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Chapter 13

Chapter 13. Desert Precipitation. Importance of Understanding Desert Precipitation. Flash floods, caused by rainfall events, are more likely in deserts because more rainfall runs off.

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Chapter 13

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  1. Chapter 13 Desert Precipitation

  2. Importance of Understanding Desert Precipitation • Flash floods, caused by rainfall events, are more likely in deserts because more rainfall runs off. • Water is scarce, so we need to better understand the characteristics of precipitation (frequency of events, intensity, etc.) • Population is increasing fast in arid areas • Wildfires

  3. General Properties of Desert Precipitation • Scattered and isolated – that is, there generally are not large convective complexes. • Much precipitation evaporates before reaching the ground (virga). • It can appear to be very intense. • It is scarce. • Relatively few events dominate the statistics.

  4. Desert Rainfall - A Few Events Dominate the Statistics

  5. Storm-total Rainfall Extremes Place Mean-annual Single Storm Precipitation (mm) Precipitation (mm) Chicama, Peru 4 394 Aozou, Chad 30 374 Swakopmund, Namibia 15 50 Sharjah, UAE 107 74/50 min Lima, Peru 46 1524 Tamanrasset, Algeria 27 44/3 hr Biskra, Algeria 148 210/2 day El Djem, Tunisia 275 319/3 day

  6. 24-h Rainfall Extremes (mm) Place Mean Annual Min Annual Max Annual Max 24 h Dongola, Sudan 23 0 60 36 Wadi Halfa, Sudan 3 0 33 19 Galcaio, Somalia 149 33 448 160 Luderitz, Namibia 18 1 59 31

  7. Average Monthly Precipitation Versus Greatest Monthly Precipitation 50 year record 173 year record

  8. Desert Rainfall – Is It More Intense Than Non-desert Rainfall? • Intriguing examples, like > 2.5 cm/ 1 min in San Gabriel Mountains in semi-arid California

  9. Subjective Factors That Might Make Desert Rain Seem Intense • Great psychological impact because of its rarity • The resulting flooding • The associated dust storms • One sometimes has to endure the storm without shelter • We get an unobstructed view of the approaching storm

  10. Meteorological Factors That Might Make Desert Rain Intense • The tropopause is higher in the subtropics, and convective storms can be deeper.

  11. Meteorological Factors That Might Make Desert Rain Intense • The tropopause is higher in the subtropics, and convective storms can be deeper. • Weaker large-scale winds in subtropics, compared to mid-latitudes, mean that the storm will move slower and therefore last longer

  12. Seasonality of Desert Precipitation

  13. A Satellite View of Precip. – Inter-Tropical Convergence Zone Mid-Latitude Storms

  14. Global Atmospheric Circulation

  15. Winter Precipitation (Cyclonic/Frontal) Summer Precipitation (Convective)

  16. The Number of Rainy DaysPer Year

  17. Based on a Number of Measurement Locations in Each Desert Place Mean Annual Rainy Days Average Rain Rainfall (mm) (>.1 mm) Per Year per Rainy Day (mm) Northern Sahara 1-286 1-57 3.82 Kalahari 147-592 19-68 9.55 Gobi 84-396 33-78 4.51 Patagonian 51-542 6-155 5.41

  18. Some Statistical Measures But first, let’s discuss what a “frequency distribution” is.

  19. In general – A table or graph showing how frequently different values of a quantity occur

  20. For example

  21. Measures of the central tendency, or middle, of a distribution • Mean – the arithmetic average • Median – the number such that half the values fall above it and half fall below it. • Mode – the value that occurs most often

  22. What is the mode in these distributions?

  23. Measures of the central tendency, or middle, of a distribution • Mean – the arithmetic average • Median – the number such that half the values fall above it and half fall below it. • Mode – the value that occurs most often • Inter-quartile range – the range of the middle 50% of the observations

  24. 166 years Q1 Q2 Q3 24 20 19 18 15 13 12 9 8 8 7 5 2 2 2 1 1 IQR = 95-100 to 115-120 Middle 50%

  25. Measures of the variability, or shape, of the distribution • Standard deviation (s) The “wider” the distribution, the larger the standard deviation

  26. Coefficient of variation – standard deviation divided by the mean

  27. Inter-quartilevariability factor 100 (Q3 – Q1 )/Q2

  28. What Should be the StatisticalProperties of the Distribution of Annual Precipitation in the Arid Areas? • Small mean • Large standard deviation (small values some years and large values others) • Large coefficient of variation (s/mean) • Large inter-quartile variability factor

  29. Coefficient of Variation

  30. Desert Locations

  31. Coefficient of Variation

  32. Budyko Index

  33. For Locations in Africa Coefficient of variation Inter-quartile variability factor

  34. Inter-quartile variability

  35. Budyko Index

  36. Is Desert Precipitation More Intense?

  37. Summer Precipitation Intensities in North America (15 min totals)

  38. Causes of Desert RainfallOrographic Effects

  39. Northern Sonoran Desert

  40. JJA Frequency of Radar Echoes in Northern Chihuahuan Desert

  41. Mean Annual Rainfall Between Syrian Desert and Mediterranean 600-900 m ASL 200-300 m BSL

  42. Convection over the Yemen Mountains

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