Junhong (June) Wang
Dave Parsons, Julie Caron and Jim Hack
NCAR ATD and CGD
Acknowledgement:
Kate Young (NCAR/ATD) and Aiguo Dai (NCAR/CGD)
Thanks NCAR Water Cycle Initiative Support

Outline
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

• 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:
Twenty days of wind speed profiles: two modes

T
Temperature and MR in two modes
MR

Diurnal cycle at surface

Diurnal Cycle of
LLJ and non-LLJ cases: Winds
Whiteman et al. (1997)

U wind anomaly
Day
Night solenoidal circulation

Temperature: Anomaly

Sonde Mixing Ratio: Anomaly

Radiosonde (Dai et al. 2002)
JJA
MWPR
JJA
NCAR - Atmospheric Technology Division

23.5
Data from objective variational analysis of the ARM IOP data (Zhang et al.
2000) horizontal advection
(g/kg/day)
Omaga (mb/day)
Vertical advection
(g/kg/day)
11/17

Nocturnal 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: mean
• Larger CAPE for LLJ throughout the diurnal cycle
• Maximum CAPE but minimum CIN in the afternoon for LLJ
• The 2 nd small maximum at ~0.5 km around early morning

CAPE and CIN Diurnal Variations during Nauru99
-5
-10
-15
-20
0
600 700 800 900 1000 1100 1200
•Qualitative behavior and simple scaling arguments suggest a possible link between the diurnal cycles in stability and rainfall.
0500 LST
0800 LST
2300 LST
0200 LST
-25
-30
2000 LST
-35
-40
-45
1100 LST
1700 LST
1400 LST
From Dave Parsons
CAPE (J kg-1)

CAPE/CIN from large scale advection and from surface flux forcing (Zhang et al. 2003)
CAPE
Tropospheric forcing
PBL forcing
Convective
Non-Convective

Links to nocturnal precipitation maximum
Future work:
• Analysis of more data,
• Rain v.s. non-rain days?
• MWRP data?
• Large-scale circulation data?

Model 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

V anomaly
LLJ
Data
~4 hours early for the peak of LLJ
CAM

U anomaly
LLJ
Data
CAM

T anomaly
LLJ
Data CAM

Q anomaly
LLJ
Data CAM

Conclusions
1. 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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