AGCM Simulations of The Warm Season Diurnal Cycle

AGCM Simulations of The Warm Season Diurnal Cycle
Over the Continental United States and Northern Mexico
M.-I. Lee1,2, S. Schubert2, M. Suarez2, J. Bacmeister1,2, P. Pegion2, I. Held3, J. Ploshay3,
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N.-C. Lau , B. Tian , A. Kumar , H.-K. Kim , J. Schemm , K. Mo and W. Higgins
1Goddard
Earth Sciences and Technology Center, NASA/GSFC, 2Global Modeling and Assimilation Office, NASA/GSFC,
3Geophysical Fluid Dynamics Laboratory, NOAA, 4National Centers for Environmental Prediction, NOAA/CPC
Abstract
• Large diversities in the representation
of rainfall diurnal cycle among the
models
The diurnal cycle of warm season rainfall was analyzed over the continental United States and Northern
Mexico in three different atmospheric general circulation models (GFDL, NCEP, and NASA/GMAO).
• Agreements to the observation in the
East and Southeast of the US;
Disagreements in the mid-continent and
mountain regions
Despite the time-mean (summer) patterns of rainfall and low-level wind are simulated reasonably well in
three models, they exhibit notable defects in the representation of the diurnal cycle of convection. In
particular, the models commonly fail to capture the observed nocturnal peak of rainfall over the Great
Plains, even though they successfully simulate the nocturnal amplification of low-level jet. Moreover,
observed eastward propagating characteristics of convection (or delaying of diurnal peak convection time)
from the Rocky Mountains toward the Great Plains are not adequately resolved, which contributes to the
systematic time-mean biases of the models over that region.
The analysis of local diurnal variations of convective available potential energy (CAPE) show an
afternoon peak in the models as observed. On the other hand, the observed diurnal cycle of CAPE is not
in phase with rainfall, especially over the Great Plains, implying that boundary layer and free atmospheric
large-scale forcing plays a more important role in initiating or inhibiting convection than near-ground
forcing in this region.
Introduction
Fig.2. Nighttime(00-12UTC) – daytime (12-00 UTC)
rainfall amount differences. Differences are given as the
percentage by dividing daily total rainfall amounts. The
NCEP Hourly Precipitation Dataset (only available over
the United States) was used for obtaining 20 year
observed climatology (1983-2002).
• Current atmospheric/land general circulation models (AGCMs) do poorly in simulating the diurnal
cycle and consequently do poorly in simulating the mean warm season climate over much of the
United States and Mexico.
• GP show eastward moving patterns in the nighttime: clockwise rotation of the wind anomalies
that must be a favorable condition for the moisture transport by southerlies in the nighttime dynamical forcing mechanism engaged.
• Diurnal wind simulations and nocturnal low-level jets (not shown) are in good agreement to the
observed, even though they fail to capture the nocturnal rainfall over GP.
AZNM
NAME
• ARM (Atmospheric Radiation Measurement Program) Intensive Observing Period (IOP)
- Summer Cases: 1995, 1997, 1999 cases ( ~ 54+ days total)
- Location: Southern Great Plains (36.61 N, 97.49 W)
• Diagnostic test of the RAS scheme in the NASA model
• Observed CAPE: daytime
maximum in most regions
• Late afternoon peaks of rainfall are simulated 1-3 hours
earlier in most regions
SE
• Examining the response of the model physics to the “perfect” observed atmospheric forcing
Observed and Simulated Precipitation Rates
Diurnal Cycle of CAPE
Amplitude and Phase of the Diurnal Cycle
MT GP
• The diurnal cycle is a fundamental component of the warm season climate of the continental
United States and Northern Mexico. Diurnal variations exceed interannual variations in many
places and serve to define the seasonal mean.
• Observed diurnal rainfall: MT and SE show local oscillations (amplified in the daytime and
suppressed in the nighttime)
Single Column Model Test
• Observed CAPE is not in
phase with the observed
rainfall over GP: CAPE
can not explain nocturnal
rainfall
• Out of phase in the Great Plains to the observed; NCEP is
exceptional-nighttime maximum
• Ensemble spread merely reflect mean differences, not the
diurnal phase in the models.
• NASA and GFDL model shows little variation in the maximum
diurnal phase; NCEP shows much variation by location.
[meter]
Fig. 8. Observed (top) and simulated (bottom) precipitation rate (mm/day) over the ARM IOP period.
Diurnal Cycle of Rainfall
• The AGCM deficiencies in the diurnal cycle are symptomatic of our lack of understanding of the
relevant physical processes that, over the United States and Mexico, operate and interact on local,
regional, as well as continental and larger scales.
Fig. 5. Diurnal variation of the observed (NARR) convective available potential energy
(CAPE, unit: kJ/kg). Daily mean is subtracted.
• To improve our understanding of the important physical processes that drive the diurnal cycle and
to provide the guidance for the physical parameterizations, ensemble AGCM ensemble
experiments with three different models (GFDL, NCEP, NASA/GMAO) have been conducted by
prescribing climatological sea surface temperature.
Obs.
• Model Descriptions
Version
GFDL
AM2p12
NCEP
GFS v2
NASA
GMAO
AGCM v2
Resolution
Finite
2x2.5 L24
Spectral
T62 (~2x2)L64
Finite
2x2.5, L40
Cumulus Convection
Relaxed Arakawa-Schubert
(Moorthi and Suarez 1992)
Simplified ArakawaSchubert
(Pan and Wu 1994)
Relaxed Arakawa-Schubert
(Moorthi and Suarez 1992)
Obs
GFDL
NCEP
NASA
NARR
Shallow Convection
None
Diffusion type nonprecipitating (Tiedtke 1983)
None
Fig.3. Diurnal mean hourly precipitation rate (mm/day) for the five selected grid boxes. Colored thick lines (with
filled circle) indicate the ensemble mean and thin lines indicate each ensemble members. Observed are indicated
by black (NCEP HPD) and grey (NARR) lines.
• Simulated CAPE is
generally in phase with the
observed - maximum in the
daytime
• Simulated convection is
more sensitive to the local
CAPE variation rather than
large-scale dynamical
forcing (especially for GP)
rainfall amount
rainfall frequency
NARR
GFDL
NCEP
NASA
Model
Fig. 6. Comparison of diurnal CAPE variations
Diurnal Evolution of Rainfall and Low-Level Wind
• Experiments
Ensembles
Boundary condition
(SST and sea ice)
Initial Condition
Period
Observation
GFDL
GP: Eastward Propagating Convections
5 members
Climatology (1983-2002) from OISST version 2
Fig. 9. Diurnal cycle of rainfall in the observed (top) and simulated (bottom). Diurnal variations of rainfall
amount (left) and percent frequency (right) are separately shown.
00Z 01 May of selected years from their own AMIP runs
GFDL: 84, 88, 90, 92, 93
NCEP: 84, 88, 90, 92, 93
NASA: 93, 95, 96, 97, 98
5 summer months (1 May - 30 September)
hourly archive
Nighttime rainy day composite
• The RAS scheme still prefers daytime convection over the GP region:
Fundamental problem in the model convection scheme and the moist physics
Summary
• Validating Datasets
NCEP/CPC Hourly Precipitation Data (HPD) over the US domains (Higgins et al. 1996)
NCEP North American Regional Reanalysis (NARR) 3-Hourly datasets
Atmospheric Radiation Measurement Program (ARM) 3-Hourly observations in the South Great
Plains
Summer Mean Rainfall Simulations
• Diurnal cycle of warm season rainfall simulation was analyzed using three
different AGCMs (GFDL, NCEP and NASA/NSIPP).
NCEP
NCEP
NASA
NASA
• Reasonable simulations in general of
wet south/east and dry west over the
United States and North American
monsoon system
OBS
• Common dry biases over the Great
Plains (100-90W, 35-45N); shifted to
the west or northwest toward the
Rocky mountain area
• Drier in the Arizona-New Mexico
regions (110W, 32N) and wetter in the
ITCZ regions; too strong ITCZs and
weak northward penetration of the
monsoon front
GFDL
Fig. 7. Longitude-time composite of rainfall in the observation and models for the nighttime (06 pm-06 am)
rainy days over the Great Plains (between 100-95 W along the 40 N).
• Observed diurnal rainfall peaks in the evening (6 pm) over the mountains (105 W); from midnight
to morning (00-06 am) over the Great Plains (95 W)
• Convective activity moving eastward from the Rocky mountain toward Great Plains – contributed
by the eastward moving meso-scale convective systems (MCSs)?
Fig. 1. June-July-August mean precipitation rates (mm/day). Model
simulations are taken from 5-member ensemble averages and
compared with the observation (NCEP US-Mexico daily precipitation
analysis).
Contact: Myong-In Lee ([email protected]) URL: http://janus.gsfc.nasa.gov/~milee/diurnal
Fig. 4. 3-Hourly summer (JJA) mean diurnal cycle of rainfall and low-level wind vectors. Rainfall amount is represented as the
percentage (%) from the daily total amount. Diurnal time mean was removed in the wind fields and relative length of 5 m/s wind
arrow is given in the bottom of each panel.
• Quite different characteristics in models: different timing in rainfall maximum over the mountain
areas as well as over the Great Plains (model resolutions can be an issue)
• The AGCMs show large diversities in the simulation of diurnal cycle of rainfall
amplitude and phase; In general, earlier (2-3 hours in SE) or quite out of
phase (in GP) to the observed.
• Model simulation over the Great Plains:
- Fail to capture the nocturnal rainfall (NCEP was exceptional),
in spite of reasonable LLJ simulation
- Local diurnal forcing (or cape)>> large-scale dynamical forcing
- Wrong diurnal sensitivity in the single column test
Next Steps
• Extending to higher (1 deg, and ½ deg runs) resolution runs:
- Validate with High-resolution satellite dataset and upcoming
NAME field observations
• Focus detail of physical parameterizations:
- Interaction between deep convection and PBL
- Inhibition/trigger functions in the convection scheme