Diagnosis of 2m temp, dewpoint temp - 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 MYJ PBL scheme (used in 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.

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. PDF ) This method has improvement over the standard reduction method in mountainous areas and gives geostrophic winds that are more consistent with observed surface winds.

Precipitation accumulation - All precipitation values, including the 12-h total, are liquid equivalents, regardless of whether the precipitation is rain, snow, or frozen.
- 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 the Grell 3-d scheme option 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 (in web product) This fields is calculated using a 10:1 ratio from the accumulated snow water equivalent. This ratio varies in reality, but the ratio used here 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.

Snow depth - 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 (best reference: Thompson 2008, Mon. Wea. Rev.) in the RAP model. Modifications were made in January 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. 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):

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 diagnosed.
  • 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) and
    • 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 > 0.05 g/kg 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.

Freezing levels - 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 http://ruc.noaa.gov/pdf/RUC_cape_vs_RAP_Unipost_cape_apr12.pdf . 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.

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.

Reference: 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 basically a 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 very large 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:

Additional information is available at http://www.spc.noaa.gov/publications/bright/ltgparam.pdf

Soil moisture - cycled continuously in the RAP model/assimilation cycle without resetting from external models. There are 6 levels in the RUC land-surface model used in the RAP configuration of WRF, extending down to 3 m deep, but the field shown are for only the top 3 levels, at the surface, 5 and 20 cm depth. The surface value is indicitive of the top 2 cm of soil only, so this field responds quickly to recent precipitation or surface drying. In general, the deeper in the soil, the more slowly do soil conditions change..

Tropopause Pressure -

Diagnosed from
• RAP - diagnosed in standard Unipost configuration with surface-upward search for first occurrence of a 3-layer mean lapse rate less than or equal to a critical lapse rate (2 K/km) in accordance with WMO definition of tropopause.
• RUC - diagnosed from 2.0 isentropic potential vorticity unit (PVU) surface. The 2.0 PVU surface is calculated directly from the native isentropic/sigma RUC grids. First, a 3-d PV field is calculated in the layers between RUC levels from the native grid. Then, the PV=2 surface is calculated by interpolating in the layer where PV is first found to be less than 2.0 searching from the top down in each grid column. Low tropopause regions correspond to upper-level waves and give a quasi-3D way to look at upper-level potential vorticity. They also correspond very well to dry (warm) areas in water vapor satellite images, since stratospheric air is very dry.

Vertical velocity - Following NCEP unipost convention, vertical velocity in m/s is converted to omega in Pa/s using the formula omega = -rho*g*w, where rho is air density and g = 9.80665 m/s*s.

(The RAP vertical motion is at a given time step and is not time-averaged.)

PBL depth - Using vertical profile of virtual potential temperature from RAP native levels, find height above surface at which theta-v (virtual potential temperature) again exceeds theta-v at surface (lowest native level - 5 m above surface). The surface theta-v is boosted by an additional 0.5 K, which does not strongly affect the PBL height if it is already at least 100m, but does avoid a diagnosis of zero depth from a small ( < 0.5 K) inversion in the lowest 20m. Units: Distance above surface in meters.

gust wind speed - Within PBL depth, calculate excess of wind speed over surface speed at each level. Multiply this excess by a coefficient (f(z)) that decreases with height from 1.0 to 0.5 at 1 km height, and is 0.5 for any height > 1 km. Add the maximum weighted wind excess back to the surface wind. [gust = vsfc + max (f(z)*(v(k)-vsfc) ]

cloud base height (ceiling) - Lowest level at which combined cloud and ice mixing ratio exceeds 10**-6 g/g. Units - meters above sea level (ASL). Horizontal grid points without any cloud layer are indicated with -99999. Note that the RAP graphics show cloud base in above GROUND level (AGL) -- the RAP terrain elevation height is subtracted first. But in the actual GRIB files, cloud base height is in ASL.

- There are these additional extensions to ceiling calculation beyond the explicit cloud water/ice conditions.
• 1. Lowering of ceiling from falling snow. This corresponds to the "vertical visibility" sometimes reported in METAR reports. This uses the visibility calculation based on snow mixing ratio.
• 2. PBL-top cloud-top ceiling diagnosed from PBL-top RH. If PBL-top RH > 95%, a ceiling is identified at this level even if there is no explicit cloud water or ice at this level.
• 3. Avoid identifying surface fog layers as low ceiling. If cloud water is available at level 2 (~32 m AGL) and/or level 3 (~80m AGL) but not above that level, this is ascribed as a fog layer near the surface too shallow to be an aviation-affecting ceiling.

cloud top height - Top level at which combined cloud and ice mixing ratio exceeds 10**-6 g/g. Units - meters above sea level. Horizontal grid points without any cloud layer are indicated with -99999.

cloud fraction - (available in BUFR only) In the RAP, this tends to be 0 or 100% since non-zero cloud hydrometeor mixing ratios can only occur if the grid volume is saturated, but includes some horizontal smoothing not in the RUC. Currently, there is no RH-based cloud fraction in the NCEP oper RAP but this will be considered in the future (5/2012).

Prepared by Stan Benjamin stan.benjamin@noaa.gov, 303-497-6387