DOCSLIB.ORG

On the Climatology of Orographic Precipitation in the Mid-Latitudes

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On the Climatology of Orographic Precipitation in the Mid-Latitudes On the Climatology of Orographic Precipitation in the Mid-Latitudes Justin R. Minder A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2010 Program Authorized to Offer Degree: Department of Atmospheric Sciences University of Washington Graduate School This is to certify that I have examined this copy of a doctoral dissertation by Justin R. Minder and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Co-Chairs of the Supervisory Committee: Dale R. Durran Gerard H. Roe Reading Committee: Dale R. Durran Gerard H. Roe Mark T. Stoelinga Date: In presenting this dissertation in partial fulfillment of the requirements for the doctoral degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this dissertation is allowable only for scholarly purposes, consistent with \fair use" as prescribed in the U.S. Copyright Law. Requests for copying or reproduction of this dissertation may be referred to Proquest Information and Learning, 300 North Zeeb Road, Ann Arbor, MI 48106-1346, 1-800-521-0600, to whom the author has granted \the right to reproduce and sell (a) copies of the manuscript in microform and/or (b) printed copies of the manuscript made from microform." Signature Date University of Washington Abstract On the Climatology of Orographic Precipitation in the Mid-Latitudes Justin R. Minder Co-Chairs of the Supervisory Committee: Professor Dale R. Durran Department of Atmospheric Sciences Professor Gerard H. Roe Department of Earth and Space Science For mountainous regions, and regions downstream, the climatology of mid-latitude oro- graphic precipitation determines the susceptibility to hazards such as flooding and land- slides while also cotrolling the volume and timing of streamflow and fresh water resources. However, due to modeling and observational challenges, many aspects of the climatology of mountain precipitation remain poorly understood. This thesis uses a synthesis of nu- merical models, theory, and field observations, loosely focused on the Cascade and Olympic Mountains of western North America, to investigate in detail a number of general aspects of mid-attitude orographic precipitation. First, the climatology of ridge-valley scale precipitation patterns is investigated by anal- ysis of several years of data from the Olympic Mountains, both from archived operational mesoscale numerical model forecasts and a special dense observing network of precipitation gauges. By simulating and analyzing case studies, the physical processes responsible for the mean pattern and variations in the pattern are diagnosed. Large (> 50%) enhance- ment of precipitation over ridges relative to valleys a few kilometers away is found to be a very robust feature of the region's climate, and the climatological patterns are surprisingly well-simulated by a mesoscale model (despite frequent errors for individual storms). The impact of large climatological gradients in mountain precipitation, such as those found in the Olympic mountains, on patterns of landslide susceptibility is also investigated. This is accomplished using an idealized model of shallow landslides, forced by the climatol- ogy developed from mesoscale model forecasts. Results suggest that small-scale maxima in climatological precipitation may play an important role in making certain regions more sus- ceptible to slope failure. Furthermore, the use of unadjusted lowland precipitation data to characterize conditions on nearby mountain slopes may lead to a substantial underestimate of landslide hazard. Next, the controls on the sensitivity of mountain snowpack accumulation to climate warming are investigated using two idealized physically based models. Results suggest that the relationship between the climatological melting-level distribution and the topography is the principle control on the sensitivity of snowpack accumulation to climate warming. It is also shown that, while thermodynamically driven increases in precipitation with warming may moderate the loss of snowfall somewhat, for large amounts of warming increases in precipitation become unimportant, as the loss of accumulation area is too substantial. Finally, the physical mechanisms acting on the mesoscale to control the mountainside snow line are investigated. On the mountainside, the snow line is often located at an ele- vation hundreds of meters different from its elevation in the free air upwind. The processes responsible for this behavior are examined in semi-idealized simulations with a mesoscale numerical weather prediction model. Spatial variations in latent cooling from melting pre- cipitation, adiabatic cooling from vertical motion, and the melting distance of frozen hy- drometeors are all shown to make important contributions. The relative importance of these processes depends on properties of the incoming flow and terrain geometry. Results suggest an increased depression of the snow line below the upstream 0◦C level with increasing tem- perature, a relationship that, if present in nature, could act to buffer mountain hydroclimate against the impacts of climate warming. TABLE OF CONTENTS Page List of Figures........................................ iii List of Tables......................................... xiii Chapter 1: Introduction.................................1 1.1 Motivation.....................................1 1.2 Challenges......................................2 1.3 Previous work....................................3 1.4 An example: The Olympic and Cascade mountains..............5 1.5 Outline....................................... 10 Chapter 2: The climatology of small-scale orographic precipitation: Patterns and processes................................... 13 2.1 Introduction and Background........................... 13 2.2 Precipitation in the Olympic Mountains..................... 16 2.3 Case studies..................................... 29 2.4 Composite analysis of precipitation climatology................. 49 2.5 Discussion...................................... 52 2.6 Conclusions..................................... 55 Chapter 3: Spatial patterns of orographic rainfall and shallow landslide suscepti- bility..................................... 57 3.1 Introduction and Background........................... 57 3.2 Rainfall and Landslides over the Western Olympic Mountains......... 60 3.3 Methods....................................... 63 3.4 Results........................................ 67 3.5 Sensitivity Analysis................................. 74 3.6 Conclusions..................................... 77 i Chapter 4: The sensitivity of climatological mountain snowpack accumulation to climate warming.............................. 79 4.1 Introduction and background........................... 79 4.2 Focus and strategy................................. 80 4.3 Linear Theory (LT) Orographic Snowfall Model................. 83 4.4 Melting-Level (ML) Model............................. 95 4.5 Experiments: Controls on λS ........................... 98 4.6 Conclusions..................................... 107 Chapter 5: Mesoscale controls on the climatology of the mountainside snow line. 110 5.1 Introduction and background........................... 110 5.2 Numerical Model.................................. 119 5.3 Results: physical mechanisms........................... 123 5.4 Results: 2-D sensitivity experiments....................... 140 5.5 Results: 3D effects................................. 151 5.6 Discussion...................................... 156 5.7 Summary and Conclusions............................. 159 Chapter 6: Summary and conclusions......................... 161 Bibliography......................................... 165 ii LIST OF FIGURES Figure Number Page 1.1 Figure caption...................................6 2.1 Rainfall observations in the Olympic Mountains. (a) shows the terrain of the Olympic Peninsula (grayscale shading), with a maximum elevation of 2.43 km. The locations which regularly report hourly precipitation (from the RAWS, ASOS, and the SNOTEL networks) are shown with stars. Location of the Quillayute (KUIL) sounding, and the COOP stations (Forks and Sequim) discussed in the text are also denoted. (b) shows a detailed view of the Queets-Quinault gauge network. Mixed precipitation gauges are shown with white circles, while rain-only gauges are shown with black circles. The Queets and Quinault Valleys, and the Black Knob (BKBW) RAWS station are also denoted........................................ 17 2.2 MM5 annual precipitation climatology (color shading, in units of mm yr−1), and model topography (contours every 250 m) over Olympic, Cascade, and Coastal mountain ranges (water years 2001{2006)................ 18 2.3 Precipitation weighted wind roses from water years 2004{2006. The length of each radial line is proportional to the amount of precipitation falling at the BKBW station when winds are from each direction. (a) uses 6 hr 850 hPa winds from the gridpoint in the NCEP-NCAR reanalysis upwind of the site during average flow, (b) uses 6 hr averaged 10 m winds from the BKBW station........................................ 21 iii 2.4 Observed and modeled precipitation totals at sites along Queets-Quinault gauge transect for four rainy seasons (dates given in Table 2.1). Accumu- lated precipitation is plotted as
Load more

Recommended publications
  • Snow Depth on Arctic Sea Ice
    1814 JOURNAL OF CLIMATE VOLUME 12 Snow Depth on Arctic Sea Ice STEPHEN G. WARREN,IGNATIUS G. RIGOR, AND NORBERT UNTERSTEINER Department of Atmospheric Sciences, University of Washington, Seattle, Washington VLADIMIR F. R ADIONOV,NIKOLAY N. BRYAZGIN, AND YEVGENIY I. ALEKSANDROV Arctic and Antarctic Research Institute, St. Petersburg, Russia ROGER COLONY International Arctic Research Center, University of Alaska, Fairbanks, Alaska (Manuscript received 5 December 1997, in ®nal form 27 July 1998) ABSTRACT Snow depth and density were measured at Soviet drifting stations on multiyear Arctic sea ice. Measurements were made daily at ®xed stakes at the weather station and once- or thrice-monthly at 10-m intervals on a line beginning about 500 m from the station buildings and extending outward an additional 500 or 1000 m. There were 31 stations, with lifetimes of 1±7 yr. Analyses are performed here for the 37 years 1954±91, during which time at least one station was always reporting. Snow depth at the stakes was sometimes higher than on the lines, and sometimes lower, but no systematic trend of snow depth was detected as a function of distance from the station along the 1000-m lines that would indicate an in¯uence of the station. To determine the seasonal progression of snow depth for each year at each station, priority was given to snow lines if available; otherwise the ®xed stakes were used, with an offset applied if necessary. The ice is mostly free of snow during August. Snow accumulates rapidly in September and October, moderately in November, very slowly in December and January, then moderately again from February to May.
    [Show full text]
  • Comparison of Remote Sensing Extraction Methods for Glacier Firn Line- Considering Urumqi Glacier No.1 As the Experimental Area
    E3S Web of Conferences 218, 04024 (2020) https://doi.org/10.1051/e3sconf/202021804024 ISEESE 2020 Comparison of remote sensing extraction methods for glacier firn line- considering Urumqi Glacier No.1 as the experimental area YANJUN ZHAO1, JUN ZHAO1, XIAOYING YUE2and YANQIANG WANG1 1College of Geography and Environmental Science, Northwest Normal University, Lanzhou, China 2State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources/Tien Shan Glaciological Station, Chinese Academy of Sciences, Lanzhou, China Abstract. In mid-latitude glaciers, the altitude of the snowline at the end of the ablating season can be used to indicate the equilibrium line, which can be used as an approximation for it. In this paper, Urumqi Glacier No.1 was selected as the experimental area while Landsat TM/ETM+/OLI images were used to analyze and compare the accuracy as well as applicability of the visual interpretation, Normalized Difference Snow Index, single-band threshold and albedo remote sensing inversion methods for the extraction of the firn lines. The results show that the visual interpretation and the albedo remote sensing inversion methods have strong adaptability, alonger with the high accuracy of the extracted firn line while it is followed by the Normalized Difference Snow Index and the single-band threshold methods. In the year with extremely negative mass balance, the altitude deviation of the firn line extracted by different methods is increased. Except for the years with extremely negative mass balance, the altitude of the firn line at the end of the ablating season has a good indication for the altitude of the balance line.
    [Show full text]
  • Glacier (And Ice Sheet) Mass Balance
    Glacier (and ice sheet) Mass Balance The long-term average position of the highest (late summer) firn line ! is termed the Equilibrium Line Altitude (ELA) Firn is old snow How an ice sheet works (roughly): Accumulation zone ablation zone ice land ocean • Net accumulation creates surface slope Why is the NH insolation important for global ice• sheetSurface advance slope causes (Milankovitch ice to flow towards theory)? edges • Accumulation (and mass flow) is balanced by ablation and/or calving Why focus on summertime? Ice sheets are very sensitive to Normal summertime temperatures! • Ice sheet has parabolic shape. • line represents melt zone • small warming increases melt zone (horizontal area) a lot because of shape! Slightly warmer Influence of shape Warmer climate freezing line Normal freezing line ground Furthermore temperature has a powerful influence on melting rate Temperature and Ice Mass Balance Summer Temperature main factor determining ice growth e.g., a warming will Expand ablation area, lengthen melt season, increase the melt rate, and increase proportion of precip falling as rain It may also bring more precip to the region Since ablation rate increases rapidly with increasing temperature – Summer melting controls ice sheet fate* – Orbital timescales - Summer insolation must control ice sheet growth *Not true for Antarctica in near term though, where it ʼs too cold to melt much at surface Temperature and Ice Mass Balance Rule of thumb is that 1C warming causes an additional 1m of melt (see slope of ablation curve at right)
    [Show full text]
  • (Highresmip V1.0) for CMIP6
    High Resolution Model Intercomparison Project (HighResMIP v1.0) for CMIP6 1 2 3 5 Reindert J. Haarsma , Malcolm Roberts , Pier Luigi Vidale , Catherine A. Senior2, Alessio Bellucci4, Qing Bao5, Ping Chang6, Susanna Corti7, Neven S. Fučkar8, Virginie Guemas8, Jost von Hardenberg7, Wilco Hazeleger1,9,10, Chihiro Kodama11, Torben Koenigk12, Lai-Yung Ruby Leung13, Jian Lu13, Jing-Jia Luo14, Jiafu Mao15, Matthew S. Mizielinski2, Ryo Mizuta16, Paulo Nobre17, Masaki 18 4 19 20 10 Satoh , Enrico Scoccimarro , Tido Semmler , Justin Small , Jin-Song von Storch21 1Royal Netherlands Meteorological Institute, De Bilt, The Netherlands 2Met Office Hadley Centre, Exeter, UK 15 3University of Reading, Reading, UK 4 Centro Euro-Mediterraneo per i Cambiamenti Climatici, Bologna, Italy 5LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China P.R. 6Texas A&M University, College Station, Texas, USA 7National Research Council – Institute of Atmospheric Sciences and Climate, Italy 20 8Barcelona Super Computer Center, Earth Sciences Department, Barcelona, Spain 9Netherlands eScience Center, Amsterdam, The Netherlands 10Wageningen University, The Netherlands 11Japan Agency for Marine-Earth Science and Technology, Japan 12Swedish Meteorological and Hydrological Institute, Norrköping, Sweden 25 13Pacific Northwest National Laboratory, Richland, USA 14Bureau of Meteorology, Australia 15Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, USA 16Meteorological Research Institute,
    [Show full text]
  • Orographic Effect on the Summer
    Orographic Effects on South China Sea Summer Climate Haiming Xu∗ Department of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing, China Shang-Ping Xie and Yuqing Wang International Pacific Research Center and Department of Meteorology, University of Hawaii, Hawaii, USA Wei Zhuang, and Dongxiao Wang LED，South China Sea Institute of Oceanology, Chinese Academy of Science, Guangzhou, China Submitted to Meteorology and Atmospheric Physics April, 2007 ∗ Corresponding author address: Dr. Haiming Xu, Department of Atmospheric Sciences, Nanjing University of Information Science and Technology, 114 New Street, Pancheng, Pukou District, Nanjing 210044, China. E-mail: [email protected] Abstract New satellite observations reveal several distinct features of the South China Sea (SCS) summer climate: an intense low-level southwesterly wind jet off the coast of South Vietnam, a precipitation band on the western flank of the north-south running Annam mountain range, and a rainfall shadow to the east in the western SCS off the east coast of Vietnam. A high-resolution full-physics regional atmospheric model is used to investigate the mechanism for the formation of SCS summer climate. A comparison of the control model simulation with a sensitivity experiment with the mountain range artificially removed demonstrates that the aforementioned features form due to orographic effects of the Annam mountains. Under the prevailing southwesterly monsoon, the mountain range forces the ascending motion on the windward and subsidence on the lee side, giving rise to bands of active and suppressed convection, respectively. On the south edge of the mountain range, the southwesterlies are accelerated to form an offshore low-level wind jet.
    [Show full text]
  • Snow Cover and Glacier Change Study in Nepalese Himalaya Using Remote Sensing and Geographic Information System
    26 A. B. Shrestha & S. P. Joshi August 2009 Snow Cover and Glacier Change Study in Nepalese Himalaya Using Remote Sensing and Geographic Information System Arun Bhakta Shrestha1 and Sharad Prasad Joshi2 1 International Centre for Integrated Mountain Development, Nepal E-mail: [email protected] 2 Water and Energy Commission Secretariat, Nepal ABSTRACT Snow cover and glaciers in the Himalaya play a major role in the generation of stream flow in south Asia. Various studies have suggested that the glaciers in the Himalaya are in general condition of retreat. The snowline is also found to be retreating. While there are relatively more studies on glaciers fluctuation in the Himalaya, studies on snow cover is relatively sparse. In this study, snow cover and glacier fluctuation in the Nepalese Himalaya were studied using remote sensing techniques and geographic information system. The study was carried out in two spatial scales: catchments scale and national scale. In catchments scale two catchments: Langtang and Khumbu were studied. Intermittent medium resolution satellite imageries (Landsat) were used to study the fluctuation in snow cover and glacier area in the two catchments. In the national scale study coarse resolution (MODIS) imageries were used to derive seasonal variations in snow cover. An indication of decreasing trend in snow cover is shown by this study, although this result needs verification with more data. The snowline elevation is in general higher in Khumbu compared to Langtang. In both catchments, snowline elevation are higher in east, south-east, south and south-west aspects. The areas of snow cover in those aspects are also greater.
    [Show full text]
  • World Map Outline Find and Shade: Andes, Alps, Rockies, Himalayas, Caucasus Mountains, East Africa Mountains, Alaska/Yukon Ranges, Sentinel Range, Sudiman Range
    World Map Outline Find and shade: Andes, Alps, Rockies, Himalayas, Caucasus Mountains, East Africa Mountains, Alaska/Yukon Ranges, Sentinel Range, Sudiman Range 1 The World’s Highest Peaks 2 Earth Cross-Section 3 Mountain Climates Fact Sheet • How high a mountain is affects what its climate is like. Moving 300 metres up is the same as moving 350 miles towards one of the poles! • Air pressure also changes as one gains altitude. At the top of Mount Everest (8848 m) the pressure is around 310-360 millibars, compared to around 1013 mb at sea level. • As a result of falling air pressure, rising air expands and cools (although, dry air cools faster than moist air because, as the moist air rises, the water vapour condenses – like in clouds – and this gives off some heat). The higher you are the cooler it gets. That is why we often have snow on mountaintops, even along the equator. • Mountains therefore act as a barrier to moisture-laden winds. Air rising to pass over the mountains cools and the water vapour condenses, turning into either clouds, rain, or if it is cold enough, snow. This is why on one side of a mountain you can experience a wet climate, while on the other side of the same mountain you find an arid one. • A large mountain range can affect the weather of the land beyond it. The Himalayas influence the climate of the rest of India by sheltering it from the cold air mass of central Asia. • In high mountains the first snow may fall several weeks earlier than it does in the surrounding area.
    [Show full text]
  • Computer Models, Climate Data, and the Politics of Global Warming (Cambridge: MIT Press, 2010)
    Complete bibliography of all items cited in A Vast Machine: Computer Models, Climate Data, and the Politics of Global Warming (Cambridge: MIT Press, 2010) Paul N. Edwards Caveat: this bibliography contains occasional typographical errors and incomplete citations. Abbate, Janet. Inventing the Internet. Inside Technology. Cambridge: MIT Press, 1999. Abbe, Cleveland. “The Weather Map on the Polar Projection.” Monthly Weather Review 42, no. 1 (1914): 36-38. Abelson, P. H. “Scientific Communication.” Science 209, no. 4452 (1980): 60-62. Aber, John D. “Terrestrial Ecosystems.” In Climate System Modeling, edited by Kevin E. Trenberth, 173- 200. Cambridge: Cambridge University Press, 1992. Ad Hoc Study Group on Carbon Dioxide and Climate. “Carbon Dioxide and Climate: A Scientific Assessment.” (1979): Air Force Data Control Unit. Machine Methods of Weather Statistics. New Orleans: Air Weather Service, 1948. Air Force Data Control Unit. Machine Methods of Weather Statistics. New Orleans: Air Weather Service, 1949. Alaka, MA, and RC Elvander. “Optimum Interpolation From Observations of Mixed Quality.” Monthly Weather Review 100, no. 8 (1972): 612-24. Edwards, A Vast Machine Bibliography 1 Alder, Ken. The Measure of All Things: The Seven-Year Odyssey and Hidden Error That Transformed the World. New York: Free Press, 2002. Allen, MR, and DJ Frame. “Call Off the Quest.” Science 318, no. 5850 (2007): 582. Alvarez, LW, W Alvarez, F Asaro, and HV Michel. “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” Science 208, no. 4448 (1980): 1095-108. American Meteorological Society. 2000. Glossary of Meteorology. http://amsglossary.allenpress.com/glossary/ Anderson, E. C., and W. F. Libby. “World-Wide Distribution of Natural Radiocarbon.” Physical Review 81, no.
    [Show full text]
  • Aerosol-Orography-Precipitation – a Critical Assessment T Goutam Choudhurya, Bhishma Tyagia,*, Jyotsna Singhb, Chandan Sarangic, S.N
    Atmospheric Environment 214 (2019) 116831 Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Review article Aerosol-orography-precipitation – A critical assessment T Goutam Choudhurya, Bhishma Tyagia,*, Jyotsna Singhb, Chandan Sarangic, S.N. Tripathid a Department of Earth and Atmospheric Sciences, National Institute of Technology Rourkela, Rourkela, 769008, Odisha, India b Shanti Raj Bhawan, Paramhans Nagar, Kandwa, Varanasi, 221106, India c Pacific Northwest National Laboratory, Richland, United States d Department of Civil Engineering & Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur, 208016, India GRAPHICAL ABSTRACT ARTICLE INFO ABSTRACT Keywords: The increasing anthropogenic pollution and its interaction with precipitation received much attention from the Aerosols research community and have been explored extensively for understanding the aerosol-cloud interactions. The Aerosol-orography-precipitation interactions impacts of orography and aerosols on the precipitation processes have unveiled the Aerosol-Orography- Cloud microphysics Precipitation (AOP) interaction as an essential research area. The understanding of AOP interaction is critical for Spillover effect improving the extreme rainfall events prediction over mountainous regions. The phase of clouds (warm or Orographic enhancement factor mixed) along with orography has emerged as a significant factor for influencing the AOP relations. The present work reviews the modelling and observational based studies dealing with the relationship between orography and aerosols on the precipitation. The study reveals the principal role of aerosols in shifting the precipitation pattern for orographic regions. The environmental factors, especially ambient temperature, humidity and flow patterns are also identified to affect the orographic precipitation. The review also discovers that AOP studies exist only to limited areas of the world due to limited observations, and mostly with idealised cases in the modelling framework.
    [Show full text]
  • What Even Is 'Climate'?
    Geosci. Commun. Discuss., https://doi.org/10.5194/gc-2018-11 Manuscript under review for journal Geosci. Commun. Discussion started: 13 July 2018 c Author(s) 2018. CC BY 4.0 License. What even is ‘Climate’? Oliver Bothe1 1Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Max-Planck-Strasse 1, 21502, Geesthacht, Germany Correspondence: Oliver Bothe ([email protected], [email protected]) Abstract. Although the concept of climate is easy to understand, there is not any uncontroversial definition of it. Most def- initions fall back to the simple formulation that ‘climate is the statistics of weather’. Recent attempts at a definition called versions of this saying vague. Climate is policy-relevant, and discussions on climate and climate change benefit from clarity on the topic. Beyond the policy relevance a definition should also be valid for scientific purposes and for individual views. It 5 has to account for a general concept and individual instances of climate. Here, I try to highlight why the flexibility and the immediacy of the colloquial definition fit the topic. This defence shifts the lack of a clear definition towards the term ‘weather’ and the time-scales separating weather and climate. 1 Introduction “Climate is what you expect, weather is what you get” (e.g., Geographical Association and London Geographical Institute, 10 1902; Lorenz, unpublished). Or: “Climate is the statistics of weather” (e.g., McBean et al., 1992; Easterling et al., 1999; Farmer, 2014; Molua and Lambi, 2007; von Storch, 2004; Roe and O’Neal, 2009; Roe, 2009). Public discourse and scientific literature frequently use versions of these colloquial sayings.
    [Show full text]
  • Southern African Orography Impacts on Low Clouds and the Atlantic ITCZ
    PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Southern African orography impacts on low clouds 10.1002/2017GL073098 and the Atlantic ITCZ in a coupled model Key Points: S. F. Potter1 , E. J. Dawson2, and D. M. W. Frierson2 • Namibian stratocumulus deck thins fi signi cantly due to the removal of 1Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, USA, southern African orography 2 • Atlantic ITCZ shifts south due to the Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA increase in temperature resulting from the reduction of Namibian low clouds • Use of a coupled model allows ocean Abstract We examine the influence of southern African orography on the Namibian stratocumulus deck, circulation to cool the South Atlantic, the South Atlantic ocean-to-atmosphere energy transport, and the Intertropical Convergence Zone (ITCZ), which lessens and changes the structure of the ITCZ shift using an atmosphere-only model and a coupled atmosphere-ocean model. For both models, a control simulation with realistic orography is compared to a simulation where the orography in southern Africa was removed. As in the previous studies, the removal of orography results in thinning of the Namibian stratocumulus deck. In the coupled model, the increased sea surface temperature in the southern Atlantic Correspondence to: S. F. Potter, due to the reduction of low clouds forces the Atlantic ITCZ to shift southward toward the warmer [email protected] hemisphere. However, changes in the ocean circulation cool the South Atlantic atmosphere, lessening the ITCZ shift and changing the structure of precipitation. These results show the importance of orography on Citation: shaping Atlantic rainfall and highlight the role of dynamical ocean processes in atmospheric dynamics.
    [Show full text]
  • The Impacts of Topography on Spatial and Temporal Rainfall Distribution Over Rwanda Based on WRF Model
    Atmospheric and Climate Sciences, 2016, 6, 145-157 Published Online April 2016 in SciRes. http://www.scirp.org/journal/acs http://dx.doi.org/10.4236/acs.2016.62013 The Impacts of Topography on Spatial and Temporal Rainfall Distribution over Rwanda Based on WRF Model Didier Ntwali1,2,3*, Bob Alex Ogwang4,5, Victor Ongoma4 1Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China 2University of Chinese Academy of Sciences, Beijing, China 3Rwanda Meteorology Agency, Ministry of Natural Resources (MINIRENA), Kigali, Rwanda 4Nanjing University of Information, Science and Technology, Nanjing, China 5Uganda National Meteorological Authority, Kampala, Uganda Received 13 January 2016; accepted 26 February 2016; published 1 March 2016 Copyright © 2016 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract The impact of topography on heavy rainfall during two rain seasons was investigated in order to explain their mechanisms on rainfall distribution over Rwanda. Weather Research and Forecast- ing (WRF-ARW) model was used to study two historical cases of heavy rainfall which took place over Rwanda during two rain seasons, March to May (MAM) and September to December (SOND), from April 7 to 9, 2012 (for MAM) and from October 29 to 31, 2012 (during SOND). The control experiment was done with actual topography, whereas sensitivity experiment was carried out with topography reduced by half. Results show that rainfall distribution over Rwanda significantly changes when topography is reduced. The reduction in topography leads to a decrease in rainfall amounts in both MAM and SOND seasons, with varying magnitudes.
    [Show full text]