Dear Seth,
I know that I am replying to late in the game to this e-mail from a few
weeks ago - sorry about this, but I wanted to make one comment regarding
your proposed modification to the definition for CAPE. As you might
remember, I recently proposed some new standard names which are related
to the future GOES-R platform. Another GOES-R output data product will
be CAPE, so I perused your proposed modification to the CAPE definition
to see if it was consistent with the GOES-R product.
The first sentence of your definition says "...calculated by integrating
the positive differences in virtual temperature...". My concern about
using virtual temperature in the definition is that it is too specific
since CAPE is not always calculated using the virtual temperature. For
example, in the GOES-R product, potential temperature differences are
used. The parcel potential temperature is corrected for moisture in the
parcel, but not in the same way as calculating the virtual temperature
of the parcel. I'm thinking it might be good to attempt to generalize
your proposed definition of CAPE. Perhaps you could say that it is
calculated by integrating the temperature difference between the parcel
and environment, where the parcel temperature may be corrected due to
the moisture content of the air parcel (e.g. the virtual temperature).
Using "temperature difference" as opposed to a specific temperature,
e.g. the virtual temperature, would help to generalize the definition.
Sincerely,
Jonathan Wrotny
On 5/24/2013 7:57 PM, Seth McGinnis wrote:
Greetings CF mailing list!
I would like to propose some new standard_names related to convective
instability indices.
I apologize for sending such a long proposal right before a holiday
weekend in the US, but I've been working on it for a while and it
dovetails with the recent discussion of a standard_name for Lifted
Index. In that discussion, I've proposed two new standard names to
record the starting and ending points of a lifted parcel:
air_pressure_of_lifted_parcel_at_origin
air_pressure_of_lifted_parcel_at_finish
Given this (or some other) way of specifying the lifted parcel, we can
then handle variables recording various of its properties. I would like
to propose three new standard_names for CIN, LCL, and LFC, and to
add to the existing definition for CAPE, as follows:
CIN:
name: atmosphere_specific_convective_inhibition
Convective inhibition is the amount of energy required to overcome the
negatively buoyant energy exerted by the environment on a parcel of
air. Convective inhibition is often abbreviated as "CIN" or "CINH".
It is calculated by integrating the negative differences in virtual
temperature between a parcel of air lifted adiabatically and its
surroundings. If the start and/or end points of the lifted parcel are
not specified using auxiliary coordinate variables with the
standard_names air_pressure_of_lifted_parcel_at_origin (_finish), the
parcel starts at the surface (lower boundary of the atmosphere) and
ends at the top of the atmosphere.
canonical units: J/kg
LCL:
name: atmosphere_lifted_condensation_level
The lifting condensation level is the height at which the relative
humidity of an air parcel cooled by dry adiabatic lifting would reach
100%. If the starting point of the lifted parcel is not specified
using an auxiliary coordinate variable with the standard_name
air_pressure_of_lifted_parcel_at_start, the parcel starts at the
surface (lower boundary of the atmosphere).
canonical units: m
[Note that, wikipedia notwithstanding, Google reports "liftING
condensation level" as more commonly used than "liftED condensation
level" by about 5:4. Personally, I have no strong feelings about it
one way or the other, and will gladly bow to anyone who has an opinion
based on observations in the wild.]
LFC:
name: atmosphere_level_of_free_convection
The level of free convection is the altitude where the temperature of
the environment decreases faster than the moist adiabatic lapse rate
of a saturated air parcel at the same level. It is calculated by
lifting a parcel of air dry adiabatically to the LCL (lifted
condensation level), then moist adiabatically until the parcel
temperature is equal to the ambient temperature. If the starting
point of the lifted parcel is not specified using an auxiliary
coordinate variable with the standard_name
air_pressure_of_lifted_parcel_at_start, the parcel starts at the
surface (lower boundary of the atmosphere).
canonical units: m
CAPE:
CAPE already exists in the standard name table as
atmosphere_specific_convective_available_potential_energy
There are apparently several different flavors of CAPE depending on
how one chooses the starting point of the lifted parcel. (Among
others, there's surface-based, most-unstable, and maximum-theta-e
CAPE.) Rather than propose several new standard_names for the
different flavors, I think they can be handled by simply adding an
explanatory note in the metadata -- as long as the starting height of
the parcel can be recorded.
So I propose that we update the existing CAPE definition to default to
being surface-based, and add an explanation of the whole lifted parcel
business so that other flavors can be accommodated by noting the
starting heights, as follows:
Additions to definition of
atmosphere_specific_convective_available_potential_energy:
Convective(ly) available potential energy (often abbreviated CAPE) is
a stability measure typically calculated by integrating the positive
differences in virtual temperature between a parcel of air lifted
adiabatically and its surroundings. If the start and/or end points of
the lifted parcel are not specified using auxiliary coordinate
variables with the standard_names
air_pressure_of_lifted_parcel_at_origin (_finish), the parcel starts
at the surface (lower boundary of the atmosphere) and ends at the top
of the atmosphere. [remainder of existing definition goes here]
Cheers,
--Seth
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