DOCSLIB.ORG

Operational Techniques for Forecasting Thunderstorms

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Operational Techniques for Forecasting Thunderstorms Module 4.2D2 Operational Techniques for Forecasting Thunderstorms Contents Introduction i Diagnosis and Forecasting of Thunderstorms iv 250mb (Purple).............................................................................................................................................v 500mb (Blue)...............................................................................................................................................vi 700mb (Brown)......................................................................................................................................... viii 850mb (Red)................................................................................................................................................xi Surface (Black)..........................................................................................................................................xiv Convective Parameter Review xvii Stability / Moisture Parameters................................................................................................................ xvii Dynamic or Wind Shear Parameters...........................................................................................................xx Combined Stability / Dynamic Parameters................................................................................................xxi Convective Assessment Example xxiii Summary xxvii Introduction In very general terms there are three requirements for the development of deep (surface based) convection (referring here to TCu and garden variety Cb), these are instability, boundary layer moisture (due to latent energy considerations) and some mechanism to produce vertical motion (i.e., a trigger) allowing a parcel to attain free convection. Assuming these three conditions are met we may expect deep convection to develop with its intensity dependant primarily on the degree of instability and availability of boundary layer moisture. Taking an ingredients-based approach to forecasting convection simplifies the process to one of diagnosing basic requirements to be met for thunderstorm development. Figure 1: Ingredients necessary for thunderstorm development. In the above figure we can imagine that the square represents all possible states of the atmosphere. Each circle represents possible states of the atmosphere exhibiting the indicated requirement for thunderstorms. Where the requirements for instability, moisture and some trigger mechanism are met is the most likely location for thunderstorms to develop. The difficulty for forecasters is that there are often many ways to determine if, when, and where these requirements can be met. There can be a tendency to seek a particular parameter or index that will give the final answer instead of simply looking at what physical requirements must be met for convection to develop. Using the shortcut approach can introduce even more confusion as many parameters will often suggest the same thing in different ways. Convective parameters are extremely useful tools but only if they are used correctly. The three requirements listed above are fundamental to the development of all thunderstorms from short-lived single cells to destructive supercells. What separates the environments of varying storm types from one another is the degree of vertical wind shear within the pre-storm environment. Buoyancy primarily determines updraft strength but it is wind shear that determines the resulting storm evolution and morphology. In general, a good rule of thumb is that the stronger the wind shear, the more organized the resulting convection will become. Figure 2: Ingredients necessary for long-lived thunderstorms. Similar to figure 1, the figure above illustrates the requirements for persistent thunderstorms. The addition of vertical wind shear can be the difference between short-lived cells and long- lived severe thunderstorms. Note that no thunderstorms are expected when one of the original three requirements is missing, even if wind shear is present. Forecasting persistent thunderstorms (and severe thunderstorms) can again be simplified by using this ingredients based approach. At this point it is useful to remind ourselves of the distinction between wind speed and wind shear. Wind speed of course refers to the vector magnitude of the wind at some level in the atmosphere, wind shear on the other hand refers to the change in wind speed between some level(s). This means that we can have an environment with strong winds aloft but little shear. The role of wind shear in thunderstorm development includes the following: interaction of low-level wind shear and thunderstorm outflow boundaries to generate new thunderstorm cells (this is especially important for multicell thunderstorms) tilting of thunderstorm updrafts in deeply sheared environments generation of low-level horizontal vorticity in sheared flow (this is crucial for the development of mid-level rotation in supercell thunderstorms) interactions between updrafts and environmental wind shear resulting in storm splitting generation of vertical perturbation pressure gradients in supercell thunderstorms (this is significant in the production of strong updrafts in supercells and can have implications for tornadogenesis) These processes are all discussed in earlier sections of module 6.4. Diagnosis and Forecasting of Thunderstorms Now that we have reviewed the concepts important for understanding how thunderstorms develop we will look at how to diagnose the potential for thunderstorm development from an operational perspective. It is useful to always keep in mind the four requirements we have outlined for the development of persistent thunderstorms, namely: instability moisture vertical motion (trigger) wind shear As the forecaster goes through the analysis / diagnosis process they should always be on the lookout for these four things. More importantly, regardless of methods used or how they are characterized (e.g., tephigrams, upper air / SFC charts, GRIB viewers and various convective parameters) it is always these four basic requirements that must be diagnosed when forecasting convection. Therefore, even just reciting them in your mind during your work-up will help simplify the forecasting process. Sometimes, just one missing ingredient can be the difference between no convection and long-lived damaging supercells or multicell storm complexes. In the Module 2.5D we reviewed the assessment of stability of a column using tephigrams. Now we turn our attention to diagnosis in spatial terms. We will focus on the use of SFC and upper air charts, hodographs to assess wind shear, and the use of GRIB viewers and convective parameters. In this section we will examine the use of meteorological charts (both SFC and upper level) to look for significant features and associated processes that are conducive to convective initiation. We will look at each of the standard isobaric levels in turn, highlighting important features to look for and the conventional symbols used to represent them in the composite chart. For each level we have included an example highlighting the location of some relevant features. The charts used are all analysis charts from 12Z 2 July 2000, a day when widespread convection occurred in both AB and SK. The information given below borrows heavily from both original work by Miller (1972) and the Summer Severe Weather Distance Learning Course (Training and Education Services Branch, Environment Canada 1995), hereafter referred to as SSWDLC (1995). Some modifications have been incorporated following an internal technical note (Whittle 1999) and discussions with operational forecasters. The list of features below is not an exhaustive one and does not include all of the features discussed by Miller (1972). We have instead included some of the more common parameters used operationally. Associated with each feature are methods of diagnosis, which of the necessary conditions for sustained convection (instability, moisture, vertical motion, or vertical wind shear) that it addresses, and comments regarding the physical process that it represents. It is important to note that while we may look for features using specific isobaric levels, the features themselves are not constrained to those levels. For example, the high level jet is not always at 250mb and generally exhibits vertical variability. Similarly, low-level features often associated with the 850mb level may actually exist above or below that level. As with any forecast, it is intended that the forecaster utilize all available information during the analysis / diagnosis process. This section is intended as a starting point for forecasters to develop their own personal convective assessment routine. Through experience, features not discussed here may be included and the forecaster will likely develop some of their own symbols. 250mb (Purple) 250mb Jet J 120 Diagnosis / Comments: isotach analysis and 250mb heights. Also use CMC high-level max. wind chart. The J is used to denote the jet core with its strength in knots noted below. Associated Process / Significance: VERTICAL MOTION The high level jet is significant for enhancing vertical motions in the troposphere due to the ageostrophic circulations that exist around its core (refer to MOIP notes Section 6.2 H - number 8). Qualitatively and through visual inspection, for straight jet streaks, the right entrance / left exit of the jet core are preferred regions for upward vertical motion. In the case of cyclonically
Load more

Recommended publications
  • Link Between the Double-Intertropical Convergence Zone Problem and Cloud Biases Over the Southern Ocean
    Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean Yen-Ting Hwang1 and Dargan M. W. Frierson Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195-1640 Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved February 15, 2013 (received for review August 2, 2012) The double-Intertropical Convergence Zone (ITCZ) problem, in which climate models show that the bias can be reduced by changing excessive precipitation is produced in the Southern Hemisphere aspects of the convection scheme (e.g., refs. 7–9) or changing the tropics, which resembles a Southern Hemisphere counterpart to the surface wind stress formulation (e.g., ref. 10). Given the complex strong Northern Hemisphere ITCZ, is perhaps the most significant feedback processes in the tropics, it is challenging to understand and most persistent bias of global climate models. In this study, we the mechanisms by which the sensitivity experiments listed above look to the extratropics for possible causes of the double-ITCZ improve tropical precipitation. problem by performing a global energetic analysis with historical Recent work in general circulation theory has suggested that simulations from a suite of global climate models and comparing one should not only look within the tropics for features that affect with satellite observations of the Earth’s energy budget. Our results tropical precipitation. A set of idealized experiments showed that show that models with more energy flux into the Southern heating a global climate model exclusively in the extratropics can Hemisphere atmosphere (at the top of the atmosphere and at the lead to tropical rainfall shifts from one side of the tropics to the surface) tend to have a stronger double-ITCZ bias, consistent with other (11).
    [Show full text]
  • Laboratory Studies of Turbulent Mixing J.A
    Laboratory Studies of Turbulent Mixing J.A. Whitehead Woods Hole Oceanographic Institution, Woods Hole, MA, USA Laboratory measurements are required to determine the rates of turbulent mixing and dissipation within flows of stratified fluid. Rates based on other methods (theory, numerical approaches, and direct ocean measurements) require models based on assumptions that are suggested, but not thoroughly verified in the laboratory. An exception to all this is recently provided by direct numerical simulations (DNS) mentioned near the end of this article. Experiments have been conducted with many different devices. Results span a wide range of gradient or bulk Richardson number and more limited ranges of Reynolds number and Schmidt number, which are the three most relevant dimensionless numbers. Both layered and continuously stratified flows have been investigated, and velocity profiles include smooth ones, jets, and clusters of eddies from stirrers or grids. The dependence of dissipation through almost all ranges of Richardson number is extensively documented. The buoyancy flux ratio is the turbulent kinetic energy loss from raising the potential energy of the strata to loss of kinetic energy by viscous dissipation. It rises from zero at small Richardson number to values of about 0.1 at Richardson number equal to 1, and then falls off for greater values. At Richardson number greater than 1, a wide range of power laws spanning the range from −0.5 to −1.5 are found to fit data for different experiments. Considerable layering is found at larger values, which causes flux clustering and may explain both the relatively large scatter in the data as well as the wide range of proposed power laws.
    [Show full text]
  • Tropical Weather Discussion
    TROPICAL WEATHER DISCUSSION • Purpose The Tropical Weather Discussion describes major synoptic weather features and significant areas of disturbed weather in the tropics. The product is intended to provide current weather information for those who need to know the current state of the atmosphere and expected trends to assist them in their decision making. The product gives significant weather features, areas of disturbed weather, expected trends, the meteorological reasoning behind the forecast, model performance, and in some cases a degree of confidence. • Content The Tropical Weather Discussion is a narrative explaining the current weather conditions across the tropics and the expected short-term changes. The product is divided into four different sections as outline below: 1. SPECIAL FEATURES (event-driven) The special features section includes descriptions of hurricanes, tropical storms, tropical depressions, subtropical cyclones, and any other feature of significance that may develop into a tropical or subtropical cyclone. For active tropical cyclones, this section provides the latest advisory data on the system. Associated middle and upper level interactions as well as significant clouds and convection are discussed with each system. This section is omitted if none of these features is present. 2. TROPICAL WAVES (event-driven) This section provides a description of the strength, position, and movement of all tropical waves analyzed on the surface analysis, from east to west. A brief reason for a wave’s position is usually given, citing surface observations, upper air time sections, satellite imagery, etc. The associated convection is discussed with each tropical wave as well as any potential impacts to landmasses or marine interests.
    [Show full text]
  • Boundary Layer Verification
    Boundary Layer Verification ECMWF training course April 2015 Maike Ahlgrimm Aim of this lecture • To give an overview over strategies for boundary layer evaluation • By the end of this session you should be able to: – Identify data sources and products suitable for BL verification – Recognize the strengths and limitations of the verification strategies discussed – Choose a suitable verification method to investigate model errors in boundary layer height, transport and cloudiness. smog over NYC Overview • General strategy for process-oriented model evaluation • What does the BL parameterization do? • Broad categories of BL parameterizations • Which aspects of the BL can we evaluate? – What does each aspect tell us about the BL? • What observations are available – What are the observations’ advantages and limitations? • Examples – Clear convective BL – Cloud topped convective BL – Stable BL Basic strategy for model evaluation and improvement: Observations Identify discrepancy Model Output Figure out source of model error Improve parameterization When and where does error occur? Which parameterization(s) is/are involved? What does the BL parameterization do? Attempts to integrate Turbulence transports effects of small scale temperature, moisture and turbulent motion on momentum (+tracers). prognostic variables at grid resolution. Stull 1988 Ultimate goal: good model forecast and realistic BL Broad categories of BL parameterizations Unified BL schemes Specialized BL scheme •Attempt to integrate BL (and •One parameterization for each shallow cloud) effects in one discrete BL type scheme to allow seamless transition •Simplifies parameterization for •Often statistical schemes (i.e. each type, parameterization for making explicit assumptions about each type ideally suited PDFs of modelled variables) using •Limitation: must identify BL type moist-conserved variables reliably, is noisy (lots of if •Limitation: May not work well for statements) mixed-phase or ice •Example: Met-Office (Lock et al.
    [Show full text]
  • On the Computation of Planetary Boundary-Layer Height Using the Bulk Richardson Number Method
    Geosci. Model Dev., 7, 2599–2611, 2014 www.geosci-model-dev.net/7/2599/2014/ doi:10.5194/gmd-7-2599-2014 © Author(s) 2014. CC Attribution 3.0 License. On the computation of planetary boundary-layer height using the bulk Richardson number method Y. Zhang1, Z. Gao2, D. Li3, Y. Li1, N. Zhang4, X. Zhao1, and J. Chen1,5 1International Center for Ecology, Meteorology & Environment, Jiangsu Key Laboratory of Agricultural Meteorology, College of Applied Meteorology, Nanjing University of Information Science and Technology, Nanjing 210044, China 2State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 3Program of Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ 08540, USA 4School of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China 5Department of Geography, Michigan State University, East Lansing, MI 48824, USA Correspondence to: Z. Gao ([email protected]) Received: 15 March 2014 – Published in Geosci. Model Dev. Discuss.: 24 June 2014 Revised: 12 September 2014 – Accepted: 22 September 2014 – Published: 10 November 2014 Abstract. Experimental data from four field campaigns are 1 Introduction used to explore the variability of the bulk Richardson num- ber of the entire planetary boundary layer (PBL), Ribc, which The planetary boundary layer (PBL), or the atmospheric is a key parameter for calculating the PBL height (PBLH) boundary layer, is the lowest part of the atmosphere that is in numerical weather and climate models with the bulk directly influenced by earth’s surface and has significant im- Richardson number method. First, the PBLHs of three differ- pacts on weather, climate, and the hydrologic cycle (Stull, ent thermally stratified boundary layers (i.e., strongly stable 1988; Garratt, 1992; Seidel et al., 2010).
    [Show full text]
  • How Appropriate They Are/Will Be Using Future Satellite Data Sources?
    Different Convective Indices - - - How appropriate they are/will be using future satellite data sources? Ralph A. Petersen1 1 Cooperative Institute for Meteorological Satellite Studies (CIMSS), University of Wisconsin – Madison, Madison, Wisconsin, USA Will additional input from Steve Weiss, NOAA/NWS/Storm Prediction Center ✓ Increasing the Utility / Value of real-time Satellite Sounder Products to fill gaps in their short-range forecasting processes Creating Temperature/Moisture Soundings from Infra-Red (IR) Satellite Observations A Conceptual Tutorial All level of the atmosphere is continually emit radiation toward space. Satellites observe the net amount reaching space. • Conceptually, we can think about the atmosphere being made up of many thin layers Start from the bottom and work up. 1 – The greatest amount of radiation is emitted from the earth’s surface 4 - Remember, Stefan’s Law: Emission ~ σTSfc 2 – Molecules of various gases in the lowest layer of the atmosphere absorb some of the radiation and then reemit it upward to space and back downward to the earth’s surface - Major absorbers are CO2 and H2O 4 - Emission again ~ σT , but TAtmosphere<TSfc - Amount of radiation decreases with altitude Creating Temperature/Moisture Soundings from Infra-Red (IR) Satellite Observations A Conceptual Tutorial All level of the atmosphere is continually emit radiation toward space. Satellites observe the net amount reaching space. • Conceptually, we can think about the atmosphere being made up of many thin layers Start from the bottom and work
    [Show full text]
  • Effect of Deep Convection on the TTL Composition Over the Southwest Indian Ocean During Austral Summer
    https://doi.org/10.5194/acp-2019-1072 Preprint. Discussion started: 22 January 2020 c Author(s) 2020. CC BY 4.0 License. Effect of deep convection on the TTL composition over the Southwest Indian Ocean during austral summer. Stephanie Evan1, Jerome Brioude1, Karen Rosenlof2, Sean. M. Davis2, Hölger Vömel3, Damien Héron1, Françoise Posny1, Jean-Marc Metzger4, Valentin Duflot1,4, Guillaume Payen4, Hélène Vérèmes1, 5 Philippe Keckhut5, and Jean-Pierre Cammas1,4 1LACy, Laboratoire de l’Atmosphère et des Cyclones, UMR8105 (CNRS, Université de La Réunion, Météo-France), Saint- Denis de la Réunion, 97490, France 2Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, 80305, CO, USA 3National Center for Atmospheric Research, Boulder, 80301, CO, USA 10 4Observatoire des Sciences de l’Univers de La Réunion, UMS3365 (CNRS, Université de La Réunion, Météo-France), Saint- Denis de la Réunion, 97490, France 5LATMOS, Laboratoire ATmosphères, Milieux, Observations Spatiales-IPSL UMR8190 (UVSQ Université Paris-Saclay, Sorbonne Université, CNRS), Guyancourt, 78280, France Correspondence to: Stephanie Evan ([email protected]) 15 Abstract. Balloon-borne measurements of CFH water vapor, ozone and temperature and water vapor lidar measurements from the Maïdo Observatory at Réunion Island in the Southwest Indian Ocean (SWIO) were used to study tropical cyclones' influence on TTL composition. The balloon launches were specifically planned using a Lagrangian model and METEOSAT 7 infrared images to sample the convective outflow from Tropical Storm (TS) Corentin on 25 January 2016 and Tropical Cyclone (TC) Enawo on 3 March 2017. 20 Comparing CFH profile to MLS monthly climatologies, water vapor anomalies were identified. Positive anomalies of water vapor and temperature, and negative anomalies of ozone between 12 and 15 km in altitude (247 to 121hPa) originated from convectively active regions of TS Corentin and TC Enawo, one day before the planned balloon launches, according to the Lagrangian trajectories.
    [Show full text]
  • ESSENTIALS of METEOROLOGY (7Th Ed.) GLOSSARY
    ESSENTIALS OF METEOROLOGY (7th ed.) GLOSSARY Chapter 1 Aerosols Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human (anthropogenic) sources, such as the burning of fossil fuels. Sulfur-containing fossil fuels, such as coal, produce sulfate aerosols. Air density The ratio of the mass of a substance to the volume occupied by it. Air density is usually expressed as g/cm3 or kg/m3. Also See Density. Air pressure The pressure exerted by the mass of air above a given point, usually expressed in millibars (mb), inches of (atmospheric mercury (Hg) or in hectopascals (hPa). pressure) Atmosphere The envelope of gases that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly nitrogen and oxygen. Carbon dioxide (CO2) A colorless, odorless gas whose concentration is about 0.039 percent (390 ppm) in a volume of air near sea level. It is a selective absorber of infrared radiation and, consequently, it is important in the earth's atmospheric greenhouse effect. Solid CO2 is called dry ice. Climate The accumulation of daily and seasonal weather events over a long period of time. Front The transition zone between two distinct air masses. Hurricane A tropical cyclone having winds in excess of 64 knots (74 mi/hr). Ionosphere An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist. Lapse rate The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.) Mesosphere The atmospheric layer between the stratosphere and the thermosphere.
    [Show full text]
  • Basic Features on a Skew-T Chart
    Skew-T Analysis and Stability Indices to Diagnose Severe Thunderstorm Potential Mteor 417 – Iowa State University – Week 6 Bill Gallus Basic features on a skew-T chart Moist adiabat isotherm Mixing ratio line isobar Dry adiabat Parameters that can be determined on a skew-T chart • Mixing ratio (w)– read from dew point curve • Saturation mixing ratio (ws) – read from Temp curve • Rel. Humidity = w/ws More parameters • Vapor pressure (e) – go from dew point up an isotherm to 622mb and read off the mixing ratio (but treat it as mb instead of g/kg) • Saturation vapor pressure (es)– same as above but start at temperature instead of dew point • Wet Bulb Temperature (Tw)– lift air to saturation (take temperature up dry adiabat and dew point up mixing ratio line until they meet). Then go down a moist adiabat to the starting level • Wet Bulb Potential Temperature (θw) – same as Wet Bulb Temperature but keep descending moist adiabat to 1000 mb More parameters • Potential Temperature (θ) – go down dry adiabat from temperature to 1000 mb • Equivalent Temperature (TE) – lift air to saturation and keep lifting to upper troposphere where dry adiabats and moist adiabats become parallel. Then descend a dry adiabat to the starting level. • Equivalent Potential Temperature (θE) – same as above but descend to 1000 mb. Meaning of some parameters • Wet bulb temperature is the temperature air would be cooled to if if water was evaporated into it. Can be useful for forecasting rain/snow changeover if air is dry when precipitation starts as rain. Can also give
    [Show full text]
  • DEPARTMENT of GEOSCIENCES Name______San Francisco State University May 7, 2013 Spring 2009
    DEPARTMENT OF GEOSCIENCES Name_____________ San Francisco State University May 7, 2013 Spring 2009 Monteverdi Metr 201 Quiz #4 100 pts. A. Definitions. (3 points each for a total of 15 points in this section). (a) Convective Condensation Level --The elevation at which a lofted surface parcel heated to its Convective Temperature will be saturated and above which will be warmer than the surrounding air at the same elevation. (b) Convective Temperature --The surface temperature that must be met or exceeded in order to convert an absolutely stable sounding to an absolutely unstable sounding (because of elimination, usually, of the elevated inversion characteristic of the Loaded Gun Sounding). (c) Lifted Index -- the difference in temperature (in C or K) between the surrounding air and the parcel ascent curve at 500 mb. (d) wave cyclone -- a cyclone in which a frontal system is centered in a wave-like configuration, normally with a cold front on the west and a warm front on the east. (e) conditionally unstable sounding (conceptual definition) –a sounding for which the parcel ascent curve shows an LFC not at the ground, implying that the sounding is unstable only on the condition that a surface parcel is force lofted to the LFC. B. Units. (2 pts each for a total of 8 pts) Provide the units used conventionally for the following: θ ____ Ko * o o Td ______C ___or F ___________ PGA -2 ( )z ______m s _______________** w ____ m s-1_____________*** *θ = Theta = Potential Temperature **PGA = Pressure Gradient Acceleration *** At Equilibrium Level of Severe Thunderstorms € 1 C. Sounding (3 pts each for a total of 27 points in this section).
    [Show full text]
  • NOAA Technical Memorandum NWS WR-221 UTILIZATION of the BULK RICHARDSON NUMBER, HELICITY and SOUNDING MODIFICATION in the ASSESS
    NOAA Technical Memorandum NWS WR-221 UTILIZATION OF THE BULK RICHARDSON NUMBER, HELICITY AND SOUNDING MODIFICATION IN THE ASSESSMENT OF THE SEVERE CONVECTIVE STORMS OF 3 AUGUST 1992 ~ \ Eric C. Evenson Weather Service Forecast Office Great Falls, Montana November 1993 u.s. DEPARTMENT OF I National Oceanic and National Weather COMMERCE Atmospheric Administration I Service 75 A Study of the Low Level Jet Stream of the San Joaqnin Valley. Ronald A Wlllis and NOAA TECHNICAL MEMORANDA Philip Williams, Jr., May 1972. (COM 72 10707) National Weather Service, Western Region Subseries 76 Monthly Climatological Charta of the Behavior of Fog and Low Stratus at Los Angeles International Airport. Donald M. Gales, July 1972. (COM 72 11140) The National Weather Service (NWS) Western Region (WR) Subseries provi~es an informal 77 A Study of Radar Echo Distribution in Arizona During July and August. John E. Hales, Jr.. medium for the documentation and quick dissemination of results not. appropnate, or not ~et July 1972. (COM 72 11136) ready, for formal publication. The series is used to report. o'! work '-? progreas, to des~ 78 Forecasting Precipitation at Bakersfield, California, Using Preasure Gradient Vectors. Earl technical procedures and practices, or. to relate progre_as ~ a limited. audience. These Techrucal T. Riddiough, July 1972. (COM 72 11146) Memoranda will report on investigations devoted primarily to regtonal and local problems of 79 Climate of Stockton, California. Robert C. Nelson, July 1972. (COM 72 10920) interest mainly to penionnel, and hence will not be widely distributed. 60 Estimation of Number of Days Above or Below Selected Temperatures. Clarence M.
    [Show full text]
  • The Effect of Stable Thermal Stratification on Turbulent Boundary
    The effect of stable thermal stratifcation on turbulent boundary layer statistics O.Williams, T. Hohman, T. Van Buren, E. Bou-Zeid and A. J. Smits May 22, 2019 Abstract The effects of stable thermal stratifcation on turbulent boundary layers are ex- perimentally investigated for smooth and rough walls. Turbulent stresses for weak to moderate stability are seen to scale with the wall-shear stress compensated for changes in fuid density in the same manner as done for compressible fows, sug- gesting little change in turbulent structure within this regime. At higher levels of stratifcation turbulence no longer scales with the wall shear stress and turbu- lent production by mean shear collapses, but without the preferential damping of near-wall motions observed in previous studies. We suggest that the weakly sta- ble and strongly stable (collapsed) regimes are delineated by the point where the turbulence no longer scales with the local wall shear stress, a signifcant departure from previous defnitions. The critical stratifcation separating these two regimes closely follows the linear stability analysis of Schlichting (1935) [Schlichting, Hauptaufs¨atze. Turbulenz bei W¨armeschichtung, ZAMM-J. of App. Math. and Mech., vol 15, 1935 ] for both smooth and rough surfaces, indicating that a good predictor of critical stratifcation is the gradient Richardson number evaluated at the wall. Wall-normal and shear stresses follow atmospheric trends in the local gradient Richardson number scaling of Sorbjan (2010) [Sorbjan, Gradient-based scales and similarity laws in the stable boundary layer, Q.J.R. Meteorological Soc., vol 136, 2010], suggesting that much can be learned about stratifed atmo- spheric fows from the study of laboratory scale boundary layers at relatively low Reynolds numbers.
    [Show full text]