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25 March 2002 - Updated for RUC20 version
8 Sept 2004 - Added info for cloud base/top
12 Sept 2005 - Update/corrections for 13-km RUC (RUC13) implementation in June 2005
29 Sept 2005 - Added comment on omega
Apr 2007 - More updates

Relative humidity - Defined with respect to saturation over water in the RUC isobaric fields and in the surface relative humidity field.

Diagnosis of sfc temp, dewpoint temp - Temperature and dewpoint temperatures displayed are extrapolated from the lowest RUC native level at 5m above the RUC topography to a separate "minimum" topography field to give values more representative of valley stations in mountainous areas, where surface stations are usually located. The reduction from the original model topography to the "minimum" topography uses a lapse rate from the lowest ~25 hPa from the RUC field. This lapse rate is constrained to be between dry-adiabatic and isothermal. This same method is used in "downscaling" RUC data to the RTMA (Real-Time Mesoscale Analysis) background field as of July 2006.

RUC20 - "minimum" topography field uses minimum of 10km values near each 20km grid point as first guess. Then, modified to fit METAR surface elevations with fine-scale Barnes analysis.
RUC20 - 2 m temperature and dewpoint are now reduced to 2m level instead of 5 m model computational level using similarity theory. In a likewise manner, 10 m winds are reduced to 10 m instead of the 5 m model computational level. For temp/dewpoint, the final 2 m values are a combination of the reduction to "minimum" topography and the diagnosis of the 2 m level. These two methods have been found to improve fit to METAR values in RUC analyses and forecasts.
RUC13 - The 13km topomini field is derived from a 3km terrain field over the RUC domain using the WRF-SI program.

Sea-level pressure - 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 some improvement over the standard reduction method in mountainous areas and gives geostrophic winds that are more consistent with observed surface winds.

RUC20 - Reduction method improved with bug fixes. RUC20 SLP is now more coherent in mountainous regions.

Precipitation accumulation - All precipitation values, including the 12-h total, are liquid equivalents, regardless of whether the precipitation is rain, snow, or frozen.
- 1h/3h accumulations. 1h accumulation is over last 1h period in model forecast. 3h accumulation is over last 3h period in model forecast EXCEPT that it is only for the 0-2h accumulation for a 2h forecast output, and for the 0-1h accumulation for a 1h forecast output.

Instantaneous precipitation rate Total precipitation (resolved and sub-grid-scale) in last physics time step (80 sec in 13km RUC) is written in mm/s.

Resolvable and sub-grid scale precipitation - In RUC2 (40km - 1998-02) (and RUC1 - 60km - 1994-98), the forecast model uses the Grell (1994, Mon. Wea. Rev.) (RUC20 - see Grell and Devenyi, 2001, AMS NWP Conf., Benjamin et al., 2004 MWR - RUC model article) convective parameterization scheme. This scheme tends to force 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 be reflected in the RUC model with the Grell scheme with a substantial proportion of resolvable-scale precipitation. Thus, the sub-grid scale precipitation from RUC 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 (3h period or 12h period). 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 RUC model.

Snow depth (actual, updated Aug 2008, true since 2005) - 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 RUC 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 the 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 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 is provided in the RUC grib output together with snow water equivalent and snow depth.

RUC20 - New land-surface model in RUC20 includes 2-level snow model and cold-season effects (freezing and thawing of moisture in soil). Land-use data is more detailed (derived from 1 km data). Both of these changes lead to improved snow depth in the RUC20. The RUC20 continues to cycle 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 in the RUC model. The RUC microphysics is currently equivalent to the Dec 2003 version of the Thompson/NCAR microphysics using in the WRF model. The RUC model first imported a version of the Thompson scheme (with significant help from John M. Brown of GSD) in the late 1990s, taken then from the MM5 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 RUC 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:

RUC20/RUC13 - Precipitation type output was improved in both implementations of the RUC20 and RUC13 via improvements in the RUC/MM5/Thompson mixed-phase cloud microphysics. In particular, there is less graupel diagnosed.

Freezing levels - Two sets of freezing levels are output from RUC, 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 RUC levels (up to about 50 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 RUC levels to estimate the level at which the temperature goes above or below freezing.

RUC20 - freezing level accuracy is improved, especially near the surface, from more accurate temperature forecasts and an improved representation of the diurnal cycle (land-surface, cloud physics improvements)

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 RUC 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 RUC model. After 3 h, the pressure change field appears to be better well-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 - indicates energy available for buoyant parcel from native RUC hybrid-b level with maximum buoyancy within 180 hPa of surface (changed to 300 hPa on 6 May 1999). Before the most buoyant level is determined, first an averaging of potential temperature and water vapor mixing ratio is done in the lowest 7 RUC native levels (about 45-55 hPa). This "best CAPE" is equivalent to a "most unstable" CAPE (MUCAPE).
June 05 - RUC13 - Surface-based CAPE and CIN output fields were added in June 2005 (with the 13km RUC implementation) to previous best CAPE/CIN fields. Since the lowest 7 RUC native levels are averaged (see above), the surface-based CAPE/CIN in the RUC is equivalent to a mixed-layer CAPE (MLCAPE).

CIN - convective inhibition - indicates negative buoyancy in layer through which a potentially buoyant parcel must be lifted before becoming positively buoyant. Thresholds are shown at 75 W/m*m (marginally strong capping inversion, depicted with loose cross-hatching) and 100 W/m*m (strong cap, depicted with tight cross-hatching).

Lifted index / Best lifted index - Lifted index uses the surface parcel, and best lifted index uses buoyant parcel from native RUC level with maximum buoyancy within 180 hPa of surface (changed to 300 hPa on 6 May 1999).

Precipitable water - Integrated precipitable water vapor from surface of RUC model to top level (~50 hPa). This field is influenced by all available moisture data and the dynamic and physical processes in the ongoing RUC cycle. The precipitable water calculation is performed by summing the product of the specific humidity at each level times the mass of each layer defined as that between the mid-points between each level.

Helicity and storm motion

(May 2002 - RUC20 - uses Bunkers et al. 2000, Weather and Forecasting. Corrections to helicity calculation.)
(27 May 2003 - 0-1 km helicity added to ruc_presm files in addition to previous 0-3 km helicity)

- Helicity is calculated using a technique similar to that used for the Eta. following discussion is modified from a discussion of the Eta helicity product on the NCEP/EMC FAQ web page:

Storm-relative helicity is now computed using the Internal Dynamics (ID) method (Bunkers et al, 2000). Prior to March 2000, the model used the Davies and Johns method in which supercell motion is estimated to be 30 degrees to the right and 85% of the mean wind vector for a 850-300 hPa mean wind < 15 knots and 75% of the mean wind vector for a 850-300 hPa mean wind > 15 knots. This method works very well in situations with "classic" severe-weather hodographs but works poorly in events characterized by atypical hodographs featuring either weak flow or unusual wind profiles (such as northwest flow). The ID method has been found to perform as well as the Davies and Johns method in the classic cases and much better in the atypical cases. The ID method includes an advective component (associated with the 0-6 km pressure-weighted mean wind) and a propagation component (associated with supercell dynamics) that adjusts the motion along a line orthogonal to the 0-6 km mean vertical wind shear vector. A storm motion vector is computed, and this is used to compute helicity. The relevant model fields and WMO parameter ID's are:

    VALUE   PARAMETER                       UNITS
    190     Storm-relative Helicity         m**2/s**2 
    191     U-component Storm Motion        m/s
    192     V-component Storm Motion        m/s
Again, as of May 2003, the RUC outputs both 0-1 km as well as 0-3 km helicity


Bunkers, M. J., and co-authors, 2000: Predicting supercell motion using a new hodograph technique. Wea. Forecasting, 15, 61-79.

Davies, J. M., and R. H. Johns, 1993: Some wind and instability parameters associated with strong and violent tornadoes. Part I: Wind shear and helicity. The tornado: Its Structure, Dynamics, Prediction, and Hazards, Geophys. Monogr., No. 79, Amer. Geophys. Union, 573-582.

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.

Soil moisture - cycled continuously in the RUC model without resetting from external models. There are 6 levels in the RUC land-surface model, extending down to 3 m deep, but the field shown is for the top 2 cm of soil only, so this field responds quickly to recent precipitation or surface drying and may not be indicative of deep soil moisture. The variable displayed is the soil volumetric moisture content, the ratio of water volume to total volume in the soil. Values of 0.25 or so are relatively high values.

Tropopause Pressure - 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

RUC13 update (The RUC vertical motion is at a given time step and is not time-averaged.) The RUC vertical velocity (omega = dp/dt) is diagnosed in the RUC model and is not a part of the RUC pronostic equation set. It is calculated from 3 terms: 1) vertical motion through coordinate surfaces, 2) vertical motion of the coordinate surfaces, and 3) vertical motion along the coordinate surfaces. See Benjamin et al. (2004 - MWR - RUC model) - see p.15 for more information and the omega equation used in section 4.c. Note -- since omega (dp/dt) is relative to pressure, a positive value (increasing pressure) means decreasing height and a negative valud (decreasing pressure) means upward vertical motion with respect to height. RUC13 fix - The effects of diabatic heating in term 1 above were exaggerated in previous versions before the RUC13. This is now corrected.

[Following is a write-up from 1998 on RUC vertical motion, but first read information above and Benjamin et al. 2004-MWR paper.]
The vertical motion, -omega, positive upward, in the hybrid-coordinate RUC model is diagnosed , using the formula

-omega = -Dp/Dt = -[(partial p/partial t)_s +

(vector V_H dot del_s) p + sdot*(partial p / partial s)],

where p is pressure, V_H is the horizontal velocity vector, del_s is the gradient operator on a hybrid coordinate surface sdot is,s the rate at which air parcels are moving vertically with respect to the hybrid coordinate, s, which increases vertically, and (partial p / partial s) expresses the decrease in pressure with increasing s.

Omega is not actually needed to solve the RUC's model governing equations. It is, instead, a diagnosed quantity that is provided to see the effective vertical motion in the RUC model. The three terms of the omega equation correspond to:

  1. Motion through coordinate surfaces. This term is quite large on sigma levels, but zero on isentropic levels except in the event of diabatic processes (e.g., latent heat release, evaporation, radiational heating/cooling).
  2. Local movement of the surfaces. For isentropic surfaces, this can be considerable and corresponds roughly to the phase speed of the entire weather system. For sigma levels, it is negligible.
  3. Upslope/downslope motion of the horizontal wind on the coordinate surfaces. This is the classical upglide/downglide term that makes it easy to see vertical motion on isentropic coordinates. In sigma coordintes, it corresponds primarily to terrain-forced motion.
The first two terms on the right-hand side of this equation describe mathematically the change of pressure a hypothetical air parcel would experience if it traveled along a coordinate surface with the horizontal wind velocity on that surface. The last term describes the change in pressure that results from air parcels moving through coordinate surfaces. This last term will tend to be large only in regions of strong latent heating particularly in the isentropic portion of the model domain. The first two terms, however, can be large in regions where winds are strong and where the coordinate surfaces are sloped, as just above mountain ranges in the sigma part of the vertical domain, or where isentropic "upglide" or descent is occurring. In mountain wave situations, isentropes themselves tend to follow the terrain, espcially close to the surface. Higher up, they can be very steeply sloped, even more so than the (smoothed) terrain.

An important factor to bear in mind when considering model-produced vertical motion concerns a fundamental aspect of flow patterns in the middle latitudes. That is, above the planetary boundary layer, the Coriolis force and the horizontal component of the pressure-gradient force tend to be in balance in synoptic-scale weather systems. This means that the horizontal winds tend to be approximately geostrophic and that the typical relative vorticity of these winds is typically much larger in magnitude than their horizontal divergence. As a result, vertical motions in synoptic-scale systems are usually small, particularly outside areas of heavy precipitation. However, for smaller, more rapidly changing mesoscale motion fields, this constraint toward geostrophic balance imposed by the earth's rotation is less strong, and divergence and vorticity will often have about the same magnitude. With stronger divergence, vertical motions are typically also stronger for mesoscale motions.

PBL depth - Using vertical profile of virtual potential temperature from RUC 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). As of RUC upgrade on 17 Nov 2008, 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 RUC graphics show cloud base in above GROUND level (AGL) -- the RUC terrain elevation height is subtracted first. But in the actual GRIB files, cloud base height is in ASL.

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 RUC, this is either 0 or 100% since non-zero cloud hydrometeor mixing ratios can only occur if the grid volume is saturated. (This characteristic is also found with MM5 and some physics packages with WRF. The RUC the NCAR/Thompson mixed-phase cloud microphysics.) In the RUC, if there are any levels from the native grids with cloud water or ice mixing ratio > 10**-6 g/g, then the cloud fraction is set as 100% and 0% otherwise. This approach is different from the Eta model, where saturation exists at < 100% RH and the cloud fraction varies continuously from 0 to 100% based on a function of RH.
Cloud layers in the RUC are defined as follows:

visibility - RUC extension of Stoelinga-Warner (JAM, 1999) algorithm

RUC13 update -- See http://ruc.fsl.noaa.gov/ruc13_docs/RUC13-summary.htm for updates in the visibility diagnostic in the RUC13.

pressure of maximum equivalent potential temperature in column - From RUC CAPE/CIN routine.

convective cloud top height - From RUC CAPE/CIN routine. This is the level at which negative buoyant energy cancels out the CAPE below the equilibrium level. It is also equivalent to the height at which vertical velocity goes to zero (assuming no entrainment). Height above sea level.

equilibrium level height - From RUC CAPE/CIN routine. Height at which parcel is no longer buoyant. Height above sea level.

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Prepared by Stan Benjamin stan.benjamin@noaa.gov, 303-497-6387 [an error occurred while processing this directive]