GFDL

NCEP NASA

Observation

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.

• Observed diurnal rainfall: MT and SE show local oscillations (amplified in the daytime and suppressed in the nighttime)

• 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.

• Large diversities in the representation of rainfall diurnal cycle among the models

• Agreements to the observation in the East and Southeast of the US; Disagreements in the mid-continent and mountain regions

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).

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).

Summer Mean Rainfall Simulations

Introduction

• 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.

• 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.

• 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.

• 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.

• Model Descriptions

• Experiments

• Validating DatasetsNCEP/CPC Hourly Precipitation Data (HPD) over the US domains (Higgins et al. 1996)NCEP North American Regional Reanalysis (NARR) 3-Hourly datasetsAtmospheric Radiation Measurement Program (ARM) 3-Hourly observations in the South Great Plains

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, N.-C. Lau3, B. Tian3, A. Kumar4, H.-K. Kim4, J. Schemm4, K. Mo4 and W. Higgins4

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

Contact: Myong-In Lee (milee@janus.gsfc.nasa.gov) URL: http://janus.gsfc.nasa.gov/~milee/diurnal

Ensembles 5 membersBoundary condition(SST and sea ice)

Climatology (1983-2002) from OISST version 2

Initial Condition

00Z 01 May of selected years from their own AMIP runsGFDL: 84, 88, 90, 92, 93NCEP: 84, 88, 90, 92, 93NASA: 93, 95, 96, 97, 98

Period 5 summer months (1 May - 30 September)hourly archive

• Reasonable simulations in general of wet south/east and dry west over the United States and North American monsoon system

• 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

Summary

• Diurnal cycle of warm season rainfall simulation was analyzed using three different AGCMs (GFDL, NCEP and NASA/NSIPP).

• 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

Abstract

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).

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.

Version Resolution Cumulus Convection Shallow Convection

GFDL AM2p12Finite

2x2.5 L24Relaxed Arakawa-Schubert (Moorthi and Suarez 1992)

None

NCEP GFS v2Spectral

T62 (~2x2)L64

Simplified Arakawa-Schubert (Pan and Wu 1994)

Diffusion type non-precipitating (Tiedtke 1983)

NASAGMAO

AGCM v2Finite

2x2.5, L40 Relaxed Arakawa-Schubert (Moorthi and Suarez 1992)

None

SE

GPMT

AZNMNAME

[meter]

ObsGFDLNCEPNASANARR

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.

• Late afternoon peaks of rainfall are simulated 1-3 hours earlier in most regions

• 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.

Amplitude and Phase of the Diurnal Cycle

Diurnal Evolution of Rainfall and Low-Level Wind

Fig. 5. Diurnal variation of the observed (NARR) convective available potential energy (CAPE, unit: kJ/kg). Daily mean is subtracted.

• Observed CAPE: daytime maximum in most regions

• Observed CAPE is not in phase with the observed rainfall over GP: CAPE can not explain nocturnal rainfall

• Examining the response of the model physics to the “perfect” observed atmospheric forcing

• 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

Single Column Model Test

Obs.

Model

Diurnal Cycle of Rainfall

Observed and Simulated Precipitation Rates

rainfall amount rainfall frequency

• The RAS scheme still prefers daytime convection over the GP region: Fundamental problem in the model convection scheme and the moist physics

GP: Eastward Propagating Convections

• 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)?

• 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)

NARRGFDLNCEPNASA

Fig. 6. Comparison of diurnal CAPE variations

Diurnal Cycle of CAPE

• 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)

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).

OBS

GFDL

NCEP NASA

Nighttime rainy day composite

Fig. 8. Observed (top) and simulated (bottom) precipitation rate (mm/day) over the ARM IOP period.

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.