Class Summaries
Th/Fr 30-31 Aug
(8/30): Discussed overall course philosophy and expectations of students. Referenced Starr (1948) as an example of how to conduct science. Discussed ground-breaking scientific inferences about the drivers (especially the role of horizontal eddies associated with tilted troughs) of the Earth's general circulation from Starr (1948).
(8/31): Completed discussion of Starr (1948), Palmen (1949), Starr (1949), and Rossby and Starr (1948). Key takeaways from Starr (1948): (1) observations-based testable hypotheses are necessary to drive the science bus, and (2) theoretical knowledge is critical to informingg observational research. Starr (1948) provides superb physical insights on the nature of the Earth's general circulation to include why positively tilted troughs are necessary to enable a poleward relative angular momentum transport, how midlatitude cyclones result in a net downward transport of relative angular momentum in midlatitudes, how trade-wind easterlies extract angular momentum from the earth and put it into the atmosphere by opposing the Earth's rotation rate, and how the asymmetric orientation of oceanic subtropical anticyclones permits a poleward transport of relative angular momentum. Rossby and Starr (1948) provide the necessary background equations to understand the debate between Starr (1948), Palmen (1949), and Starr (1949) over the roles of horizontal eddies versus the mean meridional circulation in maintaining the general circulation.
Atmospheric Anuglar Momentum (D. Hartmann, UWashington)
Pearson Correlation Coefficient (Wikipedia)
Summer 2018 300/850 hPa Heights
Tu 4 Sep
Ajay Raghavendra followed up on the previous Friday's make-up class on the effect of continents on the shape and location of subtropical high pressure regions. Ajay discussed idealized Planet Aqua simulations performed by John Marshall at MIT to test the sensitivity of the overall idealized general circulation to atmospheric and oceanic heat fluxes subject to the presence of different oceanic barriers.
Links to John Marshall's lecture at the International Centre for Theoretical Physics Summer School on Theory, Mechanisms and Hierarchical Modelling of Climate Dynamics: Multiple Equilibria in the Climate System | (smr 3214) (http://indico.ictp.it/event/8457)/http://indico.ictp.it/event/8318/session/0/contribution/1/material/slides/0.pdf
Discussed Q-vectors in natural coordinates (Sanders and Hoskins 1990 paper). Emphasized the scientific advances made possible by this paper through the emphasis on the physical interpretation and understanding of Q-vectors. Key takeaways: Q-vectors are proportional to the product of the magnitude of the horizontal temperature (in natural coordinates) times k cross dVg/dx where the x-axis is oriented parallel to an isotherm. Short, stubby jets in in strong baroclinic zones give the maximum Q-vector bang for the Q-vector buck through the vigor of the associated thermally direct vertical circulation. Noted the linkage of Q-vectors to frontogenetical forcing as well. A key takeaway is how interesting scientific progress can arise when two scientists with difference backgrounds and interests team-up...in this case Fred Sanders (synoptic-dynamic meteorologist and weather forecaster) and Brian Hoskins (theoretical dynamicist).
Sanders and Hoskins 1990 paper
NOAA/ESRL/PSD Interactive Climate Analysis and Plotting Web Tools
Excerpt from Scott Runyon's M.S. thesis: Examples of Skewed Temperatures
Th 6 Sep
Summary: Class was pleasantly "hijacked" by Ajay Raghavendra. He triggered an interesting and extensive discussion about the NSF to include how one goes about securing NSF funding for research, how one prepares an NSF proposal, how one evaluates an NSF proposal, how students are integrated into NSF proposals, and how to conduct NSF-funded research. Excellent class discussion overall. Students were actively engaged.
Tu 11 Sep
Summary: Briefly commented on Hoskins and Valdes (1990). Spent most of the class reviewing Lance's August 2018 NWA annual meeting powerpoint presentation on the "pi-day" storm of 14 March 2017. Key takeaways were the importance of the configuration and evolution of upstream flow conditions across the Pacific on downstream flow configuration and evolution over North America, the relatively low predictability downstream over North America when a Rex block is present over the Pacific, how multiple trough-trough and trough-ridge interactions can impact cyclone development, track, and intensity, how uncertainties with diabatically driven downstream ridge building can impact upstream cyclone intensity and track, why coupled upper-level jets as drivers of intense cyclogenesis, and why weather-regime and weather-type dynamically based forecast verification is needed to complement and inform existing statistically based forecast verification.
Hoskins, James, and White 1983 paper
Bosart et al. 2018 NWA Presentation
Bosart, Moore, Cordeira, Archambault 2017 paper
Th 13 Sep
Summary: Briefly referenced Dole (2008; section 3a, section 3b1) to highlight the use of temporal low-pass, band-pass, and high-pass filtering to identify NH storm tracks and persistent flow anomaly features of the NH general circulation. Discussed Lance's class notes on (1) jet streaks, (2) geostrophic departure, and the isallobaric winds, and (3) Fred Sanders' take on orographic forcing from a QG perspective. Used the jet streak notes to link vertical circulations obtained through the time tendency of the Bjerknes circulation theorem with jet-related vertical circulations obtained through Q-vectors. Used the isallobaric wind notes to show that two routinely neglected terms in the isallobaric wind equation, [(V/f) k cross dVg/ds] and [(w/f) k cross dVg/dz] may be as large, if not larger, than the traditional isallobaric wind equation. Showed how the traditional isallobaric wind equation indicates that dynamical effects can contribute to cross isobar flow at the surface in addition to friction. Used the QG omega equation with the RHS set to zero to deduce the motion of cyclones and anticyclones relative to major mountain barriers (after Fred Sanders).
Lance Class Notes: Jet Streaks
Lance Class Notes: Geostrophic Departure Vector & Isallobaric Wind
Lance Class Notes: Orographic Influences
Tu 18 Sep
Synopsis: Matt Vaughan led a discussion on best practices in research and professional conduct. Began with going over the importance of appropriate advisor guidance and the pitfalls of too much (or too little) guidance. Discussed how climatologies, composites, case studies, and conceptual models can be leveraged in creating successful research projects. Introduced the 3 "arrows" of research (Theory, Observations, and Modeling) and the benefits of having multiple arrows in your quiver. Emphasized that a good researcher has a mastery of multiple tools of inquiry and is able to ask good questions. Explored techniques for networking at conferences and talking about your research with others.
Powerpoint presentation on research
Th 20 Sep
Presenter: Matthew Campbell Abstract Title: Impact of Local Soil Moisture in WRF on the Great Plains Low-Level Jet Abstract: Warm season droughts are naturally occurring, high impact phenomena that frequent the Great Plains region of the United States (US). Their modern and historical cost to society has been considerable, costing of billions of dollars in extreme years. In order to mitigate and forecast such events, determining and understanding both the global and local drivers of drought evolution is paramount. Past research has linked global teleconnections to central US drought formation and evolution. However, global scale influences cannot explain all drought variability in the Great Plains. Focus must also be placed on local land conditions and drought-related processes associated with land-atmosphere interactions. This study focuses on soil moisture in the Great Plains and how it affects the summertime low-level jet (LLJ) in the Great Plains. The LLJ is important as it provides a major source of water vapor and forcing for precipitation to the region. Soil moisture is examined since it exhibits large intraseasonal and interannual variability that can alter the corresponding thermodynamic and radiative budgets in the region. The Weather Research and Forecasting (WRF) Model is utilized to explore the impacts that various prescribed soil moisture conditions have on the summertime LLJ. More specifically, the low-level jet strength, position, and northward extent of moisture transport is analyzed.
Koster et al. 2006 GLACE Article
Tu 25 Sep
Danny Reese: Abstract Application of a Vertical Profiler Network in a Squall Line Case Study In the short-term forecasting of severe weather, the dynamic and thermodynamic vertical structure of the mesoscale environment is critical with regard to the evolution of convection. While radiosondes can capture this vertical structure with a high spatial resolution, they can not depict the rapid changes to the environment over time which are common in dynamic mesoscale environments (Knupp, 2008). Instruments such as Lidar and Microwave Radiometers, while more limited in vertical resolution, are better suited to depict these rapid mesoscale changes to the vertical profile over time. The recently finished New York State Mesonet (NYSM) has a network of seventeen vertical profile stations across the state, each containing a Lidar instrument and a Microwave Radiometer.
This study examines the profiler data from across the NYSM during a severe weather event that occurred on 22 August 2017. On this day, two squall lines, each with a notable cold pool, moved across much of upstate New York, producing widespread damaging wind reports. The profiler sites spread across the state are able to capture the pre-storm vertical environment, as well as changes to these profiles as the squall lines moved across the state. Wind speed, cold pool strength and depth, and other thermodynamic and dynamic features are all considered. These features are analyzed with regards to both location and time, and to other observations such as radar and storm reports, in order to note patterns and features that may help with short term operational forecasting.
Eli Turasky: Abstract: Extreme Precipitation Associated with Landfalling Atmospheric Rivers: Every year, the west coast of North America experiences significant economic damage and societal disruption due to the extreme precipitation associated with landfalling atmospheric rivers (ARs). ARs not only may produce significant economic and societal impacts, but also may contribute disproportionately to precipitation anomaly statistics along the west coast of the North America. Recent research suggests that the frequency of landfalling ARs along the west coast of North America may increase, due to increased horizontal moisture transport, in a warmer climate. In view of these considerations, it is important to diagnose the physical processes and the antecedent environments that govern landfalling ARs in order to improve medium-range forecasts of extreme precipitation events along the west coast of the North America. This study employs an archive of landfalling ARs along the west coast of North America constructed by the Center for Western Weather and Water Extremes in order to examine the large-scale upper-tropospheric flow patterns associated with landfalling ARs during September-May 1979-2017. In this study, the large-scale upper-tropospheric flow patterns prior to landfalling ARs are examined in the context of the two-leading empirical orthogonal functions (EOFs) of 250-hPa zonal wind over the North Pacific Ocean. The first EOF corresponds to a zonal extension or retraction of the climatological exit region of the North Pacific Jet (NPJ), while the second EOF corresponds to a poleward or equatorward shift of the climatological exit region of the NPJ. The projection of 250-hPa zonal wind anomalies at one or multiple times prior to AR landfall onto these two leading EOFs provides an objective characterization of the instantaneous state or evolution of the upper-tropospheric flow pattern over the North Pacific prior to AR landfall, which may be represented on a two-dimensional NPJ Phase Diagram. The analysis leverages the NPJ Phase Diagram to examine the degree to which AR landfall latitude and AR intensity vary as a function of the structure and evolution of the NPJ prior to AR landfall along the west coast of North America.
Th 27 Sep
Eli Turasky: Completed his discussion from the Tu 25 Sep class
Lexi Henny: Abstract: The Contribution of Atmospheric Rivers to Extreme Rainfall in Taiwan and the Northeastern U.S. This study will characterize the contribution of ARs to extreme rainfall, defined as the top 1% of daily rainfall. In order to get the complete picture, I will use several ingredients: first, the dataset of extreme rain and typhoon rain events that I have created as part of my masters thesis work on extreme rainfall in Taiwan. Second, an AR dataset developed by researchers in Taiwan. And third, a dataset of extreme precipitation events in the northeast U.S. created by Macy Howarth, as part of her completed masters research (also in the climate group of the U.S. - Taiwan PIRE project). By combining these ingredients, we can answer such questions as (1) how important are ARs for extreme rainfall in these locations; (2) what weather types are more frequently associated with ARs, and when do these events typically occur; and (3) how do the differences and similarities between the northeast U.S. and Taiwan environments affect the extent and nature of extreme precipitation-producing ARs?
Lexi Henny: Supporting Documentation:Kamae et al. (2017): Supplementary Material
Tu 2 Oct
Marshall Pfahler: Abstract The Role of the Subtropical Jet in Dry Season Caribbean Rainstorms Although significant high-impact heavy rainfall events in the Caribbean are typically associated with tropical cyclones during the wet season, significant heavy rainfall events can also occur during the Caribbean dry season absent of characteristics of a tropical cyclones (TCs). TCs can produce infrastructure and life-threatening conditions through flooding, mudslides, and damaging winds. The most significant Caribbean dry season rainstorms can require global aid for residents to have access to vital recovery resources. The Caribbean experiences a bimodal climate with a wet season extending from May/June-November/December and dry season outside of those months. Significant rainstorms that occur during the dry season are considered anomalous by their ability to produce rainfall totals that surpass climatological monthly mean precipitation in a relatively short duration (2-16 days). The subtropical jet plays an important role in the generation and severity of dry season rainfall events in the Caribbean. An analysis will be presented of the synoptic-scale evolution of the subtropical jet (STJ) preceding, during, and proceeding these extreme rainfall events utilizing the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) and Climatic Research Unit (CRU) precipitation reanalysis datasets along ERA-Interim global gridded reanalysis dataset. The evolution and position of the STJ can enable significant Caribbean rainstorms to form. Antecedent synoptic-scale conditions such as upper tropospheric anticyclonic wave breaking (AWB) over the eastern United States, Central America, or Atlantic Ocean can provide an efficient equatorward pathway for the STJ to approach the Caribbean Sea. Simultaneous formation of a potential vorticity streamer (PVS) from the AWB can promote forcing for ascent along its equatorward flank through QG dynamics. When this forcing can become juxtaposed with deep tropical moisture, intense convection can result. In some cases, strong and persistent AWB leads to eventual fracturing of athe PVS from large-scale upper tropospheric deformation. Subsequent cutoff cyclogenesis can occur from satisfaction of barotropic instability. Lowered static stability can additionally aid in generation of convection in these cases. A quantitative analysis of the role of STJ evolution and position along with the contributions of AWB and PVS formation and/or fracturing to dry season Caribbean rainstorms will be conducted. This investigation will include the development of a climatology of Caribbean rainstorms from the PERSIANN-CDR and CRU datasets Composite analyses of associated synoptic conditions will be derived from the ERA-Interim dataset. Marshall Pfahler: Supporting Documentation: Stephenson et al. (2014) talks about changes in temperature and precipitation in the Caribbean from 1961-2010. Their results suggest that the intensity of rainfall is significantly increasing and thus solidifying the importance of further understanding how these high-impact rainfall events occur. DeVries et al. (2017) looked at interactions between high values of potential vorticity (PV) and integrated vapor transport (IVT) over the Middle East and concluded that incursions of these two components are linked to extreme rainfall events in that region (northern tropics). A growing number of rainstorm events in the Caribbean have been identified to be influenced in the same manner.Links:
Th 4 Oct
31st Anniversary of the Eastern New York Surprise Snowstorm (10/4/87): Bosart & Sanders (1991): An Early-Season Coastal Storm: Conceptual Success and Model Failure
Marshall Pfahler: The Role of the Subtropical Jet in Dry Season Caribbean Rainstorms (see 2 Oct section)
Minghao Zhou: Abstract: Eyewall Replacement Cycles Eyewall replacement cycle (ERC) is a phenomenon commonly observed in intense tropical cyclones (TCs). During an ERC, TC rainbands axisymmetrize to form an outer eyewall at a larger radius, robbing the inner eyewall of moisture and momentum inflow. As the inner eyewall weakens and gradually dies away, the outer eyewall becomes the new primary eyewall. TC usually weakens during an ERC and could reintensify afterwards if environmental conditions are favorable, but the new eye diameter can hardly contract to that of the original size. ERC is an especially important forecast matrix for mature/intense TCs, as the associated TC intensity fluctuation and wind field expansion are vital to coastal impact and storm surge potential assessments. ERC behaviors of TCs within the North Atlantic basin have been studied extensively using both satellite and in-situ data, while for the Western North Pacific (WNP) which produces the most of intense TCs in the globe, documentations of ERCs are not as clear and deserves further study. This study uses microwave sensor product from the Naval Research Laboratory's tropical cyclone web page to create a climatology for ERCs in the WNP, and subsequently discusses the characteristics of ERC occurrence and impacts from larger-scale environmental factors. Microwave instruments were employed since 1997 which can penetrate through TC cirrus canopy and "see" lower level convective features, such as the secondary eyewall. There's currently no formal objective definition of what marks the onset and completion of an ERC, so subjective detection of ERCs are applied with maximum discretion. Specifically, ERC events are grouped into two different bins labeled as "successful completion" and "failure to complete" in this study. Under such classification, successful completion of an ERC means an utter substitution of the old eyewall by the new eyewall, while failure to complete an ERC means the outer eyewall does not eventually take over for different reasons. Intensity and structural changes of a TC should be subsequently dependent on whether an ERC is successful or not. In order to better evaluate what type of environmental regime better facilitates an ERC process, the high resolution ECMWF ERA5 reanalysis will be used to composite sea surface temperature (SST), vertical wind shear (VWS), low-level moisture content and the like for cases in different bins (and probably finer subsets). The goal of this study is to improve our understanding of ERCs through a handful of cases and provide better guidance for operational forecasts even without the presence of aircraft reconnaissance.
Supporting documentation:
Tu 9 Oct
Minghao Zhou: Abstract: Eyewall Replacement Cycles (see 4 Oct section)
Ajay Raghavendra:
Background/Science questions: As you probably know, I am advised by Prof. Liming Zhou and I work on a project to investigate Congo rainfall variability and trends. The project I wish to complete here will most likely be a part of my PhD research work. Broadly speaking, I am interested in working in-between the weather and climate communities and hunt for research/science opportunities. For this class project, I'm interested in exploring tropical waves (including the MJO), its relationship with convective/MCS activity over the Congo rainforest, and associated extratropical forcings (including southern hemisphere wave breaking) on both convection and atmospheric equatorial waves, and future climate projections. You may find my recent paper (Raghavendra et al. 2018 also attached, and easy to read) useful to understand some of the key issues. I have also attached two papers for you to study before my presentation on Tuesday (9th October). The papers are relatively easy to read (i.e., lots of figures, and simple analysis/discussion) and are markedly different from each other. Jackson et al. (2009) uses TRMM to construct an MCS climatology for equatorial Africa, and emphasizes the importance of orographic interactions and how a few large MCS events can produce most of the observed climatology. Sinclaire et al. (2015) uses NOAA's OLR and reanalysis datasets, and the GPCP precipitation dataset to correlate synoptic kelvin wave activity to rainfall over the Congo rainforest. During my presentation, I will briefly summarize the above two papers, followed by the discussion (including limitations and uncertainties). Later, I will present an outline for my proposed research work. During this process, I'll be happy to answer related questions and encourage dialog/discussions.
References:
Th 11 Oct
Ajay Raghavendra: See Tue. Oct. 9 Notes
Tu 16 Oct
Mike Main Abstract: Significant strides have been made over the past several years in our ability to predict mid-latitude weather in both the short and long term, but a gap still remains in sub-seasonal mid-latitude predictability. The stratosphere has been a focus of recent efforts to improve our sub-seasonal forecast skill. Significant time has been spent on examining the predictability of the stratosphere itself, as well as the impact that the stratosphere has on tropospheric weather. While troposphere-stratosphere coupling has received more attention in the more recent literature, most of this research again focuses on stratospheric processes rather than tropospheric ones. A recent study by Attard and Lang examines how mid-latitude clocking ridges and bomb cyclones impact the upward wave activity flux (WAF) from the troposphere to the stratosphere. WAF convergence in the stratosphere can lead to a sudden stratospheric warming (SSW), which results in a reversal of the 10-hPa mean westerly stratospheric zonal winds at 60oN; this can have a profound impact on mid-latitude weather a few days to several weeks later.
This study will examine the 22 Feb 2008 SSW event from a tropospheric perspective. Re-forecast data will be examined for the ensembles of several numerical weather prediction models. Ensemble forecasts will be evaluated at various lead times between 10 and 30 days. It is expected that some ensemble members will be able to predict the SSW with varying degrees of accuracy, while others, especially at longer lead times, will not. The goal of this project is to determine why certain ensemble members will be able to predict the SSW while others cannot. Differences in ensemble member forecasts for various meteorological fields will be examined subjectively. Such an analysis should bring to light some of the physical tropospheric processes required in order for an SSW to occur. Further research could then examine other SSW events and compare the results to the finding of this study.
More information can be found in the two referenced papers below:
Tripathi et al. (2015) gives a lot of background on the stratosphere and stratosphere-troposphere coupling. This paper is very long, but only sections 2 through 3.3 closely relate to my research, so just focus on these sections.
Attard and Lang (2018) gives more about heat flux and mid-latitude weather. My research is in some ways a continuation/addition to what Hannah worked on for her PhD.
Links:
Th 18 Oct
Mansour Riachy Abstract: A global mutual interest in Arctic region weather derives from the standpoint of international trade and tourism from one perspective and climate response to arctic amplification via synoptic and planetary patterns from the scientific perspective. This amplification, described as the greater warming at higher latitudes compared to lower latitudes, is linked to increased cyclone-related warm air incursions that drive warm moist air from the mid-latitudes into the arctic. The intrusions create strong downward longwave flux anomalies and bottom-amplified temperature perturbations according to Woods and Caballero 2016. They also found a statistically significant upward trend in moisture intrusions crossing 70°N into the Arctic during December and January that can explain ~45% and ~30% of trends in surface air temperature and sea ice concentration, respectively, observed over the Barents Sea since the 1990s. That said, it is important to consider the Arctic-Midlatitude interactions in relation to the synoptic and planetary scale waves that set up the flow pattern conducive to extreme poleward moisture transport. The focus is then on regimes that contribute to this flow pattern by building composite maps and climatologies pertaining to increased warm air transport and polar vortex disruption. Grouping of the results will follow categories that describe different flavors of Arctic amplification based on teleconnection indices, jet patterns and Rossby wave breaking (Cyclonic and anticyclonic). This will be achieved by analyzing NAO, North Atlantic Oscillation, AO, Arctic Oscillation modes in relation to the strength of the Polar Vortex in an event-based approach. I have attached two papers and a proposal describing the topic I will address. The first paper is a BAMS journal article titled: "What is the polar Vortex and How Does it Influence the Weather?" Waugh, D. W., Sobel, A. H., & Polvani, L. M. (2017). It describes the differences between tropospheric and stratospheric polar vortices in general. My focus will address the part under the section: Extreme weather events. The second paper by Lui, C., and E. A. Barnes (2015) titled: "Extreme moisture transport into the Arctic linked to Rossby wave breaking (RWB)", talks about how midlatitude dynamics affect moisture transport into high latitudes by cyclonic and anticyclonic wave breaking. It also discusses the influence of climate variability captured by of ENSO and NAO on the interannual variability of RWB-related transport in winter.
Links:
Tu 23 Oct
Kevin Biernat Guest Lecture:
During the first half of class, Kevin Biernat led a discussion on research perspectives gained in graduate school. Began by discussing how he came to choose his research topics, emphasizing that research does not always follow a straight path. Then discussed the research process from his perspective, the usefulness of leveraging class projects and lectures, and the importance of networking and improving research communication. Lastly, Kevin discussed some takeaway points of what he has learned through conducting research and what he would have done differently. The second half of class was spent discussing Arctic meteorology. Began by discussing and comparing coherent tropopause disturbances (CTDs) and tropopause polar vortices (TPVs). Illustrated the importance of radiative cooling on TPV maintenance and intensification. Showed the dynamical importance of TPVs and CTDs on the evolution of surface cyclones, jet streaks, and cold air outbreaks. Ended by discussing and comparing polar lows and Arctic cyclones, and showing a couple illustrative examples of polar lows and Arctic cyclones linked to TPVs.
Kevin Biernat Lecture SlidesTh 25 Oct
Jeremy Berman Guest Lecture: Synopsis: Jeremy presented a discussion of his Ph.D. research. For this discussion, he began by leading a synopsis of previous work on warm conveyor belts (WCBs) and jet stream prediction. Thereafter, Jeremy walked through an illustrative case that was characterized by a strong midlatitude cyclone interacting with an amplifying midlatitude flow pattern over the North Atlantic. The predictability of this case was highlighted by analyzing model output from the ECMWF ensemble forecast system, which showcased the large forecast variability across the ensemble members. This provided the context for Jeremy's use of two research tools: empirical orthogonal functions (EOFs) and ensemble sensitivity analysis. The students asked several thoughtful and interesting questions about the utility of these techniques and how to interpret their results. Finally, these techniques were applied to the case, and the results were presented to the students. Despite the novelty of these techniques, the students readily understood the applicability of the techniques and the dynamical insights learned. Moreover, the students found several ways to apply the techniques to their own work. Overall, Jeremy was happy to hear such thoughtful questions and also be a part of the larger discussion of the utility of these techniques to other meteorological research. In conclusion, Jeremy left a few remarks to students based on his own graduate school experience: (1) take advantage of class projects: they may just lead to a Ph.D. topic; (2) be aware that you will have many moments where research fails, but as long as you keep thinking about the problem you will find reasonable solutions; (3) always stay curious and ask questions, since questions help clarify the problem to not only the students but also the presenters. Jeremy thanked the students for their attention and also extended his gratitude to Prof. Dan Keyser for attending and providing insightful comments and questions.
Links:
Tu 30 Oct
Matthew Campbell presented a mid-semester research update during class. The overall failure of his initial experimental design was presented. Subsequently, an alternative, more physically realistic approach was proposed (discussed in more detail below). With the goal of examining the Great Plains low-level jet (LLJ) in mind, two synoptically different composites of the Great Plains LLJ were constructed subjectively. Building off previous research within Matt's research group, two main types of LLJs were identified. One type of LLJ occurs under a large upper-tropospheric anticyclone located over the central USA. This can often be seen as a trough-ridge-trough pattern from west to east across CONUS at 500 hPa. The other type of LLJ occurs under zonal upper-tropospheric flow. Both LLJs are uncoupled, meaning that there are not associated with the thermally indirect circulation associated with an upper-tropospheric jet streak. In essence, both of types of LLJs should not be strongly influenced by synoptic scale forcing. After the composites were constructed, exploration on how to force WRF with the composites was conducted. In order to explore this idea, Matt first started with the anticyclonic LLJ composite. He attempted to use this composite to provide lateral boundary conditions throughout the WRF run and provide initial conditions for the run itself. The lateral boundary conditions from this 24-hour composite were to be repeated for 30 days in order for the model to come to "equilibrium" with the land, which would vary in soil moisture conditions. Essentially, a Groundhog Day experiment was created (minus the groundhog and Bill Murray). However, when taking this approach many issues quickly appeared along the boundaries during free running simulations. These errors along boundaries of the domain led to use of spectral nudging in order to prevent severe boundary errors from occurring. While spectral nudging did manage to alleviate issues along the boundaries themselves, spectral nudging experiments developed unrealistic solutions within the domain. Many different variations in spectral nudging strength, time, levels to be nudged, and wavelengths were tried. However, unrealistic conditions within the domain persisted. It was then determined that "locking in" synoptic conditions may not be the best approach to explore the impact of soil moisture on the LLJ. A new, more physically consistent approach with reality was then proposed. Instead of forcing WRF with composites in a Groundhog Day type experiment, composites will be constructed from WRF output from individual case studies. Each case for WRF will be representative of the composites. The results from the first case study using this approach were promising and physically consistent with observations unlike the results from the previous approach. Moving forward Matt will do a handful of case studies and analyze the results individually and collectively.
Th 1 Nov
Eli Turasky:
An additional literature review was conducted, which included Nieman, P. J., et al., 2008, Rutz, J. J., Steenburgh, J. W., and Ralph, M., 2014, Mundhenk, B. A., Barnes, E. A., and Maloney, E. D., 2016, and Gershunov et al., 2017. In general, these papers offer different types of climatologies of landfalling atmospheric rivers (ARs), specifically over western North America and the North Pacific. Eli Turasky's work focuses on North Pacific Jet (NPJ) regimes that foster AR landfall and uniquely complements the previous AR climatologies. After discussing previous research, a brief review of the NPJ Phase Diagram was conducted in order to remind the audience of the methodology utilized. For this specific presentation, Eli focused on composite NPJ Phase Diagram trajectories (starting 10 days prior to landfall and ending at landfall) of landfalling ARs along the west coast of North America when the Pacific North American (PNA) pattern was greater than one or less than negative one. The results yielded a strong tendency for the NPJ to trend towards an extended and poleward shifted phase for the positive PNA cases (~350 cases). The negative PNA cases resulted in the exact opposite, such that the NPJ yielded a strong tendency to trend towards a retracted and equatorward shifted phase (~250 cases). The results were consistent when the PNA criterion was changed to PNA>2 or PNA<-2.
Links:
Tu 6 Nov
Daniel Reese:
By plotting data from a microwave radiometer, the structure of a convective cold pool can be seen and analyzed. However, there are a few issues with the data that must be accounted for first. During periods of heavy rain, the data quality typically suffers, with temperature values erroneously appearing too warm. This is of particular concern as these periods of heavy rain often occur during the time the radiometer should be sampling the cold pool. To combat this, a rain flag should be created that flags data when rain falls at or above a certain rate, allowing erroneous data to be accounted for. Additionally, exact cold pool boundaries are often hard to locate based on just the observed temperature data. It is often better to look at temperature trends (generally 30 minute trends, the same time period used with surface observation trends) rather than exact temperature, in order to determine the starting and ending times for the cold pool. Cold pool depth can be determined by deriving the buoyancy at different levels, with the top of cold pool determined by change from negative to positive buoyancy. Other derived quantities (such as theta or theta-e) may further describe these cold pool features and trends.
Th 8 Nov
Josh Alland (Guest Lecture):
Josh Alland discussed his research experience at UAlbany, concentrating on successes, tribulations, and tips. He emphasized the importance of choosing a research topic you are passionate about, can effectively communicate to the general public, and can foster collaboration. Josh also emphasized the importance of becoming an interdisciplinary scientist within our increasingly interdisciplinary weather enterprise. Lastly, Josh conducted a mini-leadership workshop (based on an exercise by Susan Edwards), where each student began to develop their own individual leadership philosophy. Such a leadership philosophy is vital to understand your core values, and will serve as an effective resource when applying to jobs.
Links:
Josh Alland Lecture Notes Leadership WorkshopTu 13 Nov
Lexi Henny: The atmospheric river dataset provided by Huang-Hsiung Hsu and constructed following the methodology of Wick et al (2013) shows two maxima in AR frequency near Taiwan: one to the northwest, extending from the Taiwan Strait to the north of Taiwan, and another to the southeast of the island. The northern maximum is larger than the southern one. Separating landfalling AR events into Mei-Yu season (15 May - 14 June), typhoon season (JAS), and Fall (ON), there are no statistically-significant trends in frequency. In order to explore the characteristics of these Taiwan-landfalling AR events, the AR points are plotted together with 500 hPa thickness, 850 hPa convergence, precipitable water, sea level pressure, TCCIP (Taiwan Climate Change Intercomparison and Adaptation Information Platform) 1 km rainfall data, and IBTrACS typhoon track and intensity data. Mei-Yu season events are typically frontal, with frequent southward incursions by the jet. These events are often long and narrow, oriented from west-southwest to east-northeast. Typhoon season and fall events are mostly typhoon-associated, coming in the form of long moisture filaments linked up with the typhoon. Suggestions for continuing research include confirming the accuracy of the data, making more easily decipherable plots, and investigating the dynamical differences between these typhoon-associated "ARs" and the more classical events. These typhoon-associated events that have been identified by the automated process may not fall within the usual definition of an atmospheric river; however, they are clearly important for rainfall in Taiwan.
Th 15 Nov
Minghao Zhou:
Using the subjecively derived Eyewall Replacement Cycle (ERC) dataset from microwave imagery archive and the 6-hourly ERA-Interim reanalysis on reduced N128 Gaussian grid, 162 ERC events between 1997-2015 and associated environmental fields were first examined. 700-850hPa mean relative humidity, 200-850hPa vertical wind shear and sea surface temperature (SST) fields within 20° ×20° of the tropical cyclone (TC) center were retrieved at both the onset and cessation time of the ERC. Composite analysis showed that in 83 successful cases, areas of ?80% relative humidity near the TC center became more axisymmetric during the ERCs and the ambient environment was also more moist, compared to 79 unsuccessful ERC events. Vertical wind shear was higher in the 79 failed ERC cases, especially to the TC's north/northwestern quadrant, implying possibly more interaction with midlatitude systems (troughs). SST field in both groups are pretty warm for TC development, but the successful ERCs take place over even warmer water, with SST?28? within 3° of the TC center throughout the ERC. Nevertheless, environmental condition for individual case can vary greatly in terms of above-mentioned features, depending on TC size, latitude, season, etc. Next step would be to exclude landfalling cases and create a distribution frequency map for environmental factors in these different ERC scenarios, in the purpose of testing if such environmental condition difference is significant.
Tu 20 Nov
Ajay Raghavendra:
Thunderstorm activity and rainfall amount are poorly correlated over the Congo rainforest where recent studies have documented in increase in thunderstorm activity, but a decrease in rainfall amount. Also, some studies suggest that large, well-organized convection known as mesoscale convective systems (MCSs) are responsible for most of the rainfall (>80%) over the Congo rainforest. Therefore, a heightened understanding of the hydrological cycle over the Congo necessitates an analysis of trends and mechanisms responsible for MCS activity over the Congo. Using a combination of long-term satellite data, reanalysis products, and precipitation datasets, this presentation reports a decreasing trend in MCS activity over the Congo, and a strong correlation between MCS activity and rainfall amount. The composite evolution of MCSs studies using wind and temperature variables from ERA-I for different RMM MJO phases were also presented.
Tu 27 Nov
Mike Main:
The presentation began by reexamining the motivation for studying troposphere-stratosphere interactions. When a disturbance occurs in the troposphere, it is possible for upward wave propagation from the troposphere into the stratosphere to occur; the resulting wave breaking in the stratosphere can lead to a sudden stratospheric warming (SSW) event, which defined by the reversal of the 10-hPa mean zonal wind at 60 oN. The nature of these tropospheric disturbances will be examined in more detail, with specific attention given to why some disturbances cause an SSW while others do not. Analysis of reforecast data from the European Center (ECMWF) ensemble shows that model forecasts of the 22 Feb 2008 SSW are initially poor at larger lead times, but model runs initialized closer to the event show improvement. Model skill is greater at longer lead times than in the troposphere, with some ensemble members able to predict an SSW roughly one month in advance, and all members showing an SSW at 12 days lead time. Synoptic scale forecast plots from two members of the ECMWF reforecast ensemble initialized for 22 Jan 2008 were shown. Variables plotted included 10-hPa zonal wind, 10-hPa temperature, 300-hPa heights, and mean sea level pressure. Significant differences in each of the aforementioned fields were apparent when comparing one member that predicted an SSW around 22 Feb 2008 to another ensemble member that did not. Additional research should bring to light key differences between ensemble members that predict this particular SSW compared to those that do not. The statistical significance of these differences will also be examined in future research.
Mansour Riachy:
See Nov 29 Summary below
Th 29 Nov
Mansour Riachy:
Sinuosity was chosen as a metric of quantification of waviness of the 500mb geopotential height in Martin et al. 2015. It is calculated by calculating the ratio of the length of the 500mb geopotential height contour chosen to the length of the equivalent latitude corresponding to the same area enclosed by that contour. For this class, I will be applying this metric to the 324K isentrope on the 2-PVU surface to assess the waviness of the tropospheric polar vortex. A preliminary result was shown by plotting sinuosity for the late December 2015/early January 2016 extreme warm event in the Arctic. This event was characterized by a series of mid to high latitude cyclones in the Atlantic protruding deep into the Arctic, causing anomalously warm temperatures north of 82 degrees north. Two spikes in maximum sinuosity stand out at the time of the event which may suggest a direct relationship between intense warm intrusions into the Arctic and waviness of the polar vortex edge. A climatology needs to be done on a longer time series to have a reference to compare to. Then, standardized anomalies need to be calculated to evaluate how anomalous are these spikes. In addition, I will be looking at sinuosity for different longitudinal sectors at high latitude and how it is affected by cyclone activity and warm air intrusion into the Arctic.
Dry runs of final projects were given by Matt Campbell and Danny Reese
Tu 4 Dec
Dry runs of final projects were given by Eli Turaski, Marshall Pfahler, and Lexi Henny.
Eli Turasky Abstract:
An Examination of North Pacific Jet Regimes Conducive to Landfalling Atmospheric Rivers along the West Coast of North America
Abstract: Every year, the west coast of North America experiences significant economic damage and societal disruption due to the extreme precipitation associated with landfalling atmospheric rivers (ARs). ARs not only may produce significant economic and societal impacts, but also may contribute disproportionately to precipitation anomaly statistics along the west coast of the North America. Recent research suggests that the frequency of landfalling ARs along the west coast of North America may increase, due to increased horizontal moisture transport, in a warmer climate. In view of these considerations, it is important to diagnose the physical processes and the antecedent environments that govern landfalling ARs in order to improve medium-range forecasts of extreme precipitation events along the west coast of the North America. This study employs an archive of landfalling ARs along the west coast of North America constructed by the Center for Western Weather and Water Extremes to examine the large-scale upper-tropospheric flow patterns associated with landfalling ARs during September?May 1979?2017. In this study, the large-scale upper-tropospheric flow patterns prior to landfalling ARs are examined in the context of the two-leading empirical orthogonal functions (EOFs) of 250-hPa zonal wind over the North Pacific Ocean. The first EOF corresponds to a zonal extension or retraction of the climatological exit region of the North Pacific Jet (NPJ), while the second EOF corresponds to a poleward or equatorward shift of the climatological exit region of the NPJ. The projection of 250-hPa zonal wind anomalies at one or multiple times prior to AR landfall onto these two leading EOFs provides an objective characterization of the instantaneous state or evolution of the upper-tropospheric flow pattern over the North Pacific prior to AR landfall, which is represented on a two-dimensional NPJ Phase Diagram. The analysis leverages the NPJ Phase Diagram to examine the degree to which AR landfall latitude and AR intensity vary as a function of the structure and evolution of the NPJ prior to AR landfall along the west coast of North America. An examination of landfalling ARs during the positive and negative phase of the Pacific North American (PNA) pattern yields results that are statistically significantly different. Specifically, the NPJ evolves towards an extension and poleward shift regime during a positive PNA and evolves toward a retraction and equatorward shift regime during a negative PNA.
Marshall Pfahler:
The Role of the Subtropical Jet in Dry Season Caribbean Rainstorms:
Although significant high-impact heavy rainfall events in the Caribbean are typically associated with tropical cyclones during the wet season, significant heavy rainfall events can also occur during the Caribbean dry season absent of characteristics of tropical cyclones (TCs). These dry season Caribbean rainstorms (CRs) can produce infrastructure and life-threatening conditions through flooding, mudslides, and damaging winds. The most significant CRs can require global aid for residents to have access to vital recovery resources. The Caribbean experiences a bimodal climate with a wet season extending from May/June-November/December and dry season outside of those months. Significant rainstorms that occur during the dry season are considered anomalous by their ability to produce rainfall totals that surpass climatological monthly mean precipitation in a relatively short duration (2-16 days). The subtropical jet (STJ) plays an important role in the generation and severity of dry CRs in the Caribbean by providing large-scale forcing for ascent that interacts with tropical moisture. Antecedent equatorward excursions of the STJ into the Tropics can be manifested as significant upper-tropospheric troughs that can, in some cases, experience thinning and elongation from upstream anticyclonic wave breaking (AWB). The latter can result in formation of a potential vorticity streamer (PVS) that can be fractured from upper-tropospheric deformation as cutoff cyclogenesis from satisfaction of barotropic instability occurs. These extra-tropical invasions of the STJ can approach from the west/northwest or retrograde from the east/northeast. Ageostrophic motions associated with jet streaks embedded within the STJ can compound forcing for severe CRs. CR events have been additionally noted to result from stalling, weakening frontal boundaries that reach the Caribbean associated with extra-tropical cyclones over the Northeast US that form from negatively-tilted shortwave troughs of the STJ.