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Title: Precipitation Variability and Associated Wet and Dry Spell Characterization in Pakistan
Authors: Ahmed, Nabeel
Keywords: Engineering & Technology
Meterology (RS and GIS)
Engineering Meterology
Issue Date: 2020
Publisher: COMSATS University, Islamabad.
Abstract: Wet (dry) spells can cause extreme climatic conditions, such as floods (droughts) which can adversely influence the natural resources. A spatio-temporal analysis of observed wet and dry spells is carried out for vulnerable and data sparse region of Pakistan (24ᴼ–38ᴼN and 61ᴼ–78ᴼE). Observed monthly precipitation datasets from 46 weather stations are used for a length of consecutive 32 years (1976−2007). Additionally, after bias adjustment, the Asian Precipitation Highly-Resolved Observational Data Integration Towards Evaluation (APHRODITE) precipitation dataset, abbreviated as APH, is utilized to corroborate the findings. Decadal variability of precipitation for observed and APH datasets indicates that there is gradual decrease in wet spell length for arid and humid regimes, as compared to semi-arid regimes where there is no change in wet spell length. Monthly dry and wet slope difference (SD), on a log-log plot between number of spells and spell length, is used to classify precipitation regimes in Pakistan, for the first time. A weather station is categorized as humid if SD is less than -2.38 in units of number of spells per length of spells. If SD lies between -2.37 and -0.51, then the weather station is classified as semi-arid and if SD is greater than -0.51, then it is classified as arid. Thus, according to SD classification, 66% of area in Pakistan is arid, whereas 30% (4%) is semi-arid (humid). Comparison of precipitation regimes based upon observed and APH datasets with other climate classification schemes that involve both precipitation and temperature is presented. Stochastic simulation of daily precipitation at 46 different weather stations across Pakistan is assessed for the 36-year baseline period during 1976–2011 using a stochastic precipitation generator, both on seasonal and annual basis. The precipitation projections of 15 intergovernmental panel on climate change (IPCC) fourth assessment report (AR4)-based Atmosphere-Ocean General Circulation Models (AOGCMs) are assessed for three 20-year projected time periods centered at 2011–2030 or T1, 2046–2065 or T2, and 2080–2099 or T3, which are embedded in the stochastic precipitation generator. The SD technique is used to delineate the three precipitation regimes in Pakistan: arid, semi-arid, and humid. The change in (i) amount of precipitation, (ii) number of wetdays, (iii) amount of precipitation per wetday, and (iv) area of precipitation regimes are assessed for all three projection time periods relative to the baseline period (TB), under IPCC AR4 based A1B emission scenario. Pakistan has received 487.48 mm of average annual precipitation, with an average of 43.54 wetdays year−1 during 1976–2011, whereas simulated average annual precipitation is 513.70 mm with 43.60 wetdays in a year. The stochastic precipitation generator has less than 2% error in simulating the mean annual numbers of wetdays during the baseline period, whereas simulated mean annual precipitation is 11.78 mm wetday−1 versus the observed 11.70 mm wetday−1. There is an underestimation of ~1% in simulated mean wet spell length during the baseline period. The AOGCMs display varying increase (decrease) in humid and semi-arid (arid) areas in Pakistan in all the three projected time periods. Further, stochastic projections of precipitation amount, number of wetdays and precipitation per wetday from 25 IPCC AR5 based AOGCMs under Representative Concentration Pathway (RCP), RCP4.5 and RCP8.5 emission scenarios, are carried out in all climate regimes of data sparse region of Pakistan. In arid climate regime, the ensemble average annual precipitation is projected to decrease by about 5.56%/3.43%/4.94% during T1/T2/T3, relative to TB under RCP4.5, whereas average annual precipitation in semi-arid (humid) climate regime, is projected to increase by about 8.40%/8.02% (2.12%/2.61%) during T2 and T3, relative to TB, respectively. Under RCP8.5, the average annual precipitation is projected to decrease/increase in arid/semi-arid and humid climate regimes, in T1/T2 and T3, relative to TB. There is a projected increase (decrease) in precipitation during T2 (T3) in all the climate regimes, relative to T1. Under RCP4.5 (RCP8.5), precipitation on wetdays in arid/semi-arid/humid climate regime is projected to be 7.84/10.78/13.67 (7.64/10.96/13.95), 7.86/11.32/14.40 (7.83/11.55/14.73), and 7.90/11.33/14.39 (8.65/11.39/15.45) mm wetday-1 during T1, T2 and T3, respectively. Overall, under both RCPs, the average annual precipitation and number of wetdays in Pakistan are projected to progressively decrease, whereas precipitation per wetday in Pakistan is projected to progressively increase, all relative to TB. A comparison of findings in AR4 and AR5 is presented.
Gov't Doc #: 20519
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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