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Instability Indices and Heavy Rainfall Forecast in a Maritime Environment

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Instability Indices and Heavy Rainfall Forecast in a Maritime Environment Instability Indices and Heavy Rainfall Forecast in a Maritime Environment by Ram Kumar DHURMEA Dissertation Submitted in Partial Fulfilment For a Master in Applied Meteorology with Climate and Management to The University of Reading Department of Meteorology, Faculty of Physical and Mathematical Sciences September 2015 Supervisor: Dr. Pete Inness Contents List of Figures iv List of Tables viii Acknowledgements x List of Acronyms xi Abstract xiii 1 Introduction 1 1.1 Problem Statement . 1 1.2 Study Objectives . 4 1.3 Strength and Limitations . 5 1.4 Outline of Thesis . 6 2 Literature Survey 7 2.1 Topography and Rainfall of Mauritius . 7 2.1.1 Location, climate and topography . 7 2.1.2 Rainfall in Mauritius . 9 2.2 An Overview of Indices . 10 2.2.1 Potential instability indices . 11 2.2.2 Temperature and humidity indices . 12 2.2.3 Advanced derived indices . 14 2.2.4 Wind shear indices . 16 2.2.5 Conclusions . 16 i CONTENTS 3 Local Convection In Mauritius 18 3.1 Introduction . 18 3.2 Spatial Distribution of Local Convection in Mauritius . 19 3.3 Indices and Local Convection Forecast . 23 3.4 Where Indices Fail . 24 3.4.1 Stable indices - heavy rainfall event . 24 3.4.2 Unstable indices - light rainfall events . 26 3.4.3 Unstable indices - nil convection events . 27 3.5 Influence of Upper Level Synoptic Situation . 28 3.6 Conclusions . 33 4 Data analysis and Index Development 34 4.1 Introduction . 34 4.2 Data . 35 4.2.1 Rainfall data . 35 4.2.2 Radiosonde data . 36 4.2.3 Categorisation of events . 36 4.2.4 Filtering of convective events . 37 4.3 Proposed Indices . 37 4.3.1 Modification of traditional indices . 37 4.3.2 Lapse-rate and moisture based indices . 38 4.3.3 Wind shear based indices . 39 4.4 Indices Thresholds and Evaluation . 40 4.5 Validation of Indices . 41 5 Results and Discussions 45 5.1 Exploratory Analysis . 45 5.2 Evaluation of Traditional Indices . 47 5.3 Evaluation of Modified Indices . 47 5.4 Evaluation of Lapse-Rate and Moisture Based Derived Indices . 50 5.5 Evaluation of the Wind Shear Indices . 51 5.6 Validation of Indices . 54 5.6.1 Performance of individual indices . 55 ii CONTENTS 5.6.2 Performance of forecast schemes . 57 6 Conclusions and Recommendations for Future Works 63 Bibliography 66 Appendices i A Correlation Between Rainfall Events and Indices ii A.1 Traditional indices . ii A.2 Modified indices . iv A.3 Lapse-rate and moisture indices . vi A.4 Wind shear indices . viii B More Examples of Indices Failures x C Cross Validation Statistics xiii iii List of Figures 2.1 Location of Mauritius in the South West Indian Ocean (Atlas of Mauritius). 8 2.2 Topography of Mauritius. The Vacoas Synoptic Station, located on the western slope of the Central Plateau at a height of 425 m, is shown by the white arrow (http://mauritiusattractions.com/)............................. 8 3.1 MeteoSat-7 visible imagery at 0600 UTC and 1200 UTC showing local convec- tion over different region of Mauritius (http : ==www:sat:dundee:ac:uk=geobrowse). 20 3.2 MeteoSat-7 visible imagery at 0600 UTC and 1200 UTC showing intensity of local convection over Mauritius (http : ==www:sat:dundee:ac:uk=geobrowse). 21 3.3 MeteoSat-7 visible imagery at 0600 UTC and 1200 UTC showing convection due to forcing from a frontal system (top) and a meso low (bottom) (http : ==www:sat:dundee:ac:uk=geobrowse). 22 3.4 Domain of the ALADIN limited area model over the SWIO showing a 12 hour forecast of accumulated precipitation (mm) for a convective event on 30 March 2013. The model analysis is at 30 @ 0000 UTC and is valid for 30 @ 1200 UTC (http : ==www:meteo:fr=extranets=). 23 3.5 Skew-T plots of soundings at Vacoas synoptic station @ 1200 UTC showing temperature and dew point profile with (a, b) moist mid-troposphere (c, d) dry mid-troposphere. Calculated indices are shown on top right corner (http : ==weather:uwyo:edu=upperair=sounding:html). 25 3.6 Skew-T plots of soundings at Vacoas synoptic station @1200 UTC showing tem- perature and dew point profile (a) dry low level (b) moist low level. Calculated indices are shown on top right corner (http : ==weather:uwyo:edu=upperair=sounding:html). 26 iv LIST OF FIGURES 3.7 MeteoSat-7 visible imagery t 0600 UTC and 1200 UTC showing evolution of a meso scale system in the vicinity of Mauritius dampening any local convection. 27 3.8 Skew-T plots of soundings at Vacoas synoptic station @1200 UTC showing tem- perature and dew point profile of a rather dry atmosphere. Calculated indices are shown on top right corner (http : ==weather:uwyo:edu=upperair=sounding:html). 28 3.9 500 hPa geopotential height (m) composite mean over the SWIO for the days with stable indices and heavy rainfall (H: high, L: Low) (http : ==www:esrl:noaa:gov=). 29 3.10 500 hPa air temperature (K) composite anomaly (1981-2010) over the SWIO for the days with stable indices and heavy rainfall (W: warm pool, C: cold pool). 30 3.11 500 hPa geopotential height (m) composite mean over the SWIO for the days with unstable indices and light rainfall (H: high, L: Low) (http : ==www:esrl:noaa:gov=). 31 3.12 500 hPa air temperature (K) composite anomaly (1981-2010) over the SWIO for the days with unstable indices and light rainfall (W: warm pool, C: cold pool) (http : ==www:esrl:noaa:gov=). ............................ 31 3.13 500 hPa geopotential height (m) composite mean for the days with unstable indices without convection (H: high, L: Low) (http : ==www:esrl:noaa:gov=). 32 3.14 500 hPa air temperature (K) composite anomaly (1981-2010) over the SWIO for the days with unstable indices without convection (W: warm pool, C: cold pool) (http : ==www:esrl:noaa:gov=). 32 4.1 Location of the 20 AWS (A) and 20 manned stations (M))used in this study. The site of radiosonde launch (Vacoas synoptic station) is encircle in red. 35 5.1 Box plots of (a) HgRr60 (b) HgAcRr (c) Drtn and (d) relationship between HgRr60 and HgAcRr for the 64 rainfall events from 2003 to 2009. 46 5.2 Scatter plots showing relationship between HgRr60 and (a) CAP E (b) P rpW tr (c) KI (d) DCI. The uneven scattering of the different category of events with CAP E and P rpW tr is quite discernible. 48 5.3 Scatter plots of HgRr60 and modified K indices showing only 3 overlapping o o weak events in Ksfc−850 ≥28 C and Ksfc−500 ≥50 C. 49 5.4 Scatter plot of HgRr60 and mixing ratios calculated between surface and (a) 925 hPa (b) 850 hPa (c) 700 hPa (d) 500 hPa. Note the absence of weak events beyond certain threshold in rsfc−850 ≥ 16, rsfc−700 ≥ 14 and rsfc−500 ≥ 12. 51 v LIST OF FIGURES 5.5 Scatter plots of rainfall event categories and conjunction between mixing ra- tios and lapse rate in different layers showing most weak events confined in (rΓd)sfc−500 < 0.07. 52 5.6 Scatter plot showing an ambiguous relationship between HgRr60 and direc- tional and speed shear and shear vector calculated for the layer 850-500 hPa and 700-500 hPa. 53 5.7 Hourly rainfall intensities of 48 selected cross validation events. 54 5.8 Schematic using traditional indices to forecast rainfall of ≥ 20 mmh−1 over Mauritius. A better CSI and lower F AR is achieved by applying the Θ ≥ 299.5. 58 5.9 Schemes with less complexity and better score using traditional indices. 58 5.10 Schemes to forecast rainfall of ≥ 20 mmh−1 using modified indices . 59 5.11 Schemes showing combined traditional indices, modified indices and 500 hPa variables scores in forecasting rainfall of ≥ 20 mmh−1 over Mauritius. 59 A.1 Scatter plots HgRr60 against traditional indices. The uneven scattering of the different category of events with TTI, Ko, PII, CII and LCLpp is quite dis- cernible . iii A.2 Scatter plots of HgRr60 against modified indices. Most of the plots show un- even scattering of the different category of events with the indices . v A.3 Scatter plots of HgRr60 against lapse-rate and moisture based indices. Most of the plots show uneven scattering of the different category of events with the indices . vii A.4 Scatter plot showing an ambiguous relationship between HgRr60 and direc- tional shear, speed shear and shear vector calculated for the layer 850-500 hPa and 500-250. ix B.1 500 hPa geopotential height (m) composite mean over the SWIO for the days with stable indices and heavy rainfall. The island is under the influence of a trough on 27 Feb 2004 and 19 Feb 2005 and high geopotential height on 26 and 27 Feb 2005(http : ==www:esrl:noaa:gov=)....................... x B.2 Skew-T plots of soundings at Vacoas synoptic station @1200 UTC showing un- stable profile with (a, b) moist mid-troposphere (c, d) dry mid-troposphere (http : ==weather:uwyo:edu=upperair=sounding:html). xi vi LIST OF FIGURES B.3 500 hPa geopotential height (m) composite mean over the SWIO for the days with unstable indices and light rainfall (http : ==www:esrl:noaa:gov=). xii B.4 Skew-T plots of soundings at Vacoas synoptic station @1200 UTC showing un- stable profile with (a, b) moist mid-troposphere (c, d) dry mid-troposphere (http : ==weather:uwyo:edu=upperair=sounding:html). xii vii List of Tables 4.1 Contingency table of a dichotomous verification scheme .
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