From: Subject: Tuesday Weather Topic in Greater Depth Date: Thu, 5 Nov 2009 10:54:42 -0800 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 3 = November 2009=20

SPECIFICS OF CHARACTERISTIC SPACE & TIME = SCALES

The accompanying description of the characteristic space and time = scales,=20 arranged from the largest space scale to the smallest, follows = traditional=20 nomenclature. Ultimately, the types and spacing of weather instruments = must be=20 chosen for the scale of motion under study.

PLANETARY SCALE -

This space scale is the largest scale that meteorologists consider, = with=20 features having a diameter that would be at least the size of a = continent, and=20 often would encompass a hemisphere. The planetary scale involves the = general=20 atmospheric circulation features that are somewhat permanent. We can = detect some=20 aspects of the planetary scale circulation regime on the individual = daily=20 weather maps, using the current observation networks. Inspection of a = sequence=20 of weather maps over several days shows those features that remain over = an area.=20 On many of these maps, a large semi-permanent subtropical high pressure = system=20 can be detected, such as the one that resides over the Atlantic Ocean = termed the=20 Bermuda high and another over the Pacific Ocean called the Hawaiian or = Pacific=20 High. Another example of a planetary scale feature is the mid latitude=20 prevailing westerlies, exemplified by the polar front jet streams that = are found=20 on the upper tropospheric constant pressure charts (such as the current = 300 mb=20 charts on the DataStreme Atmosphere website). The seasonal = monsoon=20 circulations of both the Indian sub-continent and the Southwestern = United States=20 are examples of features approaching the size of the planetary scale. =

Atmospheric phenomena that have continental sized dimensions = typically are=20 the ones that persist for the longest time interval. The typical time = scale for=20 such phenomena ranges from several weeks to millennia; this scale = captures not=20 only the seasonal cycle but other long term phenomena, such as the El = Ni=F1o and=20 Southern Oscillation. The subtropical high pressure systems, the trade = winds,=20 the prevailing westerlies and the polar front jets are permanent = planetary scale=20 features that can be found not only as distinct features on annual = average=20 charts, but on most weather maps of the given region. The intensity and = the=20 exact geographic location of these systems may change seasonally. The = monsoon=20 circulations over the Indian sub-continent and the South western United = States=20 are examples of planetary scale circulation regimes that undergo = distinct=20 seasonal reversals in the wind flow. Some would argue that the shifts = between an=20 ice age type circulation regime and one more typical of the present = interglacial=20 epoch could represent an atmospheric phenomenon with a very long time = interval.=20

MACRO- or SYNOPTIC SCALE -

Weather features in this scale subdivision are the high-pressure and=20 low-pressure systems that appear on the daily surface weather maps. The = term=20 "macro", meaning large scale has some acceptance, however "synoptic" = scale has=20 been used quite often. The word "synoptic" (from the Greek for "seen = together",=20 or a broad overview) has a varied meaning in meteorology. On the one = hand,=20 synoptic refers to a scheme of data collection and weather analysis, but = it also=20 refers to a size scale that is on the order of 1000 to 2000 km. In other = words,=20 the scale has dimensions of several states or provinces, which is larger = than=20 the countywide thunderstorms, but smaller than the continental sized = monsoon=20 circulation regimes. Macro- (synoptic) scale weather systems can be = detected=20 using traditional surface and upper air weather observation networks, = such as=20 those maintained by the National Weather Service.

These macroscale features typically have life spans that range from a = day to=20 at least one week. On rare occasion, a tropical cyclone (if sufficiently = intense, a hurricane) may persist for over two weeks. On such a = macroscale time=20 scale, the traditional hourly sampling of surface weather conditions = together=20 with the twice daily observations of upper air conditions detect most of = the=20 synoptic scale features and allow the meteorologist to track such = features.

MESOSCALE -

This "medium" scale ("meso" is Greek for middle) contains features = ranging=20 from approximately 1 km to 100 km, or the space scale that would cover = several=20 counties. Weather systems, such as large thunderstorm cells -- some = times called=20 "Mesoscale Convective Complexes", are examples of the "organized = turbulence" of=20 mesoscale systems. The localized lake (or sea) breeze phenomenon that = develops=20 on a warm summer afternoon along the shoreline is another mesoscale = example.=20 Until recently, observations and analysis of mesoscale phenomena have = been poor.=20 However, increased research and operational efforts have meant that = mesoscale=20 weather systems are now being detected by various new, sophisticated = systems=20 such as WSR-88D (Doppler) radar and wind profilers. Some states, such as = Oklahoma, have mesoscale observing networks, with at least one automatic = weather=20 station per county.

Weather features that are classified as mesoscale systems have = typical time=20 scales that range from an hour to nearly a day. Ordinary thunderstorm = cells may=20 undergo a complete life cycle on the order of an hour, while some of the = clusters of thunderstorms that form "Mesoscale Convective Complexes" may = persist=20 for anywhere from 12 to 18 hours. Other mesoscale phenomena include the = lake (or=20 sea) - land breeze circulation regime that undergoes a diurnal reversal = in the=20 wind flow from an onshore breeze during daylight hours to an offshore = breeze=20 during the evening. The traditional hourly surface observations and = radar=20 summaries will detect many mesoscale phenomena. Additional intermediate = updates=20 are often needed to obtain a more complete analysis. Newer automatic = weather=20 stations and other sophisticated instruments, such as the WSR-88D radar = units=20 will detect short lived mesoscale phenomena.

MICROSCALE -

The term "micro" is from the Greek word for small and refers to a = scale=20 ranging from centimeters to a kilometer. One end of this scale includes = the wind=20 swirls that may develop downwind of a building, while a large tornado = may be=20 near the upper end of the scale. On this scale, motion is dominated by=20 turbulence, which is often very chaotic. Special networks, such as those = at an=20 agricultural experimental station , are needed to observe microscale = phenomena.=20

Weather features that are classified as microscale atmospheric = phenomena are=20 very short-lived phenomena. Small little gusts of wind that lift litter = off the=20 ground may persist for less than several seconds. On the other hand, a = typical=20 tornado may persist for as long as an hour. Rapid and essentially = continuous=20 measurements are needed to capture many microscale phenomena. Special=20 instrumentation usually is required.

Return to the Tuesday = Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =