Diurnal Variations in Southern Great Plain during IHOP -- data and NCAR/CAM

Junhong (June) WangJunhong (June) WangDave Parsons, Julie Caron and Jim HackDave Parsons, Julie Caron and Jim Hack

NCAR ATD and CGDNCAR ATD and CGD

Acknowledgement: Kate Young (NCAR/ATD) and Aiguo Dai (NCAR/CGD)

Thanks NCAR Water Cycle Initiative Support

1. Document and understand diurnal variations in SGP using IHOP data

• Two modes: LLJ v.s. non-LLJ

• Diurnal variations and differences between two modes

• Understand humidity variations

2. Understand nocturnal precipitation maximum in SGP

3. Compare with CAM simulations

Outline

Why study diurnal cycle? Why study diurnal cycle? • The diurnal cycle is one of the most obvious

and reliable signals of the climate. • Most physical processes (such convection and

radiation) are non-linear and can not be simulated adequately using daily mean values.

• The diurnal cycle can be used as an excellent test bed for evaluating model physics because of its large amplitude and short time scale.

• Some earlier GCMs did not even include the diurnal cycle to save computer time.

• There is a lack of data with high resolution for studying the diurnal cycle on the global scale.

Courtesy of Aiguo Dai (NCAR/CGD)

ARM Data

• IHOP: 20-day (May 26 to June 14, 2002) 3-hrly data at five stations

• 3-hrly ARM radiosonde data during IOPs (315 soundings from 1996-2000)

Observations:Observations:Twenty days of Twenty days of

wind speed wind speed profiles: two modesprofiles: two modes

Temperature and MR in two modes Temperature and MR in two modes

T

MR

Diurnal Diurnal cycle at cycle at surfacesurface

Diurnal Cycle of Diurnal Cycle of LLJ and non-LLJ LLJ and non-LLJ

cases: Windscases: Winds

Whiteman et al. (1997)

U wind anomalyU wind anomaly

Day

Night

solenoidal circulation

Temperature: AnomalyTemperature: Anomaly

Sonde Mixing Ratio: AnomalySonde Mixing Ratio: Anomaly

NCAR - Atmospheric Technology DivisionNCAR Atmospheric Technology DivisionNCAR Atmospheric Technology Division

Comparisons of diurnal variations in WV

profilesRadiosonde (Dai et al. 2002)

MWPRJJA

JJA

Data from objective Data from objective variational analysis variational analysis

of the ARM IOP of the ARM IOP data (Zhang et al. data (Zhang et al.

2000)2000)

11/1723.5

Omaga (mb/day)

horizontal advection (g/kg/day)

Vertical advection (g/kg/day)

Nocturnal precipitation maximumNocturnal precipitation maximum

Why?• Eastward propagation of mountain-generated systems from the previous afternoon (Riley et al. 1987, Carbone et al. 2002)

• Large-scale convergence as a result of solar-driven diurnal and semidiurnal cycles of surface pressures (Dai et al. 1999)

• …

From Dai et al. (1999)

CAPE/CIN: meanCAPE/CIN: mean

• Larger CAPE for LLJ throughout the diurnal cycle

• Maximum CAPE but minimum CIN in the afternoon for LLJ

• The 2nd small maximum at ~0.5 km around early morning

CAPE and CIN Diurnal Variations during Nauru99

1100 LST

0800 LST

0500 LST

0200 LST2300 LST

2000 LST

1700 LST1400 LST

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

600 700 800 900 1000 1100 1200

CAPE (J kg-1)

CIN

(J

kg-1

)

•Qualitative behavior and simple scaling arguments suggest a possible link between the diurnal cycles in stability and rainfall.

From Dave Parsons

CAPE/CIN from large scale advection and CAPE/CIN from large scale advection and from surface flux forcing (Zhang et al. 2003)from surface flux forcing (Zhang et al. 2003)

CAPE

PBL forcing

Tropospheric forcing

Convective

Non-Convective

Links to nocturnal precipitation maximumLinks to nocturnal precipitation maximumFuture work:• Analysis of more data,

• Rain v.s. non-rain days?

• MWRP data?

• Large-scale circulation data?

Model and RunsModel and Runs

Community Atmospheric Model (CAM) Version 3.0

Model run

- with climatological SSTs

-- hourly output

-- over an IHOP season (May, June)

-- 1 realization (so far)

-- hourly data averaged to 3-hourly for consistency with obs

-- results shown are for T42 resolution

-- T85 (150km) and T170 (75km) explorations

-- Ensembles, as opposed to single realizations

Winds from the Climate ModelWinds from the Climate Model

V V anomaly anomaly

LLJLLJ

~4 hours early for the peak of LLJ

DataCAM

U U anomaly anomaly

LLJLLJ

DataCAM

T T anomaly anomaly

LLJLLJ

Data CAM

Q Q anomaly anomaly

LLJLLJ

Data CAM

ConclusionsConclusions1. The 20-day, 3-hrly radiosonde data at five stations during IHOP show that the atmosphere

is in two modes, LLJ v.s. non-LLJ. The atmosphere is warmer, humid, and more unstable for LLJ days than non-LLJ days.

2. Southerly LLJ exists throughout the night and reaches the maximum wind speed at 500-600 m AGL around 2 am CST. The height of the maximum wind speed does not change a lot during night.

3. The phase of diurnal cycles of temperature, CAPE/CIN and MR in BL is similar for LLJ and non-LLJ cases, but the magnitude is larger for LLJ case. The diurnal cycle of MR in the free troposphere is different for two cases.

4. The diurnal cycle of CAPE/CIN shows that the most favorable condition for convections occurs at the late afternoon, so can’t explain the nocturnal precipitation maximum. Different processes contributing to diurnal cycle of stability will be examined separately. The limited data show that rain often follows the end of LLJ events.

5. Preliminary results show good agreements between CAM and data in wind and T, but humidity. More analysis and comparisons with model need to be done.

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