The nature of rain events in summer vs. winter at the SGP

ARM Facility

MPO 581 Class ProjectEmily Riley, Siwon Song, & Brian Mapes

Background• ARM – Atmospheric Radiation Measurements

– Several Sites (facilities) funded by DOE

Background• ARM – Atmospheric Radiation Measurements

– SGP-Southern Great Plains

Data• Climate Modeling Best Estimate (CMBE) Data

– 1996 – 2009– Data averaged over one hour time intervals

Data• Climate Modeling Best Estimate (CMBE) Data

– 1996 – 2009– Data averaged over one hour time intervals

* Cloud fraction profiles* Total, high, middle, and low

clouds* Liquid water path and

precipitable water vapor* Surface radiative fluxes* TOA radiative fluxes

* Soundings* NWP analysis data* Surface sensible and latent heat fluxes* Surface precipitation* Surface temperature, relative humidity, and horizontal winds

Methods

• Composite • Weighted Composite• Regression

July 1996 – RH and PrecipitationRelative Humidity (RH) at surface – ψ(t)

Days on July 1996

Days on July 1996

[mm

/hr]

[%]

Precipitation Rate at surface – p(t)

: Rain Event

non-weighted composite method

• non-weighted composite

€

ψi(tlag )i= 0

N

∑N

=ψ (tlag )

tlag = [-7*24, 7*24] hoursN = total number of selected rain events

non-weighted composite

-7 days +7 days

Relative Humidity (RH) at surface – ψ(t)

Days on July 1996

[mm

/hr]

[%]

Precipitation Rate at surface – p(t)

-7 days +7 days-7 days +7 days-7 days +7 days-7 days +7 days-7 days +7 days

: Rain Event

Weighted composite method

• weighted composite

tlag = [-7*24, 7*24] hoursN = total number of selected rain events

€

pi(t0)ψ i(tlag )¡=1

N

∑

pi(t0)i=1

N

∑

Non-weighting

Weighted composite

-7 days +7 days

Relative Humidity (RH) at surface – ψ(t)

Days on July 1996

[mm

/hr]

[%]

Precipitation Rate at surface – p(t)

-7 days +7 days-7 days +7 days-7 days +7 days-7 days +7 days-7 days +7 days

: Rain Event

Composite vs. Weighted Compositem

m/h

rm

m/h

r

Regression method

• Regression Coefficient

€

cor p(t),ψ (t + tlag )( )sd ψ (t)( )sd p(t)( )

t = whole time series tlag = [-7*24, 7*24] hours

Regression method: t = 0 hourRelative Humidity (RH) at surface – ψ(t)

Days on July 1996

[mm

/hr]

[%]

Precipitation Rate at surface – p(t)

Regression method: t = -10 hourRelative Humidity (RH) at surface – ψ(t)

Days on July 1996

[mm

/hr]

[%]

Precipitation Rate at surface – p(t)

Regression method: t = +10 hourRelative Humidity (RH) at surface – ψ(t)

Days on July 1996

[mm

/hr]

[%]

Precipitation Rate at surface – p(t)

Comparison: Regression vs. weighted composite

• Regression Coefficient

• Weighted composite€

cor p(t),ψ (t + tlag )( )sd ψ (t)( )sd p(t)( )

=cov p(t),ψ (t + tlag )( )

sd p(t)( )2 =

cov p(t),ψ (t + tlag )( )

sd p(t)( )2 =

′ p (t) ′ ψ (t + tlag )

′ p (t)2

€

pi(t0)ψ i(tlag )¡=1

N

∑

pi(t0)i=1

N

∑=

p(t0)ψ (tlag ) + ′ p (t0) ′ ψ (tlag )

p(t0)=ψ (tlag ) +

′ p (t0) ′ ψ (tlag )

p(t0)=ψ (tlag ) +

cov p(t0),ψ (tlag )( )

p(t0)

t = whole time series tlag = [-7*24, 7*24] hoursN = total number of selected rain events

Weighted Composite vs. Regression %

RH on precipitation

Weighted Composite vs. Regression

SAME, except for units

%

RH on precipitation

Relative Humidity Perturbation

Weighted[%]

Regression[%/(mm/hr)]

Non-weighted[%]

Time for some results….

• Oklahoma JJA vs. DJF precipitation

Cumulative Fraction of Rain Events

JJA

DJF

• DJF - ~45% time not raining • JJA - ~32% time not raining

Cumulative Fraction of Rain Events

JJA

DJF

• DJF - ~45% time not raining • JJA - ~32% time not raining

5

~20% rain events > 5 mm/hr

Cumulative Fraction of Rain Events

JJA

DJF

• DJF - ~45% time not raining • JJA - ~32% time not raining

5

~5% rain events > 5 mm/hr

Summer vs. Winter Precipitation

Summer vs. Winter Temperature

Strong Diurnal Cycle

Weak Diurnal Cycle

Summer vs. Winter Temperature

Strong Diurnal Cycle

Weak Diurnal Cycle

Frontal Precip

Afternoon Convection

Seasonal:

Temperature perturbation [K]

DJF: Winter

JJA: Summer

Summer vs. Winter Surface Pressure

Seasonal:

Relative Humidity perturbation [%]

DJF: Winter

JJA: Summer

Seasonal:

Relative Humidity perturbation [%]

DJF: Winter

JJA: Summer

Summer vs. Winter LWP

Seasonal:

All Cloud Fraction perturbation [%]

DJF: Winter

JJA: Summer

Summer vs. Winter Cloud Top Height

Seasonal:

Omega perturbation [Pa/s]

DJF: Winter

JJA: Summer

Summary• Summer (JJA):

– More rain events– Heavier, but shorter rain events– Stronger diurnal cycle– Higher cloud tops

• Winter (DJF):– Tilted vertical structure for RH and Cloud Fraction

• THOUGH, hourly time composites might reveal tilted structure in summer