Post by Alicia Bentley on Sept 6, 2013 22:39:07 GMT
UAlbany's first Friday map discussion of the academic year was led by Dr. Lance Bosart earlier this afternoon. The discussion was titled "Recap of Summer 2013" and was intended to 1) provide a global overview of this past summer's observed weather patterns and 2) highlight potential scientific opportunities inherent in those observed weather patterns.
Dr. Bosart began with a discussion of the general circulation over the United States from mid-June to the end of August. Dr. Bosart noted that this period could be broken down into three separate regimes. The first regime, which existed from 21 June to 20 July, was characterized by an anomalous ridge in the 300-hPa geopotential height field over the western United States, trough over the southeastern United States, and ridge over much of New England. The northwestward shift in the position of the 850-hPa Bermuda high relative to climatology, coupled with the position of the anomalous trough in the 300-hPa geopotential height field, facilitated the transport of deep tropical moisture into the southeast United States from the Gulf of Mexico and western Atlantic. This setup lead to cloudy/soggy conditions and anomalously cool 850-hPa temperatures for much of the Southeast. Meanwhile, confluent upper-tropospheric flow over eastern Canada prevented cooler air from traveling southward and resulted in anomalously warm conditions over the Northeast. The second regime, which existed from 21 July to 20 August, was dominated by a broad upper-tropospheric trough over much of the eastern United States and ridge over northwestern Canada. The eastward shift of the subtropical high into the central North Atlantic facilitated the development of anomalous northerly flow and cooler temperatures over the central and eastern United States. The third regime, which existed from 21 August to 30 August, was dominated by an impressive ridge over the center of the country, with troughs over the eastern North Pacific and western North Atlantic. The northwest extension of the 850-hPa Bermuda high allowed for southwesterly flow from Arizona/Utah/Colorado to reach Minnesota/Wisconsin and resulted in 6 sigma 850-hPa temperature anomalies over the western Great Lakes region.
Discussion briefly shifted to the general circulation of the tropical Pacific from 1 June to 31 August. Dr. Bosart noted that the northerly upper-tropospheric winds that typically cross from the Northern Hemisphere into the Southern Hemisphere to the northeast of Australia were substantially weaker this year. This resulted in a weaker subtropical jet over Australia. Dr. Bosart also noted the anomalous high precipitable water values and low OLR values over the maritime continent. Dr. Paul Roundy stated that this setup was typical of La Nina, but that ocean temperatures had not cooperated with the atmosphere for most of the summer.
Finally, discussion shifted to the tropical Atlantic where a combination of unfavorable shear profiles and dry air were blamed for the lack of tropical cyclones this hurricane season. It was determined that this year's dry air was primarily associated with midlatitude subsidence and not Saharan dust. Dr. Bosart also noted the presence of semi-permanent "tropical trash" (aka: upper-tropospheric potential vorticity streams associated with precusor anticyclonic wave breaking events) over the central and western Atlantic that made conditions unfavorable for tropical cyclogenesis. Discussion ended with a quick look at the absence of a believable tropical cyclogenesis event in the 192 h GFS forecast. We are only a few days away from setting a record for the latest "first hurricane" to development in the North Atlantic basin. The current record is 11 September. I think we've got it.
You can see all of the links included in UAlbany's 6 September map discussion here: Friday Map Discussion Master Link. Please feel free to continue the discussion in the thread below!
Dr. Bosart began with a discussion of the general circulation over the United States from mid-June to the end of August. Dr. Bosart noted that this period could be broken down into three separate regimes. The first regime, which existed from 21 June to 20 July, was characterized by an anomalous ridge in the 300-hPa geopotential height field over the western United States, trough over the southeastern United States, and ridge over much of New England. The northwestward shift in the position of the 850-hPa Bermuda high relative to climatology, coupled with the position of the anomalous trough in the 300-hPa geopotential height field, facilitated the transport of deep tropical moisture into the southeast United States from the Gulf of Mexico and western Atlantic. This setup lead to cloudy/soggy conditions and anomalously cool 850-hPa temperatures for much of the Southeast. Meanwhile, confluent upper-tropospheric flow over eastern Canada prevented cooler air from traveling southward and resulted in anomalously warm conditions over the Northeast. The second regime, which existed from 21 July to 20 August, was dominated by a broad upper-tropospheric trough over much of the eastern United States and ridge over northwestern Canada. The eastward shift of the subtropical high into the central North Atlantic facilitated the development of anomalous northerly flow and cooler temperatures over the central and eastern United States. The third regime, which existed from 21 August to 30 August, was dominated by an impressive ridge over the center of the country, with troughs over the eastern North Pacific and western North Atlantic. The northwest extension of the 850-hPa Bermuda high allowed for southwesterly flow from Arizona/Utah/Colorado to reach Minnesota/Wisconsin and resulted in 6 sigma 850-hPa temperature anomalies over the western Great Lakes region.
Discussion briefly shifted to the general circulation of the tropical Pacific from 1 June to 31 August. Dr. Bosart noted that the northerly upper-tropospheric winds that typically cross from the Northern Hemisphere into the Southern Hemisphere to the northeast of Australia were substantially weaker this year. This resulted in a weaker subtropical jet over Australia. Dr. Bosart also noted the anomalous high precipitable water values and low OLR values over the maritime continent. Dr. Paul Roundy stated that this setup was typical of La Nina, but that ocean temperatures had not cooperated with the atmosphere for most of the summer.
Finally, discussion shifted to the tropical Atlantic where a combination of unfavorable shear profiles and dry air were blamed for the lack of tropical cyclones this hurricane season. It was determined that this year's dry air was primarily associated with midlatitude subsidence and not Saharan dust. Dr. Bosart also noted the presence of semi-permanent "tropical trash" (aka: upper-tropospheric potential vorticity streams associated with precusor anticyclonic wave breaking events) over the central and western Atlantic that made conditions unfavorable for tropical cyclogenesis. Discussion ended with a quick look at the absence of a believable tropical cyclogenesis event in the 192 h GFS forecast. We are only a few days away from setting a record for the latest "first hurricane" to development in the North Atlantic basin. The current record is 11 September. I think we've got it.
You can see all of the links included in UAlbany's 6 September map discussion here: Friday Map Discussion Master Link. Please feel free to continue the discussion in the thread below!
