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Common Terminology Used in Synoptic Meteorology

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Common Terminology Used in Synoptic Meteorology Common terminology used in Synoptic Meteorology SYNOPTIC: From Greek words meaning “seeing together”. In meteorology, it often refers to a scale of 1000 km in distance and several days in time, that is, the scale of extratropical cyclones and anticyclones. This is in contrast to the mesoscale, which would include thunderstorm complexes with a scale of ~100 km and a few hours, or the microscale (also called storm-scale or cloud-scale), which would include individual thunderstorms, or a tornado, with a scale of a few kilometers and a few minutes. Synoptic meteorology also has come to mean the branch of meteorology that has to do with weather forecasting. One of the first steps in forecasting the weather is to analyze it. To perform meteorological analysis, we must have a good understanding of data and measurements, conceptual models of the atmosphere, and numerical representations of the atmosphere. To have this understanding, we should become familiar with the terminology of synoptic meteorology. TIME CONVENTIONS: Because the weather doesn’t care what the local time zone is, and since synoptic-scale weather systems extend across multiple time zones, all times in synoptic meteorology are given in Coordinated Universal Time (UTC). To find out what the current time in UTC is, you can tune your shortwave radio to 10 MHz, or you can use the following procedure: 1. Convert the current local time to a 24-hr based time (so, 5 AM is 0500, but 5 PM is 1700). 2. Assuming we are in the Eastern Time Zone, a. If Daylight Saving Time (DST) is not in effect, add 5 hours (500) to the current local time. b. If DST is in effect, add 4 hours to the current local time. 3. If the result is >= 2400, subtract 2400. Example. 2:30 PM EST corresponds to 1930 UTC. UTC is sometimes referred to as Greenwich Mean Time (GMT) or Z time. Using the military alphabet, A = Alpha, B = Bravo, …, Z = Zulu, so Z time is sometimes referred to as “Zulu” time. Thus, 12Z = 12 “Zulu” = 1200 GMT = 1200 UTC = 7 AM EST. LONGWAVES AND SHORTWAVES: Atmospheric flow consists of a series of waves around the globe. Waves that are rather broad (so that only 3–7 fit around the globe) are called longwaves, and these tend to move slowly and influence the general weather trends. Embedded within these long waves are much smaller-scale waves called shortwaves. These systems move quickly and cause stronger vertical motions, and therefore have a more important effect on what we think of as “weather”. CYCLONE: A weather system with lower pressure than the surrounding areas. In the Northern Hemisphere, winds circulate counterclockwise around a cyclone. ANTICYCLONE: A weather system with higher pressure than the surrounding areas. In the Northern Hemisphere, winds circulate clockwise around an anticyclone. TROUGH: A region of relatively low pressure (or relatively low heights if the vertical coordinate is isobaric) that usually does not have closed contours around it. Most low-pressure systems at the surface are associated with troughs aloft. RIDGE: A region of relatively high pressure (or relatively high heights if the vertical coordinate is isobaric) that usually does not have closed contours around it. Most high-pressure systems at the surface are associated with ridges aloft. COL: The hyperbolic point between a pair of lows or a pair of highs. Also called a saddle point. SHORT-RANGE: A term applied to forecasts valid over the next 2–3 days. LONG (or MEDIUM or EXTENDED) RANGE: A term applied to forecasts valid at least 3 days in the future. ADVECTION: The process that results in a local change of a quantity due to the transport of differing values into the region. ISOPLETH: A line of constant something. Specific examples include: • Isotherm Temperature • Isentrope Potential Temperature • Isodrosotherm Dewpoint • Isobar Pressure • Isotach Wind Speed • Isogon Wind Direction • Isallobar Pressure Tendency • Isohypse Geopotential Height (also called a height contour) BOUNDARY LAYER: That portion of the atmosphere, typically a kilometer thick, where friction plays an important role. GEOSTROPHIC FLOW: The condition, typically observed above the boundary layer, where the wind flows parallel to isobars or isohypses. ZONAL FLOW: An airflow characterized by winds predominantly oriented in the east-west direction. MERIDIONAL FLOW: An airflow characterized by winds predominantly oriented in the north-south direction. DISTURBANCE: A departure of the atmospheric flow from the long-term average. Since the long-term average is usually zonal, disturbances are typically characterized by significant meridional flow. BLOCKING: A highly meridional, long-lived pattern that is nearly stationary. The pattern serves to “block” the zonal flow that you’d typically see. SHEAR: Any change in the wind in some direction. Vertical wind shear is the change in the wind with height, whereas horizontal wind shear is the change in the wind in the horizontal direction. VORTICITY: A kinematic property of the wind field describing how the flow is rotating. Vorticity can be diagnosed by placing an imaginary paddlewheel into the flow and “seeing” how it would rotate. DIVERGENCE: A kinematic property of the wind field describing how an object in the flow would be changing size. The divergence is the combination of the effects of diffluence (“spreading-outness”) and stretching. Standard Upper-Air Maps By convention, upper-air maps are prepared for the following levels: 1) 200 mb, 250 mb, and/or 300 mb These pressure surfaces are near the jet stream, and thus near the tropopause. This makes them ideal to study how the jet stream is currently behaving. During the winter, the jet stream is typically near 300 mb (although the subtropical jet stream may be more apparent at 200 mb). During the summer, 200 mb best captures the jet stream(s). As a year-round compromise, 250 mb is sometimes used. 2) 500 mb The 500-mb map depicts shortwaves and longwaves (higher levels like 300 mb often show the longwaves only), as well as blocking patterns. Vorticity and vorticity advection are typically diagnosed at this level. 3) 850 mb The 850-mb map depicts near-surface conditions without the complications of friction. Fronts and near-surface cyclones and anticyclones can be seen at this level. Temperature and temperature advection are often key variables at this level. .
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