He wrote that a surge resembles a large sea breeze. John Hales, formerly of the Phoenix National Weather Service office, wrote in the April 1972 edition of Monthly Weather Review that gulf surges are related to large areas of cloud masses that are transported northward up the Gulf of California and spill into southern Arizona. Gulf of California moisture surges were first scientifically documented in the early 1970s. The high pressure area over the northern gulf tends to push the moisture surge to the east towards the Tucson area. As this moist air travels northward, it encounters the already present southerly winds and gets pushed into southern Arizona. Some of this circulation will make its way into the gulf and get funneled northward towards the southwestern United States like a waveguide. Flow around such a low pressure system is cyclonic, which corresponds to counter-clockwise. Typically, during a traditional surge, a large mesoscale convective system is located off the southern tip of the Baja peninsula. Winds will blow from the south due to the pressure gradient force. During the monsoon, there is a pressure difference between the heat low that is present over the Southwestern United States and the relative high pressure over the northern portion of the gulf. This flow tends to get wider and deeper as the surge progresses. The first indication of a gulf surge is a change in the surface wind direction at Yuma, Arizona, with the winds switching from westerly to south-easterly. This instrument has the capability to measure wind speed and direction at several altitudes in the atmosphere in what is known as a vertical wind profile. The best data currently available that indicates moisture arriving from the gulf is from the NEXRAD radar in Yuma, Arizona. However, due to a lack of observations in the area, the exact cause is uncertain. There have been several proposed mechanisms for the development of gulf surges including gravity currents, ageostrophic flows, Kelvin waves, or Rossby waves. The North American Monsoon is not as strong or persistent as its Indian counterpart, mainly because the Mexican Plateau is not as high or as large as the Tibetan Plateau in Asia. Rains from the monsoon typically start in May or June along the western slope of the Sierra Madre Occidental and move northward, reaching southern Arizona sometime in July. The low develops over the Mexican Plateau and gradually moves northward towards the four corners region. During the monsoon months, the subtropical ridge moves northward due to the development of a thermal low from the intense solar radiation. During the winter months, the weather patterns in the Southwest United States are characterized by a semi-permanent high-pressure system with quasi-weekly weather systems moving through the area a cold front will move through the area, followed by a gradual building of the ridge. Onset is usually in early July when the winds start to shift due to intense solar heating of the Southwest United States. The North American Monsoon is experienced as a seasonal reversal of the prevailing winds, which is usually accompanied by an increase in rainfall.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |