Factors Influencing Precipitation Efficiency PE in a Continental MidLatitude Location

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Factors Influencing Precipitation Efficiency
(PE) in a Continental Mid-Latitude Location

• by
• Mohd Hisham Mohd Anip
• Advisor
• Dr Patrick Market

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Content

• Introduction
• Literature review
• Methodology
• Results Discussions
• Conclusions
• Questions/comments?

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Introduction

• What is PE?
• The efficiency of a system in eliminating the
  moisture it ingests from the atmosphere (Doswell
  et al. 1996)
• total available moisture
• Braham (1952) total inflow water vapor into
  the cloud base
• Ferrier et al. (1996) total condensation over
  column of air

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• Why study the PE?
• Give an indication of how efficient a system is
  eliminating the moisture it ingests from the
  atmosphere
• Heavy rainfall systems are associated with
  higher PE value (Fankhauser 1988, Doswell et al.
  1996, Scofield 2000)
• Use as a forecasting tool to forecast heavy
  rainfall flash flood events

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Literature Review

• First study has been conducted in the early
  1950s,
• - after the Thunderstorm Project
• Braham (1952)
• - determined thunderstorm water budget.

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Literature Review

• Findings from later observational studies reveal
• i. negative correlation
• between wind shear and PE value
• (Marwitz 1972)

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Literature Review

• ii. Larger cloud base area and cloud base mixing
  ratio, PE, higher cloud base height, PE
• (Fankhauser 1988)

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Literature Review

• iii. PE instantaneous values vary with the
  lifetime of a system (Doswell et al. 1996)

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• (Market et al. 2003)

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Literature Review

• iv. Relatively strong negative correlation
  between Convective Inhibition (CIN) and PE
• (Market et al. 2003)

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Literature Review

• Significant findings from modeling study
• i. PE in convective system is control by
  vertical orientation of the updrafts (Ferrier et
  al. 1996)
• ii. PE is higher in a heavy rain regime under
  warm environmental condition and strong
  convection (Li et al. 2002)

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• Objective of this study
• i. Examine PE variation with seasons
• ii. Examine PE variation with cloud types
  (stratiform vs. convective)
• iii. To find the relations of environmental
  factors to PE value, if any, and used them to
  support the earlier findings

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• Area of Study - Columbia, MO
• Why choose Columbia?
• i. Good annual amount of rainfall 1000mm
• ii. Sanborn KCOU Weather Station
• iii. SuomiNet system
• Criteria to select the event from Oct 1st, 2003
  to Sept 30th, 2004
• i. Total rainfall gt 0.25 in (6.4mm)
• ii. Rainfall was continuously for at least
  4hrs.

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Methodology

• Few methods to calculate PE, e.g.,
• Sellers Method (1965), P/PW
• - a climatological approach
• Scofield Method (1987), PW X RH
• - RH is a mean value taken between 1000 and
  700mb pressure level
• Water Budget Method (Braham 1952, Palmén
• Newton 1969)
• - known as true PE because it takes into
  account all the moisture in and out of a
  rainfall system

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• Using water budget method by Palmén Newton
  (1969) to calculate PE assumed to be in steady
  state system at a fixed point
• (A) (B)
• P total precipitation
• E evaporation
• local change in specific humidity tendency
• moisture flux divergence

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• PE Calculation
• Used analysis data from RUC20 model to generate
  water budget terms
• RUC20- latest meteorological numerical models,
  and features 50 layers and 20 km horizontal
  resolution that improves the accuracy and
  timeliness of current weather analyses and
  short-range weather forecasts
• RUC20 output acquired from the Atmospheric
  Radiation Measurement (ARM) of the Department of
  Energy (DOE)

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Results and Discussions

• 31 events have been identified
• PE values range from -78 to 236 were calculated
• According to Marwitz (1972), PE value over the
  lifetime of rainfall system should be between 0
  and 100, but instantaneous value can vary from
  0 to (Doswell et al. 1996)

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Results and Discussions

• PE value gt 100 (4 cases)
• - because the values were estimated toward the
  end of the system lifetime
• (Doswell et al. 1996)
• PE value lt 0 (1 case)
• - because the system was precipitating out as
  well as advecting moisture out of the system,
  experiencing a net loss in moisture ingest

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• Seasonal variation
• - land heating and cooling has played a major
  factor for these variations
• - higher temperature will result in more
  evaporation which produces more moisture and
  rainfall during warmer season

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• Seasonal variation
• - precipitable water values are typically
  greater through the warm season and lower in the
  cold season (Ross and Elliott 1996)
• - PE value is affected by relative humidity and
  precipitable water (Doswell et al. 1996)

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• Cloud/system type
• - PE value for convective system is higher
  because of the ability of updrafts and downdrafts
  to collect droplets that would become
  precipitation (Ferrier et al. 1996 )
• - Under warm environmental condition and strong
  convective system the precipitation would be more
  efficient, which produced higher PE value (Li et
  al. 2002)

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• Cloud/system type
• - Stratiform system generally produce little or
  no precipitation and what little might fall
  consist of minute particles such as drizzle
• - Convective rainfall accounts for 70 of the
  total rainfall, because its intensity is so much
  higher (Houze 1997)

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• Environmental factors
• - only involve the convective system (19 events)
• - unfortunately no correlations have been found
• - proceed with advance analysis by selecting the
  event that extremely fulfill study assumption
  have moderate unstable condition
• - 6 events were short listed which shows
• i. strong inverse correlation (-0.764)
  between PE value and environmental wind
  shear (Marwitz 1972)
• ii. high probability (86) where PE value is a
  function of precipitable water (Doswell et
  al. 1996)

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• Case study ( August 24th, 2004)
• - example of perfect steady-state system passing
  through the study area
• 1500 UTC Approaching the study area

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• Surface Analysis chart (1500 UTC)

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• 500 mb Height Vorticity chart (1500 UTC)

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• Skew-T chart from 1500 UTC RUC20 sounding

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• 1800 UTC Over the study area

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• 2000 UTC Leaving the study area

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• Facts of the Event
• i. 16.76 mm rainfall amount was recorded over
  study area from 16 to 19 UTC observation
• ii. Amount of precipitable water measure from
  UMC SuomiNet system drop tremendously as the
  system passing trough the study area
• iii. 45 of PE value was calculated with
  estimation of 37.61 mm available water between
  1000 and 300 mb pressure level
• iv. Report from National Climatic Data Center
  reveals that 13 locations in Missouri were
  experienced significant weather associated with
  this weather system

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Conclusions

• PE value is highest during summer, followed by
  spring, fall winter
• PE value is higher for convective type system
  than the stratiform
• After advance analyses
• - There is strong inverse correlation between
  environmental wind shear and PE value
• - There is high probability that precipitable
  water is a function of PE value
• Case study shows that 45 PE was estimated
• for the perfect steady-state convective
• weather system

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• Questions/Comments??
• Special Thanks Dr Market