Factors Influencing
Precipitation Efficiency (PE)
in a Continental Mid-Latitude Location
by
Mohd Hisham Mohd Anip
Advisor
Dr Patrick Market
Content
Introduction
Literaturereview
Methodology
Results & Discussions
Conclusions
Questions/comments?
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
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
Literature Review
First study has been conducted in the early
1950’s,
- after the Thunderstorm Project
Braham (1952)
- determined thunderstorm water
budget.
Braham (1952) Mid-latitude thunderstorm Observed 10%
Sellers (1965) Climatological Observed 5-19%
Newton (1966) Mid-latitude squall line Observed 50%
Fankhauser (1966) Mid-latitude thunderstorm Observed 60%
Auer and Marwitz (1968) Mid-latitude hailstorms Observed 21-120%
Chisholm (1968) Mid-latitude thunderstorm Observed 21%
Hartzell (1969) Mid-latitude hailstorm Observed 45%
Chisholm (1970) Mid-latitude squall line Observed ~100%
Marwitz (1972) Mid-latitude hailstorm Observed ~4%
Foote and Fankhauser (1973) Mid-latitude hailstorm Observed 15%
Chalon et al. (1976) Mid-latitude hailstorm Observed 40%
Houze et al. (1976) Mid-latitude frontal system Observed 60-90%
Caracena et al. (1979) Mid-latitude thunderstorm Observed 85%
Gamache and Houze (1983) Tropical MCS Observed ~59%
Heymsfeld and Schotz (1985) Mid-latitude squall line Observed 25-40%
Lipps and Hemler (1986) Tropical convection Modeled ~40%
Fankhauser (1988) Mid-latitude thunderstorms Observed 19-47%
Ryan et al. (1989) Mid-latitude cold front Observed 0%
Chong and Hauser (1989) Tropical squall line Observed 45-57%
McBean and Stewart (1991) Mid-latitude frontal system Observed 70%
Ferrier et al. (1996) Tropical and mid-latitude Modeled 24-45%
squall lines
Rauber et al. (1996) Tropical convection Observed 20-30%
Szeto et al. (1997) High-latitude frontal system Modeled 0-80%
Li et al. (2002) Tropical convection Modeled 20-130%
Market et al. (2003) Mid-Latitude MCS Observed 4-48%
Literature Review
Findings from later
observational studies
reveal
i. negative correlation
between wind shear
and PE value
(Marwitz 1972)
Literature Review
ii. Larger cloud base area and cloud base
mixing ratio, PE, higher cloud base
height, PE
(Fankhauser 1988)
Literature Review
iii. PE instantaneous values vary with the
lifetime of a system (Doswell et al. 1996)
(Market et al. 2003)
Literature Review
iv. Relatively strong negative correlation
between Convective Inhibition (CIN) and PE
(Market et al. 2003)
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)
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
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 > 0.25 in (6.4mm)
ii. Rainfall was continuously for at least 4hrs.
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
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
(A) = local change in specific humidity tendency
(B) = moisture flux divergence
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)
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)
Results and Discussions
PE value > 100% (4 cases)
- because the values were estimated
toward the end of the system lifetime
(Doswell et al. 1996)
PE value < 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
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
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)
Cloud/system type
Rainfall No of Events Summer Fall Winter Spring PE average (%)
System
Convective 19 6 7 1 5 56
Stratiform 11 2 0 5 4 29
- 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)
Cloud/system type
Rainfall No of Events Summer Fall Winter Spring PE average (%)
System
Convective 19 6 7 1 5 56
Stratiform 11 2 0 5 4 29
- 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)
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)
Case study ( August 24th, 2004)
- example of perfect steady-state system passing
through the study area
1500 UTC : Approaching the study area
Surface Analysis chart (1500 UTC)
500 mb Height & Vorticity chart (1500 UTC)
Skew-T chart from 1500 UTC RUC20 sounding
1800 UTC: Over the study area
2000 UTC: Leaving the study area
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
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
Questions/Comments??
Special Thanks: Dr Market