The vertical measurements, or
soundings, are taken when a weather balloons released with a rawinsonde—a light-weight instrument
equipped with a radio transmitter that sends measured data back to a receiver
on the earth. Whereas a radiosonde, often used synonymously, consists of the
measuring devices rather than the rawindsonde, which refers to the addition of
radio tracking capabilities to determine wind speeds and directions as well. Rawinsonde
observations, or raobs, are taken twice daily at 00:00 and 12:00 UTC (Universal
Time Clock) or equivalently, GMT (Greenwich Mean Time), 365 days per year. (For
an excellent description of rawinsondes, see http://www.aos.wisc.edu/~hopkins/wx-inst/wxi-raob.htm).
Thermodynamic
diagrams allow for analysis of temperature, moisture, pressure and
wind in the atmosphere. By plotting the soundings of temperature and dew point,
one can investigate how adiabatic processes determine instability and may be
used to help predict severe weather—certain profiles indicate certain weather
to be expected. The psuedo-adiabatic diagram at first looks very confusing. However,
once we take apart the diagram and look at each line individually, we will
determine the purpose of each line. When you get past the part of searching for
the right line, the information is easily attainable.
Find Temperature
(T)—lifted
dry adiabatically—by starting at the initial point (given temperature and
pressure) and travel upwards parallel to the nearest dry
adiabat. When lifting the parcel wet
adiabatically it is important to stay equidistant between two wet
adiabats since the wet adiabats diverge with decreasing pressure. Do not go
parallel to just one!
Find
Mixing Ratio (w) by locating
the value of the constant mixing ratio, w, line at the given pressure and
dew point. Interpolate—approximate between the known values by the
fractional distance between each one—if necessary.
Find
Saturation Mixing Ratio (ws) by locating the value of the constant mixing ratio, w,
line at the given pressure and temperature.
Interpolate—approximate between the known values by the fractional distance
between each one—if necessary.
Thickness
of a Layer (ΔZ) is the vertical distance between two levels of
constant pressure. In usage, it is the vertical distance between to isobaric
surfaces. Since warm air is less dense than cold air (at the same atmospheric
pressure), to travel through a layer of air that is warmer it will require a
greater vertical distance to drop a given amount of pressure.
Find Lifting Condensation Level (LCL)—the level
at which air, dynamically lifted, reaches saturation—by locating the
intersection of the constant mixing ratio, w,
line through the dry adiabat line.
Find
Equivalent Potential Temperature (θe) by lifting
a parcel dry adiabatically until it reaches
the lifting condensation level (LCL) then lift it wet
adiabatically until all the vapor is condensed out. The wet adiabat will be parallel to the dry adiabat, since all vapor is removed causing no
latent heat to be released. When this occurs, follow a dry
adiabat back to 1000 mb.
Find Convection Condensation Level (CCL)—the
height at which a parcel of air, if heated sufficiently from below, will rise
adiabatically until condensation begins—by locating the intersection of the
constant mixing ratio (w) line through the
surface dew-point temperature, TD (with the observed temperature
sounding—as measured by a radiosonde). In the most common case this is the
height of the base of cumulus clouds (which are produced by thermally-induced
turbulent eddies—i.e. convection solely from surface heating).
Find Convective
Temperature (Tc)—the surface temperature that must be
reached to start the formation of convective clouds by solar heating of the
surface layer—by locating the convection condensation level (CCL) and following
it dry adiabat down to the surface pressure
isobar.
Find
the Level of Free Convection (LFC)—the level at which a lifted
parcel of air becomes unstable (when the temperature of the parcel becomes
warmer than the environmental temperature; Tparcel
> Tenvironment)—by
lifting the parcel dry adiabatically until
you reach the LCL (lifting condensation level) then lifting it wet adiabatically thereafter. The LFC is the
beginning of CAPE (convective available potential energy) / PBE (positive
buoyant energy).
Find Equilibrium
Level (EL)—the point
of intersection where the temperature of the parcel becomes colder than the
environmental temperature (Tparcel
< Tenvironment = stable
air). The EL indicates the end of CAPE (convective available potential energy).
Find
Equivalent Potential Temperature (θe)—from
the LCL, follow a saturation adiabat up to a
pressure where the saturation adiabat
parallels the dry adiabat. Follow the dry adiabat down to 1000 hPa, the temperature at this
level is the equivalent potential temperature.
Find
Equivalent Temperature (Te)— from the LCL, follow a
saturation adiabat up to a pressure where the saturation
adiabat parallels the dry adiabat. Follow
the dry adiabat down to 1000 hPa, then follow a dry adiabat back up to the original pressure. The
isotherm at this point is the equivalent temperature.
Find
Wet-Bulb Temperature (Tw)—from the LCL, proceed down a saturation adiabat to the original pressure level.
The isotherm at this point is the wet-bulb temperature.
Find
Wet-Bulb Potential Temperature (θw)—at a
given pressure level, find the LCL (for that level), then proceed down a saturation adiabat to 1000 hPa. The temperature at
this point is the wet-bulb potential temperature.
Convective
Available Potential Energy (CAPE)—the region where the air will
experience positive buoyant energy (PBE),
which indicates instability.
Convective
Inhibition (CIN)—the region where the air experiences negative buoyant energy (NBE), which
indicates stability (resisting vertical movements).
Parcel
Stops when NBE (above the EL) is equal to the PBE (between the
LFC and EL).
