Thursday, 18 January 2007: 4:00 PM
Land-atmosphere feedbacks: precipitation recycling in the NAMS region
209 (Henry B. Gonzalez Convention Center)
The main focus of this work is to study precipitation recycling as part of the dynamic North American Monsoon System (NAMS), and understand how different land and atmospheric variables within the system modulate recycling. In order to do this, we take a set of 18 land-atmosphere variables derived from North American Regional Recycling (NARR) data to represent the hydroclimatology of the monsoon. One of these variables is the recycling ratio, or ratio of recycled to total precipitation, calculated at the daily timescale using the dynamic recycling model (Dominguez et al, 2006). Multichannel Singular Spectrum Analysis (M-SSA) is used to extract trends in the data while at the same time selecting only the variability common to all of the variables. The first five M-SSA modes of average 1985-1995 daily data capture many of the characteristics of the monsoon that have been widely discussed in the literature, such as the abrupt increase in precipitation and precipitable water, the temperature peak prior to the rains, the shift from westerly to easterly winds and consequent shift in zonal moisture flux, and the increase in northerly moisture flux confined to the Gulf of California coastal region. Furthermore, we see an increase in evapotranspiration and recycling ratio and a decrease is sensible heat, humidity index, and the cloud base height.
The climatological analysis of NAMS precipitation recycling reveals a positive feedback mechanism between monsoon precipitation and subsequent increase in precipitation of recycled origin. In agreement with previous work using water vapor traces (Bosilovich et al., 2003), our study finds that evapotranspiration within the NAMS region significantly contributes to monsoon rainfall after NAMS onset. While monsoon rainfall and evapotranspiration are predominantly located in the southwestern part of the domain, recycling is enhanced northwest of this region, indicating a relocation of soil moisture further inland to drier regions in the north.
Focusing our analysis on the three years with longest monsoons in the eleven-year period, an interesting a-synchronous pattern between precipitation and recycling ratio is revealed. The longest monsoons present a characteristic double peak in precipitation. Intense precipitation during late June and early July is followed by a period of dry conditions and a subsequent peak in late August precipitation. Contrary to what one might expect, recycling peaks during the intermediate dry period. Further inspection of other land-atmosphere variables during a long monsoon reveals that the period of decreased precipitation is accompanied by changes in many other land-atmospheric conditions that lead to the recycling peak.