was used for the analysis. Our results show that changes in upper quantiles of the distributions of the extreme precipitation have occurred in the southeastern United States. Analysis of the potential changes in the distribution of the extreme precipitation by separating the historical record into two periods, that is, before and after 1981, reveals that upper‐quantile trends have increasing magnitude in most of the sites for the latest time period. Analysis of the impact of tropical cyclones in the extreme precipitation distribution shows that overall the heavy rainfall events in the recent decades may have been caused by tropical cyclones. Such results are particularly useful for water managers who are more concerned with extreme values rather than the averaged one. Hence, our study has significant implication in environmental and infrastructural assessment as well as disaster risk management.

ACS Style

Nirajan Dhakal; Bhikhari Tharu. Spatio-temporal trends in daily precipitation extremes and their connection with North Atlantic tropical cyclones for the southeastern United States. International Journal of Climatology 2018, 38, 3822 -3831.

AMA Style

Nirajan Dhakal, Bhikhari Tharu. Spatio-temporal trends in daily precipitation extremes and their connection with North Atlantic tropical cyclones for the southeastern United States. International Journal of Climatology. 2018; 38 (10):3822-3831.

Chicago/Turabian Style

Nirajan Dhakal; Bhikhari Tharu. 2018. "Spatio-temporal trends in daily precipitation extremes and their connection with North Atlantic tropical cyclones for the southeastern United States." International Journal of Climatology 38, no. 10: 3822-3831.