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Orographic Rain Index (ORI) - Basic Information

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Fig. 1. Example of the Orographic Rain Index image product (0300 UTC 5 May 2009) over the CONUS west coast County Warning and Forecast Areas. The units of the product are kg m-1 s-1.The product represents areas where moist air is blowing up mountain slopes. The higher the index,the more likely there will be flash flooding, either at the location or in the river valley below.(Click on figure for full resolution.)

Who is developing and distributing this product?

The Cooperative Institute for Research in the Atmosphere (CIRA) in Fort Collins, Colorado is developing and distributing the ORI product.

Who is receiving this product, and how?

The ORI products are available on the Web at, on a McIDAS server at CIRA, and via an experimental AWIPS connection. They are regularly analyzed by forecasters at the NESDIS Synoptic Analysis Branch (SAB).

What is the product size?

The size of one image in GIF format is approximately 300 KB, in McIDAS AREA format, it is 2.5 MB. Updates are available every 3 hours.

Purpose of this product.

The Orographic Rain Index is designed to indicate to forecasters where there is short-term (0-3 hours) potential for heavy orographic rain. The product has a horizontal resolution of approximately 1 km.

Why is this a GOES-R Proving Ground Product?

This product combines water vapor measurements from various sensors/satellites to determine Total Precipitable Water (TPW). In this way the measurements that will be available during the GOES-R era are simulated. The ORI product thus represents an application that will be available with GOES-R data.

How is this product created now?

Three data sources are used to create the product: Blended TPW from CIRA, which indicates the strength and location of atmospheric rivers impinging on the U.S. West Coast, GFS 850 mb winds (V), which are used to advect the water vapor to a forecast time (every 3 hours), and USGS Global 30 Arc-Second Elevation Data (GTOPO30) terrain elevations (H) (horizontal resolution of approximately 1 km).

CIRA developed the Blended TPW product by combining several data sources. These include Special Sensor Microwave / Imager (SSM/I) passes from the Defense Meteorological Satellites Program (DMSP) and the Advanced Microwave Sounding Unit (AMSU) from Polar Operational Environmental Satellites (POES), and information from Global Positional Satellite (GPS) meteorological data. GOES-West and GOES-East Sounders are used to fill in missing land locations over the continental United States. By combining all of these observations, a smooth product is created.

The blended product provides information over the continental United States primarily from GPS data, and over observation poor regions, such as the oceans, from the SSM/I and AMSU data. The Blended TPW eliminates the bias between data sets and provides an easier and faster product to analyze. This product has been used by forecasters and satellite analysts for years, and they have found it of value in helping to analyze and predict heavy rain and flooding, and also the transfer of moisture from ocean to land. The Blended TPW product is available on AWIPS. The ORI product uses this Blended TPW to calculate an index that represents the amount of moisture blowing up higher terrain. The basic calculation is:

ORI = TPW * V•∇H

TPW has units of kg m-2, V has units of m s-1, and ∇H is unitless (m/m). ORI, then, has units of kg s-1 m-1. Based roughly on the work of Nieman et al. 2008 (J. Hydrometeorology, 9, 22-47) we set 50 kg s-1 m-1 as a threshold below which no rain is likely, to 250 kg s-1 m-1, which definitely deserves the forecasterís attention. (Maximum values are probably in the 500 kg s-1 m-1 range.)

Product Examples and Interpretation.

V•∇H is an indicator of vertical motion caused by wind blowing up hill. (If the wind is blowing down hill, ORI is set to zero.) In other words, V•∇H is an indicator of terrain-induced "lift." TPW is an index of atmospheric moisture. ORI, then, is the product of moisture and terrain-induced lift.

Figure 2 shows the TPW field, the GFS 850 mb winds, and the GTOPO30 terrain used to calculate the ORI product shown in Fig. 1. Figure 3 shows the atmospheric river which was impacting the West coast. The ORI product in Fig. 1 is quite detailed, since the topography used has a horizontal grid resolution of approximately 1 km.

Fig. 2. Winds, TPW, and topography (0300 UTC 5 May 2009) used to produce the ORI in Fig. 1. (Click on figures for full resolution.)

Fig. 3. CIRA Blended TPW (ocean only) showing the tongue of tropical moisture impacting the U.S. West Coast.