The Role of Sea Surface Temperature and Vegetation Characteristics in the Seasonal Evolution of Summer Rains over Northwestern Mexico
Michael W. Douglas,
Institution National Severe Storms Laboratory
Project Duration: Three years
Introduction
Previous observations have shown that a seasonal march of the region of maximum precipitation from the slopes of the Sierra Madre Occidental (SMO) towards the Gulf of California (GOC) in Mexico occurs as the summer progresses. This project evolved to improve the understanding of the mechanisms involved in this migration. We speculate that this seasonal progression is caused by the changes in the vegetation characteristics during the onset period of the North American Monsoon (NAM) together with the seasonal increase in the GOC SSTs.
A variety of observing networks were deployed before and during the first stages of the North American Monsoon Experiment (NAME) Field Campaign, carried out in the summer of 2004. The networks included one flux tower, 7 temporary pilot balloons stations, a network of simple raingauges, and a special sounding site at Tezopaco, Sonora where thermodynamic soundings and additional surface observations were made. The entire network operated during a 4-month period, from June through September, with the exception of the flux tower, which started operation on July 10th.
The special pilot balloon sounding network was designed to better describe circulation characteristics associated with the diurnal cycle, to describe the characteristics of the seasonal evolution of the sea-land breeze that occurs on the east side of the GOC, and to complement the other sounding sites installed in western and northern Mexico for the NAME. The pilot balloon soundings were made twice daily and synchronized with the existing upper air networks. The thermodynamic observations at Tezopaco were made together with the surface observations during four intensive observation periods (IOPs) designed to capture in more detail different stages of the NAM.
The flux tower, equipped with meteorological sensors at different levels, was installed at a dry forest site about 6 km east of Tezopaco, in southeast Sonora. It was designed to measure the evolution of the canopy-top fluxes associated with the vegetation changes of the tropical dry forest. The flux data have been collected almost continuously since the date of installation. The raingauge network consists of about 300 simple raingauges. These rain gauges are designed to measure daily rainfall about the central GOC region, thus they were deployed both in parts of the Baja California peninsula and in the coastal and foothill regions on the eastern side of the GOC. Both the tower sensors and the raingauge network continued their operations after 30 September 2004.
Except for the raingauge network data, all of the different data sets were gathered in November 2004. By the time this report was created most of the data (except those from the flux tower) were still in a raw format given the short time since they were brought to the NSSL. Since the quality control process is just starting, the results currently available are preliminary. However, we are including some of these findings in this document, in particular the ones related to the thermodynamic soundings and the evolution of the dry forest foliage. A powerpoint presentation (poster) is also included, showing work that Chris Watts and collaborations in northwestern have done with data from the flux towers. This document ends with a statement of the next year’s planned activities.
Project Goals
The fundamental goal of this project is to explain the seasonal migration of the zone of maximum rainfall away from the foothills of the Sierra Madre Occidental and towards the Gulf of California as the warm season evolves. Additional objectives include developing a better understanding of whether the changes in the tropical dry forest vegetation are involved in this seasonal evolution and to determine what the role of the vegetation leaf cover change is in the overall NAM climate.
Method
Our project required making sufficient measurements to confirm the coastward migration of precipitation, and to describe the appropriate physical processes involved in producing the hypothesized rainfall migration. This involved a variety of special measurement systems and also use of some routinely available information. This is detailed in the accompanying report.
Results and Accomplishments
The field observation activities were successfully carried out for the most part. Please see the attached documents for details.
Future Work
The data analysis phase is just beginning, so the next step is to prepare a completed data base. Then the seasonal evolution of the diurnal cycle of the windfield will be determined from the pilot balloon and automated surface station data.
Publications from this project
No publications to date. Posters have been presented at meetings in Albuquerque, San Francisco (AGU) and will be presented at the AMS Annual meeting in January.
Contacts: Michael Douglas (NSSL), Christopher Watts (University of Sonora), Russell Scott (USDA, Tucson)
Work plan for coming year
Data processing
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Pilot balloon data processing
During the coming months all of the pilot balloon data will be placed in an electronic format and then fully quality controlled. This task will be carried out with the help of two undergraduate students, one of whom participated in making the observations in Tesopaco. The design of the pilot balloon array will then allow us to calculate a variety of meteorological quantities over different triangles and polygons. The data will be used to identify the seasonal evolution of land-sea breezes, and infer the changes in the intensity of the land breezes, sea breezes and the patterns of vertical motion.
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Rainfall data processing
The rainfall data stored in forms still needs to be collected by each municipality in Mexico and then gathered by Proteccion Civil from Sonora. When this task is completed, all the forms and information about the location of the gauges will be transferred to the NSSL for quality control and analysis. The information then needs to be introduced into an electronic format and further quality controlled.
The final rainfall dataset will complement other indirect measures of rainfall available such as the GOES visible and IR imagery and the Mexican Weather Service (SNM) radar data from the Guasave and Obregon radars.
We expect the rainfall data to be useful in identifying the date of onset of the rains for comparison with the vegetation and diurnal breeze changes. The stations will also be composited with respect to distance from the coast to see if the hypothesized mountain – to - coast rainfall propagation was clearly observed during the summer of 2004.
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Additional Data
Additional available datasets include the radiosonde and tethersonde observations, hourly surface observations, cloud photography and vegetation photography. These data, together with the rainfall and pilot balloon information, will be quality-controlled and made available via the UCAR JOSS website and at the following website: http://www.nssl.noaa.gov/projects/pacs.
Research foci
The research activities during the next year will focus on identifying the seasonal changes in the diurnal circulations at all of the sounding sites and distinguishing the seasonal evolution from the more rapid change that might be expected with the onset of the rains. We expect to be able to develop a composite of the boundary layer structure and diurnal cycle at the Tezopaco site that represents pre- and post onset conditions.
One graduate student will begin a MS degree program in January 2005 at the University of Oklahoma. The student, from Mexico, spent two months at the Tezopaco site in August and September 2004, and is familiar with the data set collected. Another Mexican student will visit NSSL in January-March 2005, to work towards finishing a thesis required by student of the Bachelor's degree program in Meteorology at the University of Veracruz in Xalapa, Mexico. This student also spent two months at the Tezopaco site.
Funds are being requested to continue operation of the flux tower for a second summer. These funds should cover the site's operation and associated data processing and research activities at the University of Sonora under the supervision of the co-PI Chris Watts. A preliminary report showing some of the results from the flux tower observations is attached to this report.
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Last Updated August 3, 2005