Eugene
W. McCaul, Jr., Charles Cohen and Donald J. Perkey
USRA, Huntsville, Alabama
Severe convective storms are among the most dramatic manifestations of
Earth's weather, and have attracted the attention of numerous research
meteorologists - and the general public - for a long time. The complexity and
variety of these storms have, however, posed major challenges to those trying
to understand and forecast them better. For instance, it is commonly observed
that one synoptic-scale frontal cyclone will produce a concentrated outbreak
of severe weather and tornadoes, while another, equally intense, cyclone
might spawn storms that produce only heavy rainfall and scattered wind damage.
There continues to be great difficulty in distinguishing which weather systems
will produce which mix of the various types of severe convective storms.
Indeed, achieving detailed and reliable forecasts of the convective storm
component of every large weather disturbance is one of the Holy Grails of
modern weather forecasting.
Part of the difficulty lies in the incompleteness and imperfection of
weather observations. Even with the magnificent technological advances
that have led to satellite and Doppler radar monitoring of weather systems,
coverage is still often lacking in certain details crucial to the convective
storm problem. Yet, perhaps an even larger difficulty lies in the inherent
complexity of atmospheric convective motions, which can range in size and
intensity from small cumulus clouds up through giant systems of cumulonimbus
clouds organized into linear systems thousands of miles long. It is believed
that the diversity of convective storm structures must ultimately derive from
the sensitivities of convective storm dynamics to variations in either the
vertical or horizontal structure of the local atmosphere, or both.
In the COMPASS project, we have devised a relatively simple way to classify
the relevant vertical structure parameters of an idealized atmosphere, to
construct distinct atmospheric profiles for different combinations of values
of these parameters, and to build and conduct a parameter space numerical
simulation study using a state-of-the-art cloud model to explore how convective
storms look and behave in different parts of the parameter space. The number
of key parameters in this study turns out to be eight. They are:
-
Convective available potential energy (CAPE); this is a measure of bulk
energy that clouds can turn into vertical kinetic energy; in this project,
we use the standard definition of CAPE based on assumed pseudoadiabatic
ascent of air parcels after they become saturated; thus we ignore the
complexities of the latent of heat of fusion, which becomes a subject of
study in itself within this project;
-
Hodograph size parameter, here realized as a hodograph radius; this is
a measure of the bulk magnitude of horizontal wind and its vertical shear
in the storm environment;
-
Altitude of maximum parcel buoyancy; this is implemented here using a
buoyancy profile shape parameter acting on idealized analytical profiles
of parcel buoyancy;
-
Shape of the vertical shear profile; this is implemented here using a
wind profile shape parameter analogous to that for buoyancy; by varying
this parameter, we can generate significant additional vertical shear at
low levels in the storm environment;
-
Depth of the subcloud mixed layer; this corresponds roughly to the
height of cloud base (the lifted condensation level, or LCL); in all
simulations we use a boundary layer having a subcloud layer that is as
close to well-mixed as possible, without encouraging spontaneous
turbulence in the starting environment;
-
Depth of the moist layer feeding the updraft; this corresponds roughly
to the height of the level of free convection (LFC); here we specify a
moist layer having constant equivalent potential temperature throughout
the subcloud mixed layer, and sometimes above the mixed layer; in those
cases where the moist layer is allowed to be deeper than the mixed
layer, we specify a nearly-saturated moist adiabatic environmental
profile up through the specified depth of the moist layer;
-
Temperature at cloud base, or approximately equivalently, the amount of
precipitable water, in the environment; cooler environments contain
less water vapor and less precipitable water than warmer environments,
all other things being equal;
-
Free tropospheric relative humidity; for simplicity, we specify this
humidity to be a constant everywhere in the troposphere above the moist
layer; our default simulations use a value of 90%, but we are also
examining other lower values.
Our buoyancy profiles are shaped with an analytical function of the
form m2z e(-mz). This function, while not the only one that could
have been used, has several convenient mathematical properties, especially
the constancy of its full vertical integral under arbitrary changes to the
shape parameter "m." We employ a similar function to shape the y-component
of wind when building our environmental hodographs.
For carefully chosen limiting values of each of the eight parameters,
we can do a series of numerical simulation experiments that is just tractable
with current computer technology, but which should embrace most of the range
of variability found in convective storms, resulting from variations in the
vertical thermodynamic and kinematic structure of the storm's environment.
Findings from the COMPASS project will thus help provide a new, comprehensive
and logical framework for understanding and interpreting the behavior of deep
convective storms in the real atmosphere.
Because COMPASS uses the already-existing dynamical and physical framework
of a cloud model, no new terms in the dynamical equations are being looked for.
Rather, the emphasis in COMPASS is on storm morphology, the combined effects
of storm intensity and structure that are the manifestations of how a storm
circulation responds to its environment. Preliminary findings indicate that
simple changes to a few parameters from among the list of eight can yield
very large differences in storm intensity and size. In weakly unstable
environments having only 800 J/kg of convective available potential energy
(CAPE), for example, storms fail to sustain themselves when parcel buoyancy is
not concentrated to sufficient strength near cloud base. If, however, the same
buoyancy is concentrated near cloud base, and the level of free convection is
raised in conjunction with specification of a cooler cloud base temperature,
updrafts approaching 40 m/s in strength become possible! These huge increases
in updraft strength are the result of stronger accelerations from perturbation
pressure gradient effects near cloud base, a deeper moist layer feeding the
updraft when the LFC is high, the reduced water loading, and earlier release
of the latent heat of fusion in the updraft afforded by the cooler storm
environment. This same trend in storm intensity is found at larger CAPE values
as well, except that storms are not so easily suppressed for unfavorable
buoyancy profile shapes when CAPE is large. The strength of the storms in
cooler environments seems surprising, mainly because of awareness that CAPE
tends to be smaller in cool environments, a limitation that idealized numerical
simulation studies can easily circumvent. The rigor of the conceptual framework
of COMPASS encourages better general understanding of convective storm behavior
by eliminating the possible sources of confusion resulting from the atmosphere's
tendency to visit the various parts of the eight-dimensional parameter space
very unevenly.
This website is intended to provide an overview of the COMPASS project
and its scientific results. COMPASS is the largest and most comprehensive
parameter space study of convective storm morphology ever attempted. Because
it deals with eight independent parameters, the simulation experiment
nomenclature, explained in the next section, is necessarily somewhat
cumbersome. However, after a little practice, the simplicity and utility
of the nomenclature will become apparent. The reader will find it beneficial
to become acquainted with this nomenclature before proceeding to an examination
of the simulation results. Results are presented in graphical form, in a
series of four-panel maps of the structure of the mature storms, each from four
separate but closely-related experiments. However, the reader is cautioned
that storm structure is seldom time-independent, and the snapshots of storm
structure shown in these maps cannot be taken as definitive. Additional
graphics will be added as this research effort proceeds, and more results are
obtained.
COMPASS is supported by a grant, ATM-0126408, from the National Science
Foundation, under the supervision of Dr. Stephan Nelson.