RH with respect to precipitable water - calculated as
ratio between precipitable water and PW if full column was saturated
(described more in upcoming manuscript by Benjamin).
Diagnosis of 2m temp, dewpoint temp, 10m wind
- 2m temperature and dewpoint temperatures in RAP are no longer
diagnosed using a more detailed "minimum topography" field as in the RUC.
- 2m temp - diagnosed internally in a surface_diagnostics code in HRRR/RAP
configuration of WRF-ARW model
using lowest atmospheric temp,
skin temp, and fluxes.
- 2m dewpoint - calculated directly from
temperature, specific humidity, and pressure at lowest
prognostic level in model (currently 0.999-sigma).
- 10m wind - calculated directly from
the lowest prognostic level in model (currently 0.999-sigma,
about 8m at sea-level, slightly less for higher elevations)
Sea-level pressure using MAPS reduction (MAPS SLP)
- This reduction is the one used in previous versions of RUC/MAPS
using the 700 hPa temperature to minimize unrepresentative
local variations caused by local surface temperature variations.
This reduction is described in Benjamin and Miller (1990, October,
Monthly Weather Review, pp. 2099-2116.
) This method has
improvement over the standard reduction method in mountainous
areas and gives geostrophic winds that are more consistent
with observed surface winds.
- All precipitation values, including the model run total, are liquid
equivalents, regardless of whether the precipitation is rain, snow,
- 1h accumulations. 1h accumulation is over last 1h period in
model forecast. RAP does not output 3-h accumulated precip, only
in 1h buckets.
Instantaneous precipitation rate
Total precipitation (resolved and sub-grid-scale) in last
physics time step is written in mm/s.
Resolvable and sub-grid scale precipitation -
RAP uses some version of the Grell scheme (Grell-Freitas as of 2015)
as its convective parameterization scheme.
As in most other convective parameterization schemes used at similar horizontal grid spacings, this scheme is not designed to completely eliminate grid-scale saturation in its feedback to temperature and moisture fields. One result of this is that the precipitation from weather systems that might be considered to be largely convective will nevertheless be reflected in the RAP model with the Grell 3-d scheme with a substantial proportion of resolvable-scale
precipitation. Thus, the sub-grid scale precipitation from RAP should
not be considered equivalent to "convective precipitation".
Snow accumulation for RAP and HRRR
Options (as of Nov 2015)
- Variable density snow accumulation - uses variable snow density,
can vary from less than 5:1 to up to 13:1.
- Fixed 10:1 snow accumulation
Background on options:
- For 10:1 snow, this total uses snow only (no graupel).
- For variable density, this accumulation uses both snow and graupel (sleet)
and even includes a subtraction from melt of fresh snow to obtain an estimate of what might
be measured with a stick on a snow board.
- Starting 3/3/2015, snow accumulation in ESRL experimental versions
of RAP and HRRR now use only actual snow accumulation and do not
use graupel/sleet accumulation any longer.
This change was propagated to the NCEP versions of RAP and HRRR
(RAPv3, HRRRv2) implemented in Aug 2016.
The Thompson microphysics used in RAP and HRRR calculates explicitly the
fall of snow mixing ratio, graupel mixing ratio, and rain mixing ratio
reaching the surface, using separate fall speeds for each.
This allows separate diagnosis of accumulation for each variable.
Snow/sleet accumulation (in web product)
- For NCEP RAP/HRRR forecasts until summer 2015, snow and sleet
are combined into a single product called "snow accumulation".
This fixed-density snow accumulation is calculated using a 10:1 ratio from the
accumulated snow water equivalent with both snow and graupel/sleet combined.
This ratio varies
in reality, but the ratio used for this product was set at this constant value so
that users will know the water equivalent exactly. The snow accumulation
(through the snow liquid water equivalent)
is explicitly forecast
through the mixed-phase cloud microphysics in the model
and specifically from snow mixing ratio and graupel mixing ratio fall out
to the surface. The graupel field means that this snow/sleet
accumulation field includes both sleet and even graupel from
convective storms, especially in the 3-km HRRR model.
This addition of sleet/graupel into snow accumulation
was ended for RAP/HRRR runs by ESRL as of 3/3/2015 and was ended
at NCEP for RAPv3/HRRRv2 in Aug 2016.
Frozen precip percentage
- This field uses the explicit precipitation (rain, snow or graupel)
produced from the multi-species Thompson cloud microphysics.
It is calculated as (snow-accumulated + graupel-accumulated)
divided by (snow-accumulated + graupel-accumulated + rain-accumulated).
No rime factor is used in this explicit calculation.
- This field is the current estimated snow depth using
the latest snow density, which is also an evolving variable.
(Snow water equivalent cycles internally within the RAP 1-h cycle.)
The 10:1 ratio is kept
only for fresh snow falling on the ground surface when 2-m air temperature is below -15 C. When 2-m temperature is above -15 C the density of falling snow is computed using an exponential dependency on 2-m temperature, and usually the ratio will be less than 10:1, but not less than 2.5:1. The density of snow pack is computed as the weighed average of old and fresh snow, and it changes with time due to compaction, temperature changes, melted water held within the snow pack
and addition of more fresh snow.
(See Koren et al., 1999, J. Geophys. Res., for snow density formulations.
Snow density was provided in the RUC grib output (but not in RAP)
together with snow water equivalent and snow depth. Snow density in RAP (Kg/m**3) = Snow-water equivalent [kg/m**2] / snow depth [m].
RAP uses 2011 version of RUC land-surface model with 2-level snow model and
cold-season effects (freezing and thawing of moisture in soil).
The RAP cycles
snow depth/cover, as well as snow temperature in the top 5 cm and below
that top snow layer.
Categorical precipitation types - rain/snow/ice
pellets/freezing rain -
These yes/no indicators are calculated from the
3-d hydrometeor mixing ratios calculated in the
explicit cloud microphysics parameterization (references: Thompson 2008, Mon. Wea. Rev.,
Thompson/Eidhammer 2014 MWR)
in the HRRR and RAP models.
2016 Wea. Forecasting article on precip-type diagnostic used in HRRR/RAP:
Explicit precipitation-type diagnosis from a model using a mixed-phase bulk cloud-precipitation microphysics parameterization
Benjamin, Brown, Smirnova,
NOAA Earth System Research Laboratory, Boulder, CO
These p-type values from the post-processing
are not mutually exclusive. More than
one value can be yes (1) at a grid point. Here is how the
diagnostics are done (all precipitation rates below are at the ground and in liquid-water equivalent):
- Jan 2011 - to the ESRL versions of the
RUC, Rapid Refresh, and HRRR post-processing to eliminate a bug
in which the snow/rain ratio was ignored. Changes are shown below to
include the use of snow/rain ratio, which was already intended
but rendered ineffective due to a unit error in a condition
for precip rate, which should have been very small but was very large.
- Jan 2014 - IP diagnostic change - integrated rain water requirement changed from 0.05 g/kg to
0.005 g/kg. Before this change, little IP was being diagnosed. Change made
immediately to ESRL RAP and HRRR. IP diagnostic change included NCEP HRRR
with HRRR implementation in Sept 2014. NCEP RAP will not be fixed for this
until the RAPv3 implementation in summer 2015.
Diagnostic logic for precipitation types
- A snow ratio is calculated as snow mixing ratio fall rate
divided by the total fall rates of rain+snow+graupel over the previous hour.
- Snow -
There are a few conditions under which snow precip type will be
- If the above snow ratio > 0.25 and either the current snow precipitation rate > 0.00072 mm/h
(0.2 E-9 m/s) (in liquid equivalent)
or total precipitation during the previous hour > 0.01 mm, snow is diagnosed.
- If current fall rate for graupel > 0.0036 mm/h
(1.0 E-9 m/s)
- sfc temp is < 0 deg C, and max rain mixing ratio
at any level < 0.05 g/kg or the graupel rate at the sfc is
less than the snow fall rate, snow is diagnosed.
- sfc temp is between 0 - +3 deg C
- Diagnose snow (and not rain) if snow/rain ratio > 0.60
- Rain - If the snow ratio < 0.6 and temperature at the surface is > 273.15, and either the current rain rate at ground is at least 0.01 mm/h or there has been at least 0.01 mm total precipitation during the previous hour, then rain is diagnosed.
- Diagnose rain, not snow or ice pellets if graupel fall rate is > 0.0036 mm/h and temperature at the surface > +3C.
- Freezing rain - Same as for rain, but if the temperature at
the surface is < 0 deg C and some level above
the surface is above freezing, freezing rain is diagnosed.
- Ice pellets - If the graupel fall rate at the surface is
at least 1.0 x 10**-6 mm/s and the sfc temp is < 0 deg C and
the max rain mixing ratio in the column is > 0.005 g/kg (modified 2014) and
the graupel fall rate at the sfc is greater than that for snow, then ice pellets are diagnosed. If in addition, the fall rate for graupel is greater than that for rain, ice pellets only are diagnosed, not freezing rain, not rain and not snow.
- Two sets of freezing levels are output from RAP, one searching from
the bottom up, and one searching from the top down. Of course, these
two sets may be equivalent under many situations, but they may sometimes
identify multiple freezing levels.
The bottom-up algorithm will return
the surface as the freezing level if any of the bottom 3 native levels
(up to about 80 m above the surface) are below freezing (per instructions
from Aviation Weather Center, which uses this product). The top-down
freezing level returns the first level at which the temperature goes
above freezing searching from the top downward. For both the top-down
and bottom-up algorithms, the freezing level is actually interpolated
between native levels to estimate the level at which the temperature
goes above or below freezing.
3-h surface pressure change
- These fields are determined by differencing surface pressure fields
at valid times separated by 3 h. Since altimeter setting values (surface pressure)
are used in the RAP analyses, this field reflects the observed 3-h
pressure change fairly closely over areas with surface observations.
It is based on the forecast in data-void regions.
-The 3-h pressure change field during the
first 3 h of a model forecast often shows some non-physical
features resulting from gravity wave sloshing in the model, despite
use of digital filter initialization (DFI) in RAP/WRF model.
After 3 h, the pressure change field are better behaved.
The smaller-scale features in this field appear
to be very useful for seeing predicted movement of lows, surges, etc.
despite the slosh at the beginning of the forecast.
CAPE - convective available potential
energy - RAP uses standard Unipost definition of CAPE including
use of virtual temperature. RAP CAPE vs. RUC CAPE differences are described in
CAPE values are provided for surface-based CAPE, most unstable CAPE (MUCAPE)
in lowest 300 hPa,
and mixed-layer (lowest ~50 hPa mixed) CAPE (MLCAPE). The calculation of CAPE considers only positively-buoyant contributions of the ascending air parcel, starting at the parcel's
Lifted Condensation Level and ending at the Equilibrium Level.
CIN - convective
inhibition - indicates accumulated negative buoyancy contributions for the ascending parcel, starting at the parcel's Lifted Condensation Level (LCL) and ending at it's Equilibrium Level. By this definition, CIN is mainly accumulated between the LCL and the Level of Free Convection (LFC), and represents the negative bouyant energy that must be overcome in order for the parcel to become positively buoyant once it reaches its LCL.
This is also the standard Unipost definition.
EL - equilibrium level - the highest positively buoyant level. This is also the standard Unipost definition. The EL provied is associated with the most unstable CAPE parcel (MUCAPE; using the parcel with highest theta-e in the lowest 300hPa).
Lifted index / Best lifted index -
Lifted index uses the surface parcel, and best lifted index uses
buoyant parcel from native level
with maximum buoyancy within 300 hPa of surface (also
standard Unipost definition).
Precipitable water - Integrated precipitable
water vapor from surface to top level (10 hPa).
Helicity and storm motion
Standard Unipost definition - uses
Bunkers et al. 2000, Weather and Forecasting.
Bunkers, M. J., and co-authors, 2000: Predicting supercell motion
using a new hodograph technique. Wea. Forecasting, 15, 61-79.
What about the high values of helicity?
The units of helicity are m^2/s^2. The value of 150 is generally
considered to be the low threshold for tornado formation. Helicity is
measure of the low-level shear, so in high shear situations, such as
behind strong cold fronts or ahead of warm fronts, the values will be
maybe as high as 1500. High negative values are also possible in
reverse shear situations.
Lightning / Thunder This thunder parameter is from David Bright, formerly of the NOAA Storm Prediction Center (now of NOAA Aviation Weather Center).
At any point where convective precip is forecast to occur (i.e., where the convective parameterization scheme is active), thunder is predicted if all
of the following are true: