Five maps are provided from a single case. Each map shows 1000-500mb thickness, plotted in 10's of meters (decameters) as dashed red lines. Also plotted is Sea Level Pressure (SLP) as solid black lines every 4mb. A map is provided every 12 hours. The maps can be found HERE
The 2nd lab of the semester, where you plotted isobars and fronts, was about 12 hours before the first map in the series for this lab.
On each map, draw the fronts. Remember that in general (but not in all cases), the fronts are drawn on the warm side of the packing of the thickness lines. Also note that the low which forms over the midwest and moves to the northeast goes through an occluding process during the period of the maps.
To assist with drawing the fronts, a full set of maps for this case can be found HERE.
On each map, also indicate the regions of maximum cold and warm air advection. Use the SLP contours to appriximate the wind direction and speed (eg, geostrophic wind), and remember that the thickness lines represent the mean virtual temperature for the lower half of the atmosphere. Note on the map (either by color or by some sort of legend) whether the advection is warm or cold. The advection is determined by the crossing of the two types of contour lines. Where the lines cross more perpindicular, the advection is increased. Where the isobars are closer together, the pressure gradient is higher. Where the thickness lines are closer together, the temperature gradient is higher. Greater temperature and/or pressure gradients can increase advection.
To combine these ideas -- the crossing of the contours and the spacing of the contours -- use the selenoid method described in Carlson on page 41 and 42. Although the discussion in the text is regarding PVA, the method used is applicable to graphically determining the advection of any quantity. A selenoid is formed by the crossing of two adjacent contours in both thickness and SLP, forming a curved parallelogram. Basically, the stronger the advection, the smaller will be the relative size of the selenoid. Strong advection, where the contours cross at right angles, will result in small rectangular or even square selenoids.
Three students are assigned to lead a map discussion, with different topics being assigned to each student. The motivation for the discussion is to explain the main factors entering into the National Weather Service forecast for our area for the weekend. It is appropriate to use any information available to motivate our discussion; it is appropriate for students to discuss the weather situation beforehand and coordinate their presentations, etc. All students are expected to participate in the discussion.
The forecast discussion from the local Sterling office can be particularly useful since it discusses what the NWS forecasters are seeing at the main issues with the forecast. This discussion can be found at the weather.gov web site as discussed in class.
Also potentially useful are the comments on the current weather that the lab instructor is posting HERE.
In general, the assignement is to provide a brief overview of the current situation and for the patterns over the last day or two. Then discuss the main map features for the coming weekend. It is also appropriate to discuss a particularly interesting weather situation from the prior week. A few minutes will be provided at the start of the lab for presenters to make final preparations.
The discusion is split up as follows:
Upper Air, Upper Levels: Discuss 500mb troughs and ridges and 300mb jets. If appropriate, discuss other variables at these levels, such as vorticity, vorticity advection (PVA/NVA) and relative humidity.
Upper Air, Lower Levels: Discuss the 850mb and 700mb charts. Discuss 850mb fronts, moisture, temperature patterns, and temperature advection. Discuss 700mb vertical velocity. If appropriate to the current weather situation, discuss 700mb shortwaves (when they provide more detail than the 500mb level), low level jets (850mb), and precipitable water. Also discuss atmospheric instability (CAPE, Lifited Index, etc) if appropriate.
Surface: Discuss the sea level pressure field: highs, lows, pressure troughs. Discuss temperatures, dewpoints, and the wind field. Discuss observed precipitation and radar. Discuss the satellite images. Indicate the presence and movement of major fronts. Connect discussion of clouds and precipitation to the prior discussions of the upper levels. Discuss the 1000-500mb thickness (typically plotted with SLP) and any major temperature advection patterns. If appropriate, discuss the soundings (SkewT) for the local stations (SkewT soundings can be found at the Storm Prediction Center web site under the "Forecast Tools" subheading under the "Research" subtopic, on the left-hand navigation bar found on the main page. SkewT charts will also be provided on the wx.gmu.edu web site in the near future.)
On the 500mb chart, examine the vorticity field in the region around the trough of interest. Shade those areas where there is positive vorticity advection (PVA). Vorticity advection is indicated when the lines of equal vorticity cross the height contours at an angle. When the advection of vorticity by the geostrophic wind would result in vorticity values increasing at a point, that indicates PVA. When the advection of vorticity would result in lower values, that is NVA (negative vorticity advection). In this case, it is appropriate to discount very small features in the vorticity field. Another way to think of this is as follows: consider the vorticity pattern. Move that pattern a small amount in the direction of the flow. If this results in the vorticity increasing at a fixed point, that point is experiencing PVA.
On the 300mb chart, draw a thick arrow to mark the jet stream in the area of the trough of interest. This arrow should follow the axis of maximum wind. It should start at the entrance region of the jet, and end at the exit region of the yet. (The entrance region is where wind speeds increase rapidly; the exit region is where wind speeds fall rapidly.) Mark the entrance region with an "N" and the exit region with an "X". For jets that curve cyclonicly, the area to the immediate left of the exit region is to be considered an area of divergence. This is known as the left front quadrant of the jet -- the jet LFQ. Indicate by shading or drawing a circle the area most likely to be divergent by the jet LFQ principle. Note that divergence at the 300mb level would be an indication of upward motion, convergence at the surface, and surface low pressure.
Finally, on the 500mb chart, indicate the region where the PVA and the jet LFQ overlap, either by shading or drawing a boundary. This would be the region most like to represent the maximum upper level divergence, and thus the most intense upward motion, and the largest drops in pressure at the surface.
PDFs of the maps can be found HERE
Previous Lab Assignments can be found HERE
The 500mb Chart: This is perhaps the most important weather map of them all. The 500mb level lies at about the halfway point in the atmosphere -- half the atmosphere is below, and half is above. The 500mb level varies from about 4800 geopotential meters (in polar regions) to about 6000 geopotential meters (in tropical regions). This is about 18,000 feet above mean sea level. For most places on the earth, this is comfortably above the planetary boundary layer.
The first thing to look for on the 500mb chart is the troughs and ridges. There are various ways to identify the troughts and ridges. The isoheight analysis will show the major troughs and ridges. Look for the maximum cyclonic curvature of the isoheight lines to identify troughs. Look for the maximum anti-cyclonic curvature of the lines to identify ridges. Note that ridges are sometimes not as well defined as troughs.
Also use the wind field to identify troughs and ridges. Again, look for cyclonic curvature of the winds to identify troughs, and anti-cyclonic curvature to identify ridges. The wind field can help to locate a trough or ridge when the placement is in doubt by just looking at the height field. The winds are also important for identifying small short-wave troughs. These are troughs that are smaller in scale, perhaps only 100 miles in wavelength. Because there is so little upper air data, these troughs can be hard to identify. But when they are imbedded in the jet stream (see below) they can be very important. These small features can be easier to identify at the 700mb level. So use both the 700 and 500mb charts to look for short-wave troughs. Also look at the actual observed winds at the ballon sounding stations. Sometimes these will show the turning in the wind more clearly than the computer analysis.
By old tradition, the troughs are labeled with a dashed red line, and the ridges by a zig-zag blue line. More recent practice is to label the troughs by a solid thick black line, and the ridges by a solid zig-zag black line.
Another important field to look at on the 500mb chart is the wind speed. Lines of equal wind speed are called Isotachs. If you look at an analysis of wind speed at the 500mb level, you will see that areas of higher winds tend to form bands extending from west to east. The main band of higher wind speeds is the jet stream. When there is more than one main band of higher wind, this is called split flow. Elongated areas of wind maximums, as identified by the isotach analysis, are called jets and are often labeled with a thick arrow than runs through the maximum wind area. By old tradition, the jet maxima are drawn in blue at the 500mb level. Localized intense maxima in the jets are sometimes called jet streaks.
The maximum wind speed in the jet stream is normally above the 500mb level, and is found by examining the 200mb or 300mb chart. But the main polar jets begin to be visible on the 500mb chart and it is important to identify them, particularly the way they are interactiing with troughs and ridges.
Another important variable to examine at the 500mb is something called vorticity. Vorticity is a measure of spin in the atmosphere. In the field of fluid mechanics, the localized spin of a fluid is called the curl. Meteorologists look at the component of the curl that is expressed as horizontal rotation, and they call it vorticity. This will be discussed in the lecture in the near future.
The term relative vorticity is used to refer to the rotation of the air relative to the moving earth's surface. The term absolute vorticity is used when the calculation of vorticity includes the earth's rotation. Vorticity is difficult to calculate by hand, so we look at computer analyses of this field. Vorticity units are radians per second. The values are usually multiplied by 10 to the 5th power, ie the values are plotted as radians per 100,000 seconds.
The Air Force Training Manual for weather analysis provides a lot of useful information on weather map analysis.
A complete set of GFS analysis and forecast maps for the late September heavy rain event can be found at this LINK
Analyzing fronts requires looking at every meteorological variable.
Pressure: Pressure usually drops ahead of fronts and rises after the front passes. With a strong cold front, this can be dramatic. In some cases, the pressure will rise slowly ahead of a front, but more rapidly after the front passes. If the isobars are drawn on the map, the front will usually be found in a "trough" of lower pressure. It may be appropriate to show the isobar contour lines "kinked" at the front.
Temperature: Fronts usually seperate air masses of different temperature.
Moisture: Fronts usually seperate air masses having different amounts of moisture. When the moisture is the only significant change across a front, it is sometimes called a "dryline".
Winds: The winds typically shift speed and/or direction when a front passes. On the surface map, winds will usually be different in direction on one side of a front compared to the other.
Pressure is defined as force/area. Think of cutting something with a knife. If you attempt to cut with the flat side of the blade, it will not cut. If you apply the same force on the knife, using the sharp side of the blade, it cuts easily. This is because the area of the sharp edge is much less, and thus the pressure along that edge is much higher.
The official unit of pressure is the pascal. This is one Newton per square Meter. (To put the Newton in perspective, one Newton is about the force of Earth's gravity on an object with a mass of about 102 grams. Such as, perhaps, a small apple.)
It used to be there was no official SI pressure unit in the metric system, and meteorologists adopted the Bar. This is also a metric unit of pressure, where 1 Bar equals 100,000 pascals. It turns out that one Bar is close to the typical pressure of the atmosphere near sea level.
The millibar (1000th of a bar), abbreviated mb, became the most commonly used unit of pressure in meteorology. In these units, the standard atmosphere is 1013.25mb. We still see mb used very widely in meteorology, especially in the US. However, the use of the hectoPascal, abbreviated hPa, is growing. hPa is exactly the same as the mb.
Inches of mercury is older usage, abbreviated inHg, and is the pressure required to lift a column of mercury the specified number of inches when under the influence of earth's gravity. 1 inHg equals 3,386.389 pascals at 0C and at standard gravity. The standard atmosphere, 1013.25hPa, is 29.92inHg.
In Clim301 Lab, we will be using primarily mb and/or hPa.
In meteorology, the 24 hour clock is used. The time zone normally used is Universal Time, or UTC. This is often labeled as "Z" time.
Noon in Universal Time is approximately the time that the sun is directly overhead at the zero'th meridian, which is anchored at Greenwich, England. Local time in the US is earlier than UTC.
By convention, midnight UTC -- 00Z -- is the start of the new day, not the end of the old day. Thus, 00Z Thursday is the borderline between Wednesday and Thursday.
Eastern Daylight Time (EDT) is four hours earlier than UTC.
Eastern Standard Time (EST) is five hours earlier than UTC.
Examples:
00Z Thursday is 8pm Wednesday EDT
00Z Thursday is 7pm Wednesday EST
12:00UTC Friday is 7am Friday EST
18:32Z Sunday is 2:32pm Sunday EDT
Note that Z and UTC are used interchangeably.
Sometimes seen on older charts is the use of GMT, or Greenwich Mean Time. It is the same as Z or UTC.
