Precipitation, the Rear Flank Downdraft, and Tornadoes
Mark Askelson
Department of Atmospheric Sciences
University of North Dakota
Wednesday, October 27, 2004
108 Atmospheric Sciences Building 3:30 p.m.
ABSTRACT
Given their infrequent occurrence, supercell thunderstorms produce an inordinate amount of death and damage. As a result, they have been studied intensely for the past forty years. Even so, numerous important questions concerning supercells remain. One such question regards tornadoes and observations of supercells that appear, excluding tornadic circulations, to be kinematically similar but exhibit very different tornadic behavior. Recently, analyses of data from the VORTEX project revealed that the thermodynamic properties of the rear flank downdraft (RFD) may dictate whether or not a supercell becomes tornadic. Since hydrometeors are thought to be an important driving force for the RFD, it has been postulated that they may be important to its thermodynamic properties and, possibly, to tornadogenesis.
Potential kinematic and thermodynamic properties of RFDs are investigated using a 1.5 dimension downdraft model that is driven by hydrometeor fields. This downdraft model uses conservation equations for vertical momentum, temperature, mixing ratios of water vapor and cloud water, number concentration densities of raindrops and graupel/hail, and ice-water fraction of graupel/hail. Microphysical processes include saturation adjustment, evaporation/condensation of raindrops, evaporation/condensation of melting graupel/hailstones, melting of graupel/hailstones, and shedding of melting graupel/hailstones. Environmental conditions are set using prescribed soundings that span a range of conditions to illustrate a range of simulated-RFD outcomes. The initial model state is hydrostatically balanced and subsequent evolution results only from hydrometeors, which are inserted with the same properties at the same altitude at each time step.
Results indicate that downdraft strength is strongly dependent on environmental lapse rates of temperature. Moreover, the vertical distribution of these lapse rates is a strong determinant of the maximum originating altitude of air parcels that descend to the surface. In addition to being sensitive to environmental conditions, modeled downdrafts are also sensitive to the properties of the hydrometeor fields that drive them. From these effects and the expected vertical temperature profiles of RFD environments, potential explanations of VORTEX observations will be provided. Finally, significant limitations of this study and future research directions will be discussed.
2004/10/21
Reflection on the seminar
UPDATE ON THE HYPOTHESIS OF EARLY ANTHROPOGENIC EFFECTS ON CLIMATE
Bill Ruddiman
Environmental Sciences, University of Virginia
2004/10/20
------------------------
這是個挺有趣的 seminar，談的也是現在氣候研究最 [熱門] 的問題（不論是經費或爭辯的激烈程度）。
科學家企圖在現實世界展現自己的影響力，大概已經不是什麼新聞了。在[走出象牙塔]的口號召喚之下，這些其實已經掌握相當權力與資源的人，再度勇猛的向更高的權力巔峰邁進。這其實沒什麼不好或是不對的地方，只是更凸顯了人們所共有的人性，不會因為教育或知識而改變。
Humanity, is something defaulted in our neural-structure, and is seldom changed through learning processes.
Dr. Ruddiman 從科學家轉變為歷史學家，調查了人類有史以來的農業活動與疾病傳播，結合使用[冰蕊]分析古氣候的研究資料，提出一套理論：自5000年前人類開始從事較大規模的農業活動以來，地球的氣候由原本的[走入小冰河期]已經被攔腰截住，溫室氣體的含量大幅提高，氣溫也未依照自然趨勢走入低溫。而歷史上造成人口銳減的大瘟疫，每每與冰蕊資料裡溫室氣體含量下降的時間相吻合，戰爭亦然。當然，走進工業時代之後，人類大量使用石化燃料，這個趨勢也就越演越烈，似乎已經無法回頭。
然而，讓人口[大幅]減少，自然界才會回到平衡，這個命題我國一就知道了，何需如此大費周章的研究半天？難不成是要請美國政府發動核子戰爭嗎？
顯然，這個問題不是短時間內有辦法解決的，就當成是美國的律師一樣，找件事情借題發揮，便足以提高知名度，並且撈到一些好處吧........
Bill Ruddiman
Environmental Sciences, University of Virginia
2004/10/20
------------------------
這是個挺有趣的 seminar，談的也是現在氣候研究最 [熱門] 的問題（不論是經費或爭辯的激烈程度）。
科學家企圖在現實世界展現自己的影響力，大概已經不是什麼新聞了。在[走出象牙塔]的口號召喚之下，這些其實已經掌握相當權力與資源的人，再度勇猛的向更高的權力巔峰邁進。這其實沒什麼不好或是不對的地方，只是更凸顯了人們所共有的人性，不會因為教育或知識而改變。
Humanity, is something defaulted in our neural-structure, and is seldom changed through learning processes.
Dr. Ruddiman 從科學家轉變為歷史學家，調查了人類有史以來的農業活動與疾病傳播，結合使用[冰蕊]分析古氣候的研究資料，提出一套理論：自5000年前人類開始從事較大規模的農業活動以來，地球的氣候由原本的[走入小冰河期]已經被攔腰截住，溫室氣體的含量大幅提高，氣溫也未依照自然趨勢走入低溫。而歷史上造成人口銳減的大瘟疫，每每與冰蕊資料裡溫室氣體含量下降的時間相吻合，戰爭亦然。當然，走進工業時代之後，人類大量使用石化燃料，這個趨勢也就越演越烈，似乎已經無法回頭。
然而，讓人口[大幅]減少，自然界才會回到平衡，這個命題我國一就知道了，何需如此大費周章的研究半天？難不成是要請美國政府發動核子戰爭嗎？
顯然，這個問題不是短時間內有辦法解決的，就當成是美國的律師一樣，找件事情借題發揮，便足以提高知名度，並且撈到一些好處吧........
UPDATE ON THE HYPOTHESIS OF EARLY ANTHROPOGENIC EFFECTS ON CLIMATE
UPDATE ON THE HYPOTHESIS OF EARLY ANTHROPOGENIC EFFECTS ON CLIMATE
Bill Ruddiman
Environmental Sciences, University of Virginia
2004/10/20
ABSTRACT
I recently proposed that farming has altered greenhouse-gas levels in the atmosphere for thousands of years. Clearance of forests for early agriculture raised atmospheric CO2 levels, and irrigation for rice farming increased methane levels. I also proposed that these early gas emissions were large enough to cancel most of a natural cooling that would otherwise have occurred and to stop the early stages of a glaciation. The early part of marine isotopic stage 11 near 400,000 years ago provides the closest analog to recent solar radiation levels of any interglaciation in the last million years. At the time that was most similar to the last few millennia, the CH4 concentration measured in Vostok ice fell to ~450 ppb, and CO2 values to ~250 ppm. These natural decreases contrast with the large increases (methane to 700 ppb and CO2 to 280 ppm) during pre-industrial time, thus providing strong support for the early anthropogenic hypothesis. In addition, deuterium (hydrogen-isotope) values in Vostok ice fell from typical interglacial to nearly glacial values, indicating that a major cooling occurred in Antarctica, as postulated in the hypothesis. Other evidence from both ice-core and marine-sediment sources suggests that new ice was accumulating in the northern hemisphere during the closest insolation analog to the present day, supporting the hypothesis that a glaciation is now overdue.
Bill Ruddiman
Environmental Sciences, University of Virginia
2004/10/20
ABSTRACT
I recently proposed that farming has altered greenhouse-gas levels in the atmosphere for thousands of years. Clearance of forests for early agriculture raised atmospheric CO2 levels, and irrigation for rice farming increased methane levels. I also proposed that these early gas emissions were large enough to cancel most of a natural cooling that would otherwise have occurred and to stop the early stages of a glaciation. The early part of marine isotopic stage 11 near 400,000 years ago provides the closest analog to recent solar radiation levels of any interglaciation in the last million years. At the time that was most similar to the last few millennia, the CH4 concentration measured in Vostok ice fell to ~450 ppb, and CO2 values to ~250 ppm. These natural decreases contrast with the large increases (methane to 700 ppb and CO2 to 280 ppm) during pre-industrial time, thus providing strong support for the early anthropogenic hypothesis. In addition, deuterium (hydrogen-isotope) values in Vostok ice fell from typical interglacial to nearly glacial values, indicating that a major cooling occurred in Antarctica, as postulated in the hypothesis. Other evidence from both ice-core and marine-sediment sources suggests that new ice was accumulating in the northern hemisphere during the closest insolation analog to the present day, supporting the hypothesis that a glaciation is now overdue.
2004/10/07
Not too much to talk about
結論，是一切都在未定之數。
既然熱帶地區的 biomass 無法精確測量（測量誤差大於估計的變化量），也就沒什麼好說的。
有趣的是，剛從Paris開完會回來的 Dr. Slasinger，說會議理有人提到，藉由提供發展中國家廉價的新能源，減緩熱帶地區的 deforesting，有機會讓CO2濃度回到工業時代以前的水準；然而從化學系來聽的，從事新能源研究的學生說，其實應該沒有這麼樂觀，因為採用新能源，要產生跟石化燃料等量的效率，恐怕造成的污染不會比較少（可能產生更多的廢熱）。
果然是誰也不知道答案的debate，永遠有你沒考慮到的變數........
不過，學界目前正積極的建立機制，將研究的成果能立即的反映在相關政策的制定上，不知道會不會因此發生一些慘劇？
既然熱帶地區的 biomass 無法精確測量（測量誤差大於估計的變化量），也就沒什麼好說的。
有趣的是，剛從Paris開完會回來的 Dr. Slasinger，說會議理有人提到，藉由提供發展中國家廉價的新能源，減緩熱帶地區的 deforesting，有機會讓CO2濃度回到工業時代以前的水準；然而從化學系來聽的，從事新能源研究的學生說，其實應該沒有這麼樂觀，因為採用新能源，要產生跟石化燃料等量的效率，恐怕造成的污染不會比較少（可能產生更多的廢熱）。
果然是誰也不知道答案的debate，永遠有你沒考慮到的變數........
不過，學界目前正積極的建立機制，將研究的成果能立即的反映在相關政策的制定上，不知道會不會因此發生一些慘劇？
2004/10/06
What Should We Know About Forest Biomass to Better Understand the Global Carbon Balance?
What Should We Know About Forest Biomass to Better Understand the Global Carbon Balance?
Richard.A. Houghton
Woods Hole Research Center
Woods Hole, MA
Wednesday October 6, 2004
109 Atmospheric Sciences Bldg 3:30 p.m.
ABSTRACT
The net annual flux of carbon between terrestrial ecosystems and the atmosphere is dominated by two factors: changes in the area of forests and per hectare changes in forest biomass that result from management and regrowth. While these factors are reasonably well documented in countries of the northern mid-latitudes as a result of systematic forest inventories, they are uncertain in the tropics. Recent estimates of carbon emissions from tropical deforestation have focused on the uncertainty in rates of deforestation. By using the same data for biomass, however, these studies have underestimated the total uncertainty of tropical emissions and may have biased the estimates. In particular, regional and country-specific estimates of forest biomass reported by the FAO indicate systematic changes in biomass that have not been taken into account in recent estimates of tropical carbon emissions. The ‘changes’ more likely represent improved information than real on-the-ground changes in carbon storage. In either case, however, the data have a significant effect on current estimates of carbon emissions from the tropics and, hence, on understanding the global carbon balance.
Richard.A. Houghton
Woods Hole Research Center
Woods Hole, MA
Wednesday October 6, 2004
109 Atmospheric Sciences Bldg 3:30 p.m.
ABSTRACT
The net annual flux of carbon between terrestrial ecosystems and the atmosphere is dominated by two factors: changes in the area of forests and per hectare changes in forest biomass that result from management and regrowth. While these factors are reasonably well documented in countries of the northern mid-latitudes as a result of systematic forest inventories, they are uncertain in the tropics. Recent estimates of carbon emissions from tropical deforestation have focused on the uncertainty in rates of deforestation. By using the same data for biomass, however, these studies have underestimated the total uncertainty of tropical emissions and may have biased the estimates. In particular, regional and country-specific estimates of forest biomass reported by the FAO indicate systematic changes in biomass that have not been taken into account in recent estimates of tropical carbon emissions. The ‘changes’ more likely represent improved information than real on-the-ground changes in carbon storage. In either case, however, the data have a significant effect on current estimates of carbon emissions from the tropics and, hence, on understanding the global carbon balance.
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