A33B-01 INVITED
The Low-Level Flow Along the Gulf of California During the North American Monsoon.
Six-years (1999-2004) of QuikSCAT near-surface ocean winds are used to study the flow over the northeast Pacific and the Gulf of California (GoC) during the North American Monsoon season. The wind data show that the onset of the summer season is accompanied by a reversal of the flow along the GoC, with the establishment of a mean southerly wind throughout the gulf. This reversal occurs in late spring and precedes the onset of the monsoonal rains. In the heart of the monsoon season, the time-mean flow is found to be composed of periods of enhanced southerly winds associated with gulf surges. The role that gulf surges play in modulating the GoC mean southerly flow is further explored by performing an EOF analysis of the summertime daily wind anomalies. A gulf surge mode emerges from this analysis as the leading EOF, with the corresponding principal component time series interpretable as an objective index for gulf surge occurrence. This index is used as a reference time series for regression analysis, to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon, as well as the manifestation of these transient events in the large-scale circulation. It is found that, although seemingly mesoscale features confined over the GoC, gulf surges are intimately linked to patterns of large-scale variability of the eastern Pacific ITCZ and greatly contribute to the definition of the northward extent of the monsoonal rains.
A33B-02
Influence of Sea Surface Temperature and Soil Moisture on Precipitation Over the Southwest United States
Influence of sea surface temperature anomalies (SSTAs) and soil moisture on precipitation (P) over the Southwest is examined using the reconstructed SST data and the hydrologic conditions simulated by the VIC North Land Data Assimilation System from 1915- 2003. Arizona and New Mexico belong to two different precipitation regions. They have different moisture sources and should be examined separately. The relationships between winter and summer precipitation for both AZ and NM vary from one period to another and are controlled by the SSTs. For the recent period (1960-2000), there was a strong inverse relationship between winter (January-March) and summer (July-September) P for Arizona when tropical SSTAs persist from winter to summer. When SSTAs in the North Pacific are strong, wetness or dryness in Arizona persists from winter to summer. For New Mexico, summer precipitation is modulated by SSTAs in the tropical Pacific and SSTAs in the tropical Atlantic over the western hemisphere warm pool area. For Arizona, strong evaporation in spring depreciates soil moisture.Total soil moisture reaches a minimum before the monsoon onset in July. Soil moisture does not influence monsoon onset. The variability of soil moisture over Arizona and New Mexico is small in summer. Soil moisture seems to play a secondary role in modulating monsoon rainfall.
A33B-03
Convective Heating and Moistening Within the Interior of the NAME Domain
Atmospheric sounding, aircraft, pibal, and surface observations taken during the 2004 North American Monsoon Experiment (NAME) have presented the opportunity to investigate the nature of convection and its diurnal cycle in the monsoon region of northwest Mexico, a region that has heretofore been poorly sampled. The core of the NAME domain, which is the focus of this study, is characterized by steep orography in proximity to a coastline in a moist, tropical environment. Much of the world's heaviest rainfall occurs in such environments, so the results of this study have broader implications than just those applicable to the NAME region. The sounding and surface observations have been quality-controlled and assembled to produce a gridded data set over the NAME domain. In order to obtain a proper lower boundary condition to compute vertical motion over the slopes of the central mountain range of Mexico (the Sierra Madre Occidental or SMO), surface and pibal data, albeit sparse, were used to describe the boundary-layer and low-level flow. Divergence and vertical motion were then computed within sounding polygon areas encompassing both land and ocean (Gulf of California) regions. The land polygon overlapped the domain of the NCAR S-Pol radar deployed north of Mazatlan. Results show a prominent sea breeze, land breeze cycle along the west slopes of the SMO. There is a deep return flow above the afternoon sea breeze as a consequence of the elevated SMO immediately to the east. Subsidence produces a dry layer in the afternoon over the Gulf of California, while a deep moist layer exists over the western slopes of the SMO. The diurnal cycle of apparent heating and moistening (Q1 and Q2) over the SMO is characterized by deep-convective profiles in the mid-to-upper troposphere at 1800 LT, followed by stratiform- like profiles at midnight, consistent with the observed diurnal evolution of precipitation over this coastal mountainous region. Unlike experiments such as GATE and TOGA COARE, cooling is observed in the lower troposphere in the mean profiles of Q1, likely due to more evaporative cooling from drier conditions at low levels over land compared to those over tropical oceans. The diagnosed fields and budgets from the gridded analyses, which are completely independent of model data, will be compared to those from a special North American Regional Reanalysis (NARR) prepared specifically for NAME. Explanations for the differences will be explored.
A33B-04
The Diurnal Cycle of Precipitation Over the North American Monsoon Region During the 2004 NAME Field Campaign
The structure of the diurnal cycle of warm-season precipitation and its associated fields during the North American Monsoon are examined for the core Monsoon region and for the southwestern United States, using a diverse set of observations, analyses and forecasts from the North American Monsoon Experiment field campaign of 2004. Included are rain-gauge and satellite estimates of precipitation, Eta model forecasts, and the North American Regional Reanalysis (NARR). The diurnal cycle of precipitation within the core region occurs earlier in the day at higher topographic elevations, evolving with a westward shift of the maximum. This shift appears in the observations, reanalysis, and, while less pronounced, in the model forecasts. Examination of some of the fields associated with this cycle, including convective available potential energy (CAPE), convective inhibition (CIN), and moisture flux convergence (MFC), reveals the westward shift appears in all of them, but more prominently in the latter. In general, warm-season precipitation in southern Arizona and parts of New Mexico shows a strong effect due to northward moisture surges from the Gulf of California. The non-surge diurnal cycle of precipitation lags the CAPE maximum by six hours and is simultaneous with a minimum of CIN, while the moisture flux remains divergent throughout the day. During surges, CAPE and CIN have modifications only to the amplitude of their cycles, but the moisture flux becomes strongly convergent about six hours before the precipitation maximum, suggesting a stronger role in the development of precipitation.
A33B-05
Influence of Sea-Surface Temperature on the Diurnal Cycle of the North American Monsoon System
The diurnal cycle of cloud and precipitation associated with the North American Monsoon System (NAMS) has been examined using three-hourly data sets of geostationary IR of Janowiak et al. 2001), the CMORPH satellite precipitation estimates of Joyce et al. (2004), and the Multi-Platform-Merged (MPM) SST analysis of Wang and Xie (2007) for summer 2004. A comprehensive diagnostic study is performed to describe the temporal-spatial structure of the mean state and diurnal cycle of the NAMS cloud/precipitation systems and their relationship to SST over the adjacent oceanic areas. Our results are: 1) Variations of cloudiness and precipitation associated with the North American Monsoon System (NAMS) are dominated by diurnal cycle; 2) Clouds and precipitation start from higher elevation in the morning, move toward the coast as they reach the maximum in late afternoon; 3) The phase of the diurnal cycle is relatively stable, while the magnitude presents changes of synoptic and intraseasonal time scales; 4) Maximum of deep convection and precipitation appears 50-100km west to the mountain crests, and, 5) The intensity of the NAMS convection is positively (negatively) correlated with that of the diurnal amplitude of SSTs over the Gulf of Mexico (the Gulf of California). Further work is underway to examine how the NAMS diurnal cycle and its relationship to SST are simulated by NCEP operational climate forecast models.
A33B-06
NAME Modeling and Climate Process Team
NAME Climate Process and Modeling Team (CPT) has been established to address the need of linking climate process research to model development and testing activities for warm season climate prediction. The project builds on two existing NAME-related modeling efforts. One major component of this project is the organization and implementation of a second phase of NAMAP, based on the 2004 season. NAMAP2 will re-examine the metrics proposed by NAMAP, extend the NAMAP analysis to transient variability, exploit the extensive observational database provided by NAME 2004 to analyze simulation targets of special interest, and expand participation. Vertical column analysis will bring local NAME observations and model outputs together in a context where key physical processes in the models can be evaluated and improved. The second component builds on the current NAME-related modeling effort focused on the diurnal cycle of precipitation in several global models, including those implemented at NCEP, NASA and GFDL. Our activities will focus on the ability of the operational NCEP Global Forecast System (GFS) to simulate the diurnal and seasonal evolution of warm season precipitation during the NAME 2004 EOP, and on changes to the treatment of deep convection in the complicated terrain of the NAMS domain that are necessary to improve the simulations, and ultimately predictions of warm season precipitation These activities will be strongly tied to NAMAP2 to ensure technology transfer from research to operations. Results based on experiments conducted with the NCEP CFS GCM will be reported at the conference with emphasis on the impact of horizontal resolution in predicting warm season precipitation over North America.
A33B-07
Investigating the regional influence of the monsoonal convection in Northwest Mexico
The influence of the monsoonal convection in Northwest Mexico on regional circulation and rainfall is examined by increasing the monsoonal convection in an atmospheric general circulation model and examining the regional impacts of that change relative to a control run with no modifications. The NCAR Community Atmospheric Model (CAM), version 3, is used as the atmospheric model and the monsoonal convection is enhanced by locally increasing the diabatic heating in the model, an approach that has proven successful in other regions. Previous research has linked the onset of the monsoonal convection in Northwest Mexico with decreased rainfall in a broad swath across the Northwest, Central, and western-Gulf Coast regions of the US. However, the dynamics of this relationship are not clear, especially the question of whether the monsoonal convection actively forces the suppression of US rainfall or whether some third factor, such as the interaction of the westerlies with the North American Cordillera, influences both regions. Here we assess the direct impact of the monsoonal convection by investigating the changes that occur when the monsoonal convection is substantially increased in the model. (Only the diabatic heating is altered in the model and only over Northwest Mexico; otherwise the model is unmodified and the circulation progresses freely.) The modified run with increased monsoonal convection over Northwest Mexico does indeed have suppressed rainfall in the Central US relative to the control run, demonstrating that the monsoonal convection can play a forcing role, at least in the model. The associated changes to regional circulation and their influence on US rainfall are examined in terms of dynamically-forced vertical motion, thermodynamically-forced vertical motion, moisture transport, and convective available potential energy.
A33B-08
Spatiotemporal Variability and Covariability of Temperature, Precipitation, Soil Moisture, and Vegetation in North America for Regional Climate Model Applications
Previous work has established that the dominant modes of Pacific SSTs influence the summer climate of North America through large-scale forcing, and this effect is most pronounced during the early part of the season. It is hypothesized, then, that land surface influences become more dominant in the latter part of the season as remote teleconnection influences diminish. As a first step toward investigation of this hypothesis in a regional climate model (RCM) framework, the statistically signficant spatiotemporal patterns of variability and covariability in North American precipitation (specified by the standardized precipitation index, or SPI), soil moisture, and vegetation are determined for timescales from a month to six months. To specify these respective data we use: CPC gauge- derived precipitation (1950-2000), Variable Infiltration Capacity (VIC) Model and NOAH Model NLDAS soil moisture and temperature, and the Global Inventory Modeling and Mapping Studies Normalized Difference Vegetation Index (GIMMS-NDVI). The principal statistical tool used is multiple taper frequency singular value decomposition (MTM-SVD), and this is supplemented by wavelet analysis for specific areas of interest. The significant interannual variability in all of these data occur at a timescale of about 7 to 9 years and appears to be the integrated effect of remote SST forcing from the Pacific. Considering the entire year, the spatial pattern for precipitation resembles the typical ENSO winter signature. If the summer season is considered seperately, the out of phase relationship between precipitation anomalies in the central U.S. and core monsoon region is apparent. The largest soil moisture anomalies occur in the central U.S., since precipitation in this region has a consistent relationship to Pacific SSTs for the entire year. This helps to explain the approximately 20 year periodicity in drought conditions there. Unlike soil moisture, the largest anomalies in vegetation occur in the southeast U.S. and appear more related to temperature variability. In the core monsoon region, interannual variation in vegetation growth is governed by monsoon precipitation. Future RCM work will use these patterns of long-term variability of soil moisture and vegetation in sensistivity experiments investigating land-surface interactions in the warm season.