EVALUATION OF A NEW APPROACH TO MICROWAVE REFRACTIVITY ANALYSISWilliam T. Thompson, Stephen D. Burk,

Naval Research Laboratory, Monterey, CA

Michael Jablecki and Ted RogersSPAWARS, San Diego, CA

Mesoscale numerical model analyses and forecasts of the microwave refractivity field, M, and its impact on EM propagation remains a significant challenge due to several factors: (i) the relatively small vertical scale of significant M features and (ii) subtle changes in the M-profile can dramatically im-pact the propagation environment (e.g., transition from elevated to surface-based ducting). A unique data fusion technique is used here in conjunction with NRL’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to analyze refractivity. This technique combines information from observed soundings with model forecasts valid at the same time (i.e., a model ‘back-ground’ field). A height-shifting and stretching algorithm is applied to the background refractivity field in order to bring it into close agreement with a given sounding (or soundings). The influence of this shifting and stretching is then broadcast over a region of influence, S, and used to alter the background refractivity field. Soundings were launched every 4 h (except midnight) at ~10 stations during the two-week long Variability of Coastal Atmospheric Refractivity (VOCAR) experiment in the South-ern California Bight. COAMPS forecasts are produced throughout the VOCAR period and are used to statistically evaluate this new data fusion technique. The shifting technique is applied throughout the VOCAR period at a selected sounding location and the impact of the data fusion technique on model refractivity profiles is evaluated at three surrounding sounding locations. The special VOCAR soundings are of great value here in validating this data fusion tech-nique, but once operational the approach will not require such high data density.

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Preliminary Results from VOCAR Data Fusion Tests

Statistics at SNI, PVN, and NUC when NPS Ship is the Base Point

50 Soundings Original COAMPS® Shifted COAMPS®

<DBH> 476 m 522 m 451 mDBH-Bias --- 46 m -25 m<M> 28 33 31M-Bias --- 5 3

Soundings/ Orig. COAMPS® Soundings/Shifted COAMPS®

RMSE1(DBH) 189 m 139 m

RMSE(M) 23 21

1RMSE() = 1/N(j,s - j ,f)2

N=50 soundingswhere j = DBH or M

j=1

N

• Statistics not available yet for cases with multiple Base Point soundings

COAMPS® is a trademark of the Naval Research Laboratory

26% improvementin DBH at locations~75km from sounding

This technique could be utilized to ingest soundings in the vicinity of a carrier strike group in order to provide an improved refractivity analysis for pilot briefing and asset protection

USS GEORGE WASHINGTON Strike Group