Vertically Integrated Ice (VII) - Warning Decision Training Division (WDTD)

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Vertically Integrated Ice (VII)

Short Description

Radar-derived estimate of frozen water in a vertical column.

Subproducts

None

Primary Users

NWS: WFO, SPC

Input Sources

3D Reflectivity Cube

Vertical temperature profile from the current operational NCEP/EMC mesoscale model (i.e., the RAP as of 2014).

Resolution

Spatial Resolution: 0.01^o Latitude (~1.11 km) x 0.01^o Longitude (~1.01 km at 25^oN and 0.73 km at 49^oN)

Temporal Resolution: 2 minutes

Product Creation

In an empirical study, Carey and Rutledge (2000) found the contribution of ice mass within observed horizontal reflectivity and computed a reflectivity-ice mass (Z-M) relationship. This Z-M relationship was used by Mosier et al. (2011) to calculate VII using:

where ρ_i is the density of ice (917 kg m^-3), N_o is the intercept parameter (4 x 106 m^-4) of an exponential size distribution of precipitation-sized ice, Z is radar reflectivity, and H_-10 and H_-40 indicate the heights of the -10 and -40°C environmental levels in meters, respectively.

The vertical integration is constrained to the thermodynamic layer between -10 and -40°C, which is the graupel/ice growth layer within a thunderstorm.

Technical Details

Latest Update: MRMS Version 11.5

References

Carey, L.D., and S. A. Rutledge, 2000: The relationship between precipitation and lightning in tropical island convection: A C-band polarimetric study. Mon. Wea. Rev., 128, 2687-2710.

Greene, D.R., and R.A. Clark, 1972: Vertically integrated liquid water-A new analysis tool. Mon. Wea. Rev., 100, 548-552.

Mosier, R.M., C. Schumacher, R.E. Orville, and L.D. Carey, 2011: Radar nowcasting of cloud-to-ground lightning over Houston, Texas. Wea. Forecasting, 26, 199-212.

Strengths

Like all MRMS products, the use of multiple radars is more robust than single-site radar alone. It provides faster updates and helps the forecaster integrate data from multiple radars. It also compensates for cone-of-silence, beam broadening at far ranges, and terrain blockage.

The use of mesoscale model analysis data to derive temperature information allows the temperature fields to vary across the domain of interest. This is in stark contrast to applying a single temperature altitude proxy across the entire domain, as is often done for single radar calculations. Thus, MRMS data better captures gradients in the temperature fields over space and time.

Limitations

Similar to Vertically Integrated Liquid (VIL), VII is underestimated for fast-moving and/or highly-tilted storms. However, the effect is much less pronounced than that for VIL, since VII is integrated within a much shallower vertical column.

Subject to the biases and deficiencies of the mesoscale model used to derive the vertical temperature profile.

Quality Control

This product is derived from the 3D Reflectivity Cube, which means non-hydrometeorological data has been removed including: Ground clutter, anomalous propagation (AP), chaff, interference spikes, and bioscatterers (e.g., angels and ghosts). However, bright band contamination remains.

Applications

Can be used to identify convective storm initiation and the onset of electrical activity.

Can be used to assess storm severity including hail potential.

Can be used to assess changes in updraft intensity. Sudden increases (decreases) in VII often occur when an updraft is intensifying (weakening).

Can be used to identify the region of a storm where new cell growth is occurring which is particularly useful for NWS WFO warning polygon orientation.

Example Images

Fig. 1: Vertically Integrated Ice (VII) using the updated minimum value for coloring (VII values >= 0.5 kg*m^-2) introduced in MRMS version 11.5.