HIGH-RESOLUTION RADAR MOSAICS

A DEMONSTRATION PROJECT OF THE NATIONAL WEATHER SERVICE

July 2002

INTRODUCTION

These mosaics (geographic composites) are produced in real time as a cooperative test and demonstration project between the National Weather Service (NWS) Office of Science and Technology's Meteorological Development Laboratory and the National Center For Environmental Prediction Central Operations Branch. They are currently produced on a 4-processor microcomputer running at the NOAA Science Center in Camp Springs, Maryland.

Our aim is to determine the processing requirements for preparing the full suite of mosaics at 5-minute intervals, and to develop and test methods for identifying and removing nonprecipitation echoes from reflectivity mosaics. Higher NWS management will soon review these results and render a decision on making the program fully operational.

THE MOSAIC PROCESS

Products from individual radars are disseminated as 'images' in range-direction coordinates or east-west coordinates relative to the radar site. The resolution is typically 1 degree of azimuth by 1 kilometer (km) for base reflectivity products, and 4x4 km for products such as composite reflectivity, vertically-integrated liquid, and rainfall. The latitude and longitude of each datum is calculated through standard geometric formulas. The data can then be navigated on a larger-scale national map grid, such as the Lambert Conformal projection used here. The approximate grid mesh is 2 km for based reflectivity products and 4 km for other products.

PRODUCT DISSEMINATION

As experimental products, these products are disseminated in digital form to only a limited group of NWS national forecast centers through a wide-area network that carries other products as well, and in graphical form through this web site. If implemented operationally, the products will be made available in digital form by FTP and through multicast processes.

AVAILABILITY OF PRODUCTS IN THE FUTURE

Updates will be announced at http://weather.gov/mdl/

PRODUCT CONTENT

Base Reflectivity (edited)

Reflectivity products show the location and approximate intensity of precipitation. Nonprecipitation echoes such as ground clutter and return from migrating birds have been identified and removed through an automated editing process. This process compares the reflectivity field to satellite observations and relative humidity measurements, and compares the statistical properties of the local echo spectrum to those associated with echoes other than rain or snow. Echo areas which appear unlikely to be precipitation are removed from the field.

As is often apparent, this procedure generally leaves some nonprecipitation echoes in the final product, though we are actively working on refinements. Less often, real precipitation is removed.

Base Reflectivity (unedited)

This product contains all echoes that appear in the reflectivity products from individual sites. Ground clutter is seen near most sites. Echoes from migrating birds (spring and autumn) and insects (summer) appear as large areas of low-intensity echoes surrounding most stations outside the Intermountain region during the warm season. More rarely, anomalous propagation (AP) appears as areas of high-intensity echoes with little spatial coherence. Some nonprecipitation features, such as gust front boundaries marked by clouds of insects, are important to forecasters. Therefore this unedited product is often used by meteorologists.

Vertical Composite Reflectivity (unedited)

This product shows the highest reflectivity at any level in the vertical between the ground and approximately 30,000 feet, information of importance primarily to aviation interests. The measurements are obtained by tilting the radar antenna upward by steps of about 2 , as high as 19 from the horizontal, a process called volumetric scanning. The field is unedited, and therefore returns from ground clutter and biological targets appear along with precipitation.

Vertically-integrated liquid (VIL)

The VIL product shows the vertical integral of reflectivity in terms of mass of liquid water per unit area, obtained during the volumetric scanning process. The reflectivity-to-water concentration conversion is based on an empirical formula. The standard display unit, kilograms per square meter (kg m^-2 ), is equivalent to water depth in centimeters if all the liquid were to be precipitated and collected. In this image, thunderstorm cores appear as regions with values of 10  kg m^-2 or higher. VIL values in excess of 50 kg m^-2 often indicate large hail and/or damaging winds at the surface.

Echo Tops

Another product obtained by volumetric scanning, this indicates the highest altitude above mean sea level at which the reflectivity exceeds 18 dBZ. This is roughly the altitude to which precipitation extends. In strong thunderstorms, the echo top can approach 50,000 feet.

Rainfall Accumulations

These products are obtained by converting reflectivity to rainfall rate through an empirical formula, and integrating the rainfall rate in time. Though the rain amounts are a rough approximation to measurements from gauges, the product clearly shows the location and timing of intense precipitation, and is often used in deciding on the issuance of flash flood warnings. The products are also used in preparing hydrologic forecasts of water flow on major rivers.

NOTE: Though internal editing procedures remove most nonprecipitation echoes from the rainfall calculation, some, particularly features due to migrating birds, can remain.