blur, the distinctions are often meaningful from the viewpoint of current product development techniques. The ideal system would have seamless continuity, in which a fully implemented four-dimensional data assimilation (4DDA) system could render these distinctions obsolete. The integrated observing system should provide a comprehensive 4-D database from which various users can draw those portions that are pertinent to their applications.

The following types of data limitations reduce the effectiveness of the current NEXRAD products:

• data voids,

• data corruption,

• degraded resolution at long range, and

• data latency (update rate).

Data voids have many causes (as discussed in Chapter 2), but the end result is the denial of desired data to a product algorithm in some portion of the coverage volume.

Data corruption usually results from a combination of factors, and the impact varies between minimal and severe. Product degradation can take the form of an enlarged data void, when contaminated data are detected and masked, or of erroneous product results, when incorrect data are passed on to the product algorithms. Advanced engineering techniques can reduce, but probably not totally eliminate, data corruption. Experience has shown that the integration of data quality analysis (DQA) with a data assimilation system is an effective way of detecting and masking erroneous data in order to prevent the introduction of faulty information into the product algorithms.

Data latency results from the use of a scan rate that is slow compared with the requirements of an effective product algorithm. Different products and different atmospheric situations impose different data update requirements. To alleviate data latency issues for all algorithms, it is necessary either to have a fixed scan strategy that is suitably rapid for all circumstances or to have an adaptive scan strategy that can concentrate radar resources effectively. Operational mechanical scanning radars are usually restricted to 360-degree scans, with each rotation taking 15–20 seconds. Faster scanning results in reduced sensitivity. Single-beam radar collects data from one tilt per revolution. Simple counting provides the relationship between the rotation time, the number of tilts, and the volume scan time. Continuing the current design would provide little room for improvement over the NEXRAD. Fundamental changes in the basic system, such as use of phased array technologies, would provide data updates at rates necessary for all critical products.