Land Surface Processes in NAMS
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Transcript of Land Surface Processes in NAMS
Land Surface Processes in NAMS
Dennis P. Lettenmaier
Department of Civil and Environmental EngineeringUniversity of Washington
for presentation at
Eighth Annual Meeting of WCRP/CLIVAR/VAMOS Panel (VPM8)
Mexico City
March 7, 2005
Motivation for talk
• Draws from our own work (esp. Zhu et al, 2005)
• Attempts to generalize hypotheses, and identify ways in which ongoing and future work can test them
Hypothesis for land surface role in NAMS
• Anomalously wet (dry) monsoon years tend to be preceded by anomalously dry (wet) antecedent winter precipitation (e.g. Higgins et al, 1998; 2000), and/or
• Anomalously wet (dry) monsoon years tend to be preceded by anomalously low (high) snowpack in the previous winter (e.g. Gutzler and Preston, 1997)
• Hypothesized mechanisms are different, but both lead to enhanced (suppressed) land-ocean contrast, hence strong (weak) NAMS
• Reality is, of course, more complicated – questions of lag/lead, geographic location of signal, stability over time
Study Domain
Monsoon regions are defined as in Comrie & Glenn (1998)
Monsoon West
Monsoon South
Monsoon North
Monsoon East
15-year Moving Average Correlation of PI versus MW JJAS rainfall
JFM Precipitation Index (PI)
0
40
80
120
160
1950 1960 1970 1980 1990
Year
Pre
cipi
tatio
n (m
m)
JFM PI
Winter Precipitation –
Monsoon Relationship
April Snow Index (SWE) 15-year Moving Average Correlation of SWE versus MW JJAS rainfall
Snow – Monsoon Relationship
Winter Precipitation - Monsoon Rainfall
feedback hypothesis
Higher (lower) winter precipitation & spring snowpack
More (less) spring & early summer soil moisture
Weak (strong) monsoon Lower (higher) spring & early summer surface temperature
JFM Precipitation in extreme monsoon
years
DRY WET
DRY WET
Apr-May Soil Moisture in extreme monsoon years
Dry MonsoonWet Monsoon
Soil moisture anomalies persist from spring until June
WetDry
Correlation of June Sm & JFM PI (1965-1999)
What is the feedback to the atmosphere ?
Correlation:April SWE & May-June Ts (Negative relationship )
Correlation:June Sm & June Ts (No significant relationship in MW )
SW Sm has no significant relationship with Ts
WetDry
WarmCold
June Ts in extreme monsoon years
June Sm in extreme monsoon years
× ×
Winter Precipitation - Monsoon Rainfall feedback hypothesis
Higher (lower) winter precipitation & spring snowpack
More (less) spring & early summer soil moisture
Weak (strong) monsoon Lower (higher) spring & early summer surface temperature
×?
June 30Wet years Dry years
SW desert daily precipitation in extreme years
Correlation:June Ts & July MW precipitation
Negative correlation between SW desert pre-monsoon Ts & July precipitation ?
North American Monsoon Conceptual Basis
• The combination of seasonally warm land surfaces in lowlands and elevated areas together with atmospheric moisture supplied by nearby maritime sources is conductive to the formation of a monsoon like system. ( Adams D. K and A. C. Comrie, 1997: The North American Monsoon. Bull. Amer. Meter. Soc.,2197-2213. )
• The inverse spring snow – summer rainfall relationship proposes the hypothesis intuitively: excessive spring snow means cold continental temperature, thus inhibiting the summer time land surface heating that drives the monsoon. (Gutzler D. S. and J. W. Preston, 1997: Evidence for a relationship between spring snow cover in North America and summer rainfall in New Mexico. Geophys. Res. Let., 24, 2207-2210.)
Higher (lower) June Ts Stronger (weaker)
monsoon
Z500 (m) anomalies in wet years
Wet Monsoon
Low
June
High
June Ts anomalies in wet years
Dry Monsoon
High
June
Low
Z500 (m) anomalies in dry years
June Ts anomalies in dry years
The strong positive relationship between June Ts and Z500 anomalies, suggesting that pre-monsoon Ts are not modulated by the local land-surface conditions.
Summary
● Southwest winter precipitation is a potential predictor for MW summer monsoon, even though this relationship varies with time
● Spring land surface conditions in the SW U.S. are strongly determined by the previous winter’s precipitation, and this land memory can persist through April and May into June. However, this memory appears to contribute little to the magnitude of NAM precipitation.
● June positive Z500 anomalies in dry years induce an increase in surface temperature in AZ and NV, and vice versa for wet year, which suggesting that the controlling factor for the pre-monsoon Ts anomalies may not be local
Comments• Question as to whether (lack of) persistence in
soil moisture from previous winter-spring to onset of monsoon is model artifact
• Coupled model experiments (possibly along lines of Koster-Suarez land-ocean-atmosphere coupling design) are probably needed to make further progress
• Such experiments have been hampered in the past by model-specific behavior, and/or lack of consistency between off-line and coupled land schemes.
• Latter can be resolved by multi-model ensemble framework for off-line runs, and consistent LDAS forcing data for NAMS region (which have now been assembled)