Constant Pressure Maps ATMO 4300 Spring 2008

Two Related Variables Height and Pressure On a surface weather map, height is constant and pressure varies What height is used and why?

Surface Weather Map Figure from www.rap.ucar.edu/weather/model Slide11.mp3 The surface weather map, as far as depicting horizontal pressure gradients is concerned, is really a constant level map since all the station pressures have been converted to sea level. To represent horizontal pressure gradients in the upper atmosphere we could use constant level maps such as a 5 km map or an 8 km map. However that is not what is done.

Question Where does the information come from that is plotted on a surface weather map? Mostly from ASOS (Automated Surface Observation System)

ASOS

Upper Air Maps Also called constant pressure maps On these maps, pressure is constant and height varies

500 mb Map Slide34.mp3 This is an example of a 500 mb constant pressure map. The solid dark lines connect equal height values and are called contours. They depict the variation in height of the 500 mb pressure surface. Everywhere on that surface the pressure is 500 mb. The idea of a constant pressure chart may be somewhat confusing, but we’ll return to this in the lesson on Air in Motion and discuss constant pressure charts in greater detail.

Question Where does the information come from that is plotted on a constant pressure map?

Upper Air Observations Radiosonde Measures temperature, pressure, humidity Rawinsonde - wind Figure from www.erh.noaa.gov/er/gyx Slide30.mp3 Therefore it is important to observe temperature, wind and other weather variables above the surface all the way up through the troposphere and into the stratosphere. These upper air observations are made using an instrument package called a radiosonde. It contains instruments to measure temperature, pressure, and humidity. Sometimes this device is called a rawinsonde because it also allows us to obtain wind speed and direction.

Launching a Radiosonde Photo from www. srh. noaa. gov/mob/balloon Launching a Radiosonde Photo from www.srh.noaa.gov/mob/balloon.shtml; map from www.srh.noaa.gov/bmx/upperair/radiosnd.html Slide31.mp3 The radiosonde is a relatively small instrument package that is attached to a balloon and launched twice a day at selected locations that you see on the map at the right. As the balloon carries the radiosonde up through the atmosphere the radiosonde measures temperature, pressure, humidity and wind at many levels and radios that information back to the ground station where the data are collected. The radiosonde usually ascends to an altitude of about 30 km or close to 19 miles. Once the balloon bursts, a parachute opens that allows the radiosonde to descend back to earth.

Depicting Upper Air Data 1. Constant Pressure Maps 2. Temperature/height diagram: Vertical profile of temperature, dew point and wind Slide39.mp3 A second way of depicting observations of the upper atmosphere is by plotting data from a single radiosonde site on a temperature-height diagram, also known as a thermodynamic diagram. This method shows how temperature, dew point and wind change in the vertical.

Thermodynamic Diagram Slide40.mp3 This is an example of a thermodynamic diagram. We can use pressure values as shown on the left side of the diagram to represent height since pressure decrease with height and pressure is what is measured by the radiosonde. The thicker red line shows how air temperature changes with height as measured by the radiosonde. The green line shows how dew point temperature changes with height. Over on the right side of the diagram you can see the wind direction and speed at various altitudes. Weather data plotted in this fashion is called a sounding.

Depicting Upper Air Data 1. Constant Pressure Maps Slide33.mp3 There are basically two ways of depicting observations of the upper atmosphere. One is to plot observations made by radiosondes, satellites and aircraft on a map in the form of a station model. This is similar to the way surface observations are plotted on a map. Maps on which upper air observations are plotted and then analyzed are called constant pressure maps.

Standard Upper Air Maps Pressure Height (ft) Height (m) 200 mb 39,000 ft 12,000 300 mb 30,000 ft 9000 500 mb 18,000 ft 5500 700 mb 10,000 ft 3000 850 mb 5,000 ft 1500 Slide37.mp3 The standard pressure values used to label upper air charts are 850, 700, 500, 300 and 200 mb. The approximate altitude above sea level in both feet and meters at which these pressure values are found in the atmosphere are shown here. By plotting the height of one of these pressure values at all the locations that launch a radiosonde we have a constant pressure map.

Constant Pressure Maps Pressure decreases with height Rate of decrease varies Height of a given pressure varies 500 500 Slide36.mp3 Let’s use a pressure of 500 mb for this example. The pressure at each little horizontal line is 500 mb. You can see that 500 mb occurs at a different height above each location and it’s that height that is plotted on a constant pressure chart. So on a constant pressure map we keep the pressure the same and plot the different heights of where that pressure is measured above each location. If you can imagine connecting all the points in the atmosphere where the pressure is the same, it would define an imaginary constant pressure surface. 500 up 500

500 mb Map Slide38.mp3 This is an example of a 500 mb constant pressure map. The solid dark lines connect equal height values and are called contours. They depict the variation in height of the 500 mb pressure surface. Everywhere on that surface the pressure is 500 mb. The idea of a constant pressure chart may be somewhat confusing, but we’ll return to this in the lesson on Air in Motion and discuss constant pressure charts in greater detail.

Question Why does the height of a constant pressure surface vary from place to place?

Standard Atmosphere Figure from ww2010.atmos.uiuc.edu Slide13.mp3 To explain constant pressure maps let’s look at the influence of temperature on pressure using these figures. On the left are two columns of air, A and B, both having the same average temperature. You could think of column A as being above Amarillo and column B being above Lubbock or use any 2 locations you prefer.

Non Standard Temperatures Figure from ww2010.atmos.uiuc.edu Slide13.mp3 To explain constant pressure maps let’s look at the influence of temperature on pressure using these figures. On the left are two columns of air, A and B, both having the same average temperature. You could think of column A as being above Amarillo and column B being above Lubbock or use any 2 locations you prefer.

Temperature & Density Which column has the greater density? Pressure decreases at a greater rate with height in the cold air Figure from apollo.lsc.vsc.edu/classes/met130 Slide14.mp3 Consider two columns of air. The average temperature in the column of air on the left is colder than in the column of air on the right. If the same amount of air occupies both columns, in other words both columns of air contain the same mass, which has the greater density?

Construction of a 500 mb Map upper left map from www. srh. noaa Construction of a 500 mb Map upper left map from www.srh.noaa.gov/bmx/upperair/radiosnd.html 1 3 2 500 4 Slide21.mp3 Let’s see how a constant map would be constructed. We’ll choose a 500 mb map. Twice a day at locations over the U.S. (as seen in fig. 1) a radiosonde is launched that measures pressure, temperature, humidity and wind at various altitudes. At all these locations, the altitude at which the radiosonde measures a pressure of 500 mb is determined (fig 2). That altitude can vary from place to place because of horizontal differences in temperature. 500 500 500

Use of Const. Press. Maps 850 mb map - Low level winds – Low Level Jet (LLJ) - Temperature advection – vertical motion - Moisture advection - Frontal zones

Use of Const. Press. Maps 700 mb map - Temperature advection – vertical motion - Winds - Relative Humidity T-Td = 5 or less: possible clouds/ upward motion - Quick look at possible cap: T > 12-14C

Use of Const. Press. Maps 500 mb map - Identify Long-waves & Short-waves - Vorticity – related to vertical motion - Wind flow patterns (see following slides)

Upper-Level Flow Zonal Flow – dominant W to E flow Meridional Flow – large north/south component to the flow

Meridional Flow Image from www.rap.ucar.edu/weather

Split Flow Diagram from www.theweathrprediction.com/blocking Note the northern branch and southern branch of the westerlies

Cut-Off Low Diagram from www.theweathrprediction.com/blocking

Omega Block Diagram from www.theweathrprediction.com/blocking

Rex Block Diagram from www.theweatherprediction.com/blocking

Use of Const. Press. Maps 300/250/200 mb maps - Locate Polar & Subtropical Jet Streams - Locate Jet Streaks (Jet Max)

Polar Jet