MM5/VIC Modeling Evaluation of the Influence of Antecedent Soil Moisture on Variability of the North American Monsoon System Chunmei Zhu a, Yun Qian b, Ruby Leung b, David Gochis c, and Dennis P. Lettenmaier a a Department of Civil and Environmental Engineering Box , University of Washington, Seattle, WA b Pacific Northwest National Laboratory, Richland WA c National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307

Winter Precipitation - Monsoon Rainfall feedback hypothesis Higher (lower) winter precipitation & spring snowpack More (less) spring & early summer soil moisture Lower (higher) spring & early summer surface temperature Weak (strong) monsoon Hypothesis to be tested by: (Zhu et al., J. Climate, 2005, 2007)

PNNL UW vegetation type: Single Multiple elevation band: Single Multiple Parameters: Soil, veg type dependent cell dependent initialization: Spin up 3 months Offline VIC MM5-VIC coupled model system DocumPublicatios Links Precipitation Pressure Radiation Wind Humidity Air temperature Sensible heat flux Latent heat fluxes … First coupled by Drs. Ruby Leung at PNNL and Xu Liang at University of California, Berkeley Modification of coupled MM5/VIC modeling system by UW

Domain LateEarly Regions for which winter precipitation are related to summer monsoon in MW and MSa in Zhu et al 2005, MW MS 1 MM5/VIC model setup: 150*178 grid cells at 30km resolution in a Lambert-Conformal projection NAMS (2) (1) ● Kain-Fritsch (KF) scheme ● Rapid Radiative Transfer Model (RRTM) long-wave scheme ● simple ice-explicit microphysics ● medium-range forecast (MRF) boundary layer scheme ● NCEP/NCAR Reanalysis LBC (eastern AZ and western NM) (northwestern Mexico) Soil Moisture prescribing domain

Experimental Design Initial soil moisture prescribed at OctSepAugJulyJune SM free running May 15 Field capacityWilting point ► Simulations performed on wet and dry monsoon years to represent different atmospheric circulation conditions 2 ► The initial soil wetness condition on May 15 is a surrogate for previous winter precipitation condition. ► Control simulation s. moisture prescribed from offline VIC LDAS (3 mo spin-up, Mar-Apr-May).

MSa JJAS Precipitation (dark) and Onset (gray) MW JJAS Precipitation Wet year: 1984 Dry year:1989 Selection of wet/dry years: 1989

Validation of coupled MM5/VIC modeling system 1984 wet year: Mean monthly daily precipitation Control SimulationObservation JuneJuly AugSep June July AugSep

1989 dry year: Mean monthly daily precipitation Control SimulationObservation JuneJuly AugSep June July AugSep MM5/VIC more aggressive in precipitating during ‘dry’ year

Positive Soil Moisture-Monsoon Rainfall Feedback ? 1984-wet minus 1984-dry 1989-wet minus 1989-dry JuneJuly AugSep JuneJuly AugSep mean monthly precipitation difference

Winter Precipitation - Monsoon Rainfall feedback hypothesis Higher (lower) winter precipitation & spring snowpack More (less) spring & early summer soil moisture Lower (higher) spring & early summer surface temperature Weak (strong) monsoon The reverse of the proposed negative Begin to examine 3 links……

Soil moisture differences between the wet and dry runs persist until mid-summer First LayerThird Layer JuneJuly Aug Sep JuneJuly AugSep JuneJuly AugSep June July AugSep

Land surface memory – surface thermal conditions (1984) First layer soil moisture Latent heat Surface skin temperature + -- Difference maps between 1984-wet and 1984-dry runs June July Aug Sep

Difference maps between 1989-wet and 1989-dry runs First layer soil moisture Latent heat Surface skin temperature + -- June July Aug Sep

Larger Thermal contrast– stronger monsoon JuneJuly AugSep JuneJuly AugSep Monthly mean surface skin temperature Monthly mean precipitation Difference map between 1984-wet and 1984-dry runs: ?

Difference maps between wet and 1984-dry runs 500mb Geopotential Height Surface Skin Temperature 925mb Geopotential Height In MM5-VIC increased local surface pressure weakens the Southwest surface heat low, but is related to the stronger monsoon? Southwest surface heat low – monsoon strength ? June July Aug Sep

JuneJulyAugustSeptember 1984-Wet Dry Wet Dry Monthly mean 925 mb meridional moisture flux (QV) averaged over longitude ( o W) at 32 o N Weakening of the thermal low in MM5/Vic sims results in greater moisture flux into the interior of the NAMS region, likely from increased moisture availability due to increased regional evaporation instead of increased low level winds

Monthly mean planetary boundary layer height (PBL) in the NAMS domain JuneJulyAugustSeptember 1984-Wet Dry Wet Dry Boundary layer height difference between 1984-wet and 1984-dry runs Shallower Boundary Layer Wet soil moisture conditions reduce the depth of the boundary layer, therefore increase the boundary layer moist static energy and the frequency and magnitude of rainfall from local convective storms. local land-atmosphere interaction JuneJuly Aug

Summary and Conclusions ● The MM5-VIC control sims reproduce reasonable monsoon precipitation for 1984 and 1989 over northwestern Mexico (1989 somewhat wet vs. obs) ● The model land surface has memory of the initial soil wetness that lasts for several months (until August). This land memory has a negative relationship with surface thermal conditions over the NAMS domain and its larger adjacent area. ● In contrast to the original hypothesis, the wet year 1984 and dry year 1989 experiments exhibit similar positive soil moisture – rainfall feedbacks over the NAMS domain. In essence, it appears that local-regional recycling of moisture dominates in sustaining increased precipitation in the model. However magnitude of imposed anomaly likely imparts excessive influence. ● In nature, both the large-scale circulation changes and local land- atmospheric interactions in response to soil moisture conditions likely play important roles in the soil moisture – monsoon precipitation feedback. The symbiosis of these features needs to be studied in more detail.

Limitations of the experiments Extreme wet and dry soil conditions in the sensitivity experiments extreme surface temperature anomalies exaggerated surface low (not the optimal strength and location to start monsoon) very intense local evaporation Contribute to apparent positive soil moisture – rainfall feedback Future Work Explore the relationship between antecedent soil moisture and monsoon rainfall under less extreme soil conditions, and to identify the relative importance of large-scale circulation and local evaporation.

Large-scale circulation or local land- atmosphere interaction ? shallower boundary layer Increased convective instability and potential for precipitation Changes moisture convergence and precipitation. Changes the surface pressure and the flow field Meehl G. A., 1994: J. Climate Schar C et al. 1999: J.Climate Mo K. C. and H. H. Juang, 2003: J. Geophy. Res

SM1TgrdLHSH June 1984-Wet Dry Wet Dry July 1984-Wet Dry Wet Dry August 1984-Wet Dry Wet Dry September 1984-Wet Dry Wet Dry NAMS (2) (1) Monthly means of energy components in the NAMS region Wet soil raises the latent heat and reduces the sensible heat by nearly equal amounts, resulting in decreased surface skin temperature