Influence of the North American Monsoon Experiment 2004 enhanced soundings on NCEP operational analyses

During the North American Monsoon Experiment (NAME) 2004 Field Campaign (July 1-September 15, 2004) an extensive set of enhanced atmospheric soundings was gathered over the Southwest US and Mexico. Most of these soundings were assimilated into the NCEP operational global and regional data assimilation systems in real-time. This presents a unique opportunity to carry out a series of data assimilation experiments to examine their influence on NCEP analyses and short range forecasts. To quantify these impacts, several data withholding experiments were carried out using the global Climate Data Assimilation System (CDAS), the Regional Climate Data Assimilation System (RCDAS) and the operational 12-km EDAS.

Overall, the influence of the enhanced sounding data is concentrated over the core monsoon area. While differences at upper levels are small, the differences at lower levels are more substantial. Overall, the soundings improve the analyses and forecasts over areas where the uncertainties are greatest.

The coarse resolution CDAS does not properly resolve the Gulf of California (GOC), so the assimilation system is not able to exploit the additional soundings to improve characteristics of the Gulf of California Low-Level Jet (GCLLJ) and the associated moisture transport in the GOC region. In contrast, the RCDAS has a tendency to over analyze the GCLLJ, but the soundings improve the low level winds and specific humidity. The GCLLJ with the NAME 2004 soundings is more realistic. The soundings improve the magnitude of the Great Plains low level jet.

The impact of soundings is compared with the impact of the precipitation assimilation. Their influence on short range forecasts will be discussed in the conference.