NWS Technical Procedures Bulletin

RUC20 - The 20-km version of the Rapid Update Cycle

Draft – 6 April 2002

Stanley G. Benjamin, John M. Brown, Kevin J. Brundage, Dezso Devenyi, Georg Grell, Dongsoo Kim, Barry E. Schwartz, Tatiana G. Smirnova, and Tracy Lorraine Smith, and Stephen S. Weygandt

NOAA/OAR Forecast Systems Laboratory, Boulder, CO

Geoffrey S. Manikin

Environmental Modeling Center, National Centers for Environmental Prediction, Camp Springs, MD

Summary of RUC20 vs. RUC40 (RUC-2) differences - Abstract

1. Horizontal resolution

The RUC20 has a 20-km horizontal resolution, compared to 40 km for the previous RUC40 (RUC-2). The area covered by the computational grid has not changed. The RUC20 has a 301x225 horizontal grid, compared to 151x113 for the RUC40.

2. Vertical resolution

The RUC20 has 50 computational levels, compared to 40 levels for the RUC40. The RUC20 continues to use the hybrid isentropic-sigma vertical coordinate used in previous versions of the RUC.

3. Improved moist physics

Improved quantitative precipitation forecasts have been the primary focus for changes in the RUC20 model, including a major revision in the MM5/RUC mixed-phase microphysics cloud routine, and a new version of the Grell convective parameterization with an ensemble approach to closure and feedback assumptions. The main effect of the microphysics change is to decrease overforecasting of graupel and ice and to improve the precipitation type forecast. The new Grell scheme results in considerable improvement in convective precipitation forecasts from the RUC.

4. Assimilation of GOES cloud-top data

The RUC20 includes a cloud analysis which updates the initial 3-d cloud/hydrometeor fields by combining cloud-top pressure data from GOES with the background 1-h RUC hydrometeor field. Cloud clearing and building is done to improve the initial cloud water/ice/rain/snow/graupel fields for the RUC.

5. Better use of observations in analysis

The RUC20 assimilates near-surface observations more effectively through improved algorithms for calculating observation-background differences. Assimilation of surface observations is improved by diagnosing background forecasts for surface temperature and dewpoint at 2 m and for winds at 10 m. It is also improved by matching of land-use type between the background and the observation for near-coastal stations. The RUC20 continues to use an optimal interpolation analysis as in the RUC40 – implementation of a 3d variational analysis has been deferred.

6. Improved land-surface physics

The RUC20 land-surface model is changed from that of the RUC40. It uses more detailed land-use and soil texture data, in contrast to 1-degree resolution fields used in the RUC40. It includes improved cold-season processes (soil freezing/thawing) and a 2-layer snow model. These changes improve the evolution of surface moisture and temperature and snow cover, which in turn improve forecasts of surface temperature and moisture and precipitation.

7. Lateral boundary conditions

The RUC40 used lateral boundary conditions specified from the Eta model initialized every 12 h. The RUC20 adds updates of its lateral boundaries from the 0600 and 1800 UTC Eta runs.

8. Improved post-processing

The RUC20 includes improved diagnostic techniques for 2-m temperature and dewpoint, 10-m winds, helicity, visibility, convective available potential energy, and convective inhibition.

Most significant improvements in RUC20 fields over those from RUC40 (RUC-2).

· precipitation - both summer and winter - From improved precipitation physics and higher resolution

· all surface fields - temperature, moisture, winds – Reduced bias and RMS error in comparison with METAR observations. From improved surface and cloud/precipitation physics and higher resolution

· upper-level winds and temperatures - From higher vertical and horizontal resolution, better physics

· orographically induced precipitation and circulations - From higher horizontal resolution, cloud physics, and better use of surface data near mountains.

1. INTRODUCTION

A new version of the Rapid Update Cycle (RUC) is being implemented with a doubling of horizontal resolution (40km to 20km), an increased number of vertical computational levels (40 to 50), and improvements in the analysis and model physical parameterizations. A primary goal with the 20-km RUC (or RUC20) has been improvement in warm-season and cold-season quantitative precipitation forecasts. Improvements in near-surface forecasts and cloud forecasts have also been targeted. The RUC20 provides improved forecasts for these variables, as well as for wind, temperature, and moisture above the surface.

The RUC20 provides improved short-range numerical weather guidance for general public forecasting as well as for the special short-term needs of aviation and severe-weather forecasting. The RUC20 continues to produce new analyses and short-range forecasts on an hourly basis, with forecasts out to 12 h run every 3 h. The implementation of the RUC20 in 2002 follows previous major implementations of a 60-km 3-h cycle version in 1994 (Benjamin et al 1994, 1991) and a 40-km 1-h cycle version in 1998 (Benjamin et al 1998).

The uses of the RUC summarized below continue with the implementation of the RUC20:

· Explicit use of short-range forecasts - The RUC forecasts are unique in that they are initialized with very recent data. Thus, in most cases, the most recent RUC forecast has been initialized with more recent data than other forecast model runs available. Even at 0000 or 1200 UTC, when other model runs are available, the RUC forecasts are useful for comparison over the next 12 h. Although there are a vast number of differences between the RUC and other NCEP models, the key unique aspects of the RUC are its hybrid isentropic vertical coordinate (used in analysis and model), hourly data assimilation, and model physics.

· Monitoring current conditions with hourly analyses - Hourly analyses are particularly useful when overlaid with hourly satellite and radar images, or hourly observations such as from surface stations or profilers.

· Evaluating trends of longer-range models - RUC analyses and forecasts are useful for evaluation of the short-term predictions of the Eta and AVN models.

The users of the RUC include:

· Aviation Weather Center/NCEP, Kansas City, MO

· Storm Prediction Center/NCEP, Norman, OK

· NWS Weather Forecast Offices

· FAA/DOT, including use for air traffic management and other automated tools, and for FAA workstations

· NASA Space Flight Centers

· Private sector weather forecast providers

Sections below describe changes in the RUC with the RUC20 implementation regarding spatial resolution, data assimilation, model, and diagnostics / post-processing.

Figure 1. Terrain elevation for a) 40-km RUC-2, b) 20-km RUC20

2. SPATIAL RESOLUTION

The RUC20 occupies the same spatial domain as the previous RUC40 (40-km RUC-2), as shown in Figs. 1a,b. The RUC20 grid points are still a subset of the AWIPS Lambert conformal grid (AWIPS/GRIB grid 215 for 20km) used as a distribution grid by the National Weather Service. Direct use of the AWIPS grid reduces the number of distribution grids for the RUC. The AWIPS grid ID for the RUC20 grid is 252, compared to 236 for the RUC40 grid. Thus, the 252 grid for the RUC20 is a subset of the 215 grid. The RUC20 grid size is 301 x 225 grid points (compared to 151 x 113 for RUC-2).

2.a. Horizontal resolution

The 20-km grid spacing used by the RUC20 allows better resolution of small-scale terrain variations, leading to improved forecasts of many topographically induced features, including low-level eddies, mountain/valley circulations, mountain waves, sea/lake breezes, and orographic precipitation. It also allows better resolution of land-water boundaries and other land-surface discontinuities. While the most significant differences in the terrain resolution of the RUC20 (Fig. 1b) vs. RUC40 (Fig. 1a) are in the western United States, a number of important differences are also evident in the eastern part of the domain.

The surface elevation of the RUC20, as with the RUC40, is defined as a "slope envelope" topography. The standard envelope topography is defined by adding the sub-grid-scale terrain standard deviation (calculated from a 10-km terrain field) to the mean value over the grid box. By contrast, in the slope envelope topography, the terrain standard deviation is calculated with respect to a plane fit to the high-resolution topography within each grid box. This gives more accurate terrain values, especially in sloping areas at the edge of high-terrain regions. It also avoids a tendency of the standard envelope topography to project the edge of plateaus too far laterally onto low terrain regions. Using the slope envelope topography gives lower terrain elevation at locations such as Denver and Salt Lake City which are located close to mountain ranges. As shown in Table 1, the RUC20 more closely matches station elevations in the western United States.

Rawinsonde station	Station elevation minus RUC40 elevation (m)	Station elevation minus RUC20 elevation (m)
Edwards AFB, CA	300	41
Denver, CO	354	26
Grand Junction, CO	679	323
Boise, ID	274	253
Great Falls, MT	157	29
Reno, NV	381	144
Elko, NV	352	152
Medford, OR	544	346
Salem, OR	233	51
Rapid City, SD	153	45
Salt Lake City, UT	630	438
Riverton, WY	225	119

Table 1. Terrain elevation difference between station elevation and interpolated RUC elevation for selected rawinsonde stations in western United States.

The grid length is 20.317 km at 35 deg N. Due to the varying map-scale factor from the projection, the actual grid length in RUC20 decreases to as small as 16 km at the north boundary. The grid length is about 19 km at 43 deg N. The RUC20 latitude/longitude (and terrain elevation) at each point in an ASCII file can be downloaded from http://ruc.fsl.noaa.gov/MAPS.domain.html. The lower left corner point is (1,1), and the upper right corner point is (301,225), as shown in Table 2.

An example is shown below (Fig. 2) of the improved orographic effect on low-level wind circulation comparing 3-h forecasts from RUC20 and RUC40, both displayed at 40km resolution. The RUC20 shows a better depiction of the Denver-area cyclonic circulation, strong southerly flow up the San Luis Valley into southern Colorado near Alamosa, and winds of greater than 20 knots near higher terrain in central Colorado and south central Utah. The verifying analysis in Fig. 3 shows that all of these features appear to be better depicted in the RUC20 3-h forecasts.

Figure 2. RUC 3-h surface wind forecasts from a) RUC40 and b) RUC20. Forecasts valid at 1800 UTC 3 April 2002.

Figure 3. Verifying analysis of surface winds at 1800 UTC 3 April 2002 from RUC20 for a) 40km resolution plot and b) 20km resolution plot over Colorado.

RUC20 point	AWIPS-212 point	Latitude	Longitude
(1,1)	(23,7)	16.2810 N	126.1378 W
(1,225)	(23,119)	54.1731 N	139.8563 W
(301,1)	(173,7)	17.3400 N	69.0371 W
(301,225)	(173,119)	55.4818 N	57.3794 W

Table 2. Latitude/longitude and AWIPS-212 positions of corner points for RUC20 domain.

b. Vertical resolution

The RUC20 continues to use the generalized vertical coordinate configured as a hybrid isentropic-sigma coordinate (Bleck and Benjamin 1993) used in previous versions of the RUC. This coordinate is used for both the analysis and the forecast model. The RUC hybrid coordinate has terrain-following layers near the surface with isentropic layers above. This coordinate has proven to be very advantageous in providing sharper resolution near fronts and the tropopause (e.g., Benjamin 1989, Johnson et al. 1993, 2000). Some of the other advantages include:

· All of the adiabatic component of the vertical motion on the isentropic surfaces is captured in flow along the 2-d surfaces. Vertical advection through coordinate surfaces, which usually has somewhat more truncation error than horizontal advection, is less prominent in isentropic/sigma hybrid models than in quasi-horizontal coordinate models. This characteristic results in improved moisture transport and less precipitation spin-up problem in the first few hours of the forecast.

· Improved conservation of potential vorticity. The potential vorticity and tropopause level (based on the 2.0 PV unit surface) show very good spatial and temporal coherence in RUC grids (Olsen et al 2000).

· Observation influence in the RUC analysis extends along isentropic surfaces, leading to improved air-mass integrity and frontal structure. From an isobaric perspective, the RUC isentropic analysis is implicitly anisotropic (Benjamin 1989).

The RUC20 has 50 vertical levels, compared to 40 levels in RUC40. Extra levels are added near the tropopause and lower stratosphere and also in the lower troposphere. The RUC hybrid coordinate is defined as follows:

- Each of the 50 levels is assigned a reference virtual potential temperature (q_v) which increases upward (Table 3).

- The lowest atmospheric level (k=1) is assigned to be at a pressure corresponding to an elevation of 5 m above the model terrain.

- Each of the next 49 levels is assigned a minimum pressure thickness between it and the next level below. This thickness will apply to coordinate surfaces that end up being in the lower portion of the domain where the coordinate surfaces are terrain-following. For grid points with surface elevation near sea level, the minimum pressure thickness is 2.5, 5.0, 7.5, and 10 hPa for the bottom 4 layers, and 15 mb for all layers above. These minimum pressure thicknesses are reduced over higher terrain to avoid ‘bulges’ of sigma layers protruding upward in these regions.

- The pressure corresponding to the reference q_v for each (k) level is determined for each (i,j) column. (For lower q_v values, this pressure may be determined via extrapolation as beneath the ground.)

- At this point, there are two choices for the assignment of pressure to the (i,j,k) grid point, corresponding to:

1) the reference q_v value (the ‘isentropic’ definition), and

2) the minimum pressure spacing, starting at the surface pressure (the ‘sigma’ definition)

If the isentropic pressure (1) is less than sigma pressure (2), the grid point pressure is defined as isentropic, and otherwise as terrain-following (sigma).

224	232	240	245	250	255	260	265	270	273
276	279	282	285	288	291	294	296	298	300
302	304	306	308	310	312	314	316	318	320
322	325	328	331	334	337	340	343	346	349
352	355	359	365	372	385	400	422	450	500

Table 3. Reference q_v values (K) for the RUC20 (50 levels).

The maximum q_v value in the RUC20 is 500 K, compared to 450 K for the RUC40. The 500 K surface is typically found at 45-60 hPa. As with the RUC40, a greater proportion of the hybrid levels are assigned as terrain-following in warmer regions and warmer seasons. This is shown in Figs. 4a,b below.

Figure 4. Vertical cross-sections showing RUC native coordinate levels for a) RUC40 – 40 levels, and b) RUC20 – 50 levels. Data are taken from RUC 12-h forecasts valid at 1200 UTC 2 April 2002. Cross-sections are oriented from south (Mississippi) on left to north (western Ontario) on right

In this example (Fig. 4), north-south vertical cross-sections are shown depicting the pressure at which the RUC native levels are found for a particular case. The case shown is from April 2002, with the cross-section extending from Mississippi (on the left) northward through Wisconsin (center point), across Lake Superior (slightly higher terrain on each side), and ending in western Ontario. A frontal zone is present in the middle of the cross-section, where the RUC levels (mostly isentropic) between 700 and 300 hPa are strongly sloped.

In the RUC20, seven new levels have been added with reference q_v values between 330 K and 500 K. Three new levels with reference q_v in the 270-290 K range have also been added. In the RUC20 depiction (Fig. 4b), the tropopause is more sharply defined than in the RUC40, and there are more levels in the stratosphere, resulting from the additional levels in the upper part of the domain. In the RUC20, the isentropic levels from 270-355 K are now resolved with no more than 3 K spacing.

3. FORECAST MODEL CHANGES IN RUC20

The RUC20 forecast model is similar to the RUC40 forecast model but has important changes in physical parameterizations and some regarding numerical approaches. The model continues to be based upon the generalized vertical coordinate model described by Bleck and Benjamin (1993). Modifications to a 20-line section of code in the model are sufficient to modify it from the hybrid isentropic-sigma coordinate described in section 2.b to either a pure sigma or pure isentropic model.

3.a. Basic dynamics/numerics

First, the basic numerical characteristics of the RUC model are reviewed, italicized where different in the RUC20 from the RUC40.

· Arakawa-C staggered horizontal grid (Arakawa and Lamb 1977); u and v horizontal wind points offset from mass points to improve numerical accuracy.

· Generalized vertical coordinate equation set and numerics for adiabatic part of model following Bleck and Benjamin (1993)

· No vertical staggering.

· Time step is 30 seconds at 20-km resolution.

· Positive definite advection schemes used for continuity equation (advection of pressure thickness between levels) and for horizontal advection (Smolarkiewicz 1983) of virtual potential temperature and all vapor and hydrometeor moisture variables.

· Application of adiabatic digital filter initialization (DFI, Lynch and Huang 1992) for 40-min period forward and backward before each model start. The use of the DFI in the RUC is important for producing a sufficiently “quiet” (reduced gravity wave activity) 1-h forecast to allow the 1-h assimilation cycle. A problem in application of digital filter weights is corrected in the RUC20.

The atmospheric prognostic variables of the RUC20 forecast model are:

· pressure thickness between levels

· virtual potential temperature - q_v

· horizontal wind components

· water vapor mixing ratio

· cloud water mixing ratio

· rain water mixing ratio

· ice mixing ratio

· snow mixing ratio

· graupel (rimed snow, frozen rain drops) mixing ratio

· number concentration for ice particles

· turbulence kinetic energy

The soil prognostic variables (at six levels) of the RUC-2 forecast model are:

· soil temperature

· soil volumetric moisture content

Other surface-related prognostic variables are snow water equivalent moisture and snow temperature (at 2 layers in RUC20), and canopy water.

Other differences in the RUC20 vs. RUC40 model numerics or design are as follows:

• The order of solution in each time step:

RUC40	RUC20
Continuity	Continuity
Horizontal advection of qv / moisture	Horizontal advection of qv / moisture