A discrepancy in precipitable water among reanalyses and the impact of forcing dataset on downscaling in the tropics

Abstract. Seven major reanalyses of precipitable water (PW) are compared in this paper. In addition, using a regional climate model, we also investigated the impact of the boundary conditions on downscaling simulations in the tropics with a particular focus on the differences in the absolute value of PW among reanalyses.

Results showed that the absolute amounts of PW in some reanalyses were very small compared to the observation, although most spatial patterns of PW in the reanalyses agreed closely with the observation. Particularly over the tropics, most of reanalyses tended to have dry biases throughout the annual cycle. The range of inter-reanalysis dispersion in the tropical mean PW is very large compared with their seasonal variations of the tropical mean PW. In addition, the discrepancies of the 12-yr mean PW in July over the Southeast Asian monsoon region among the reanalyses exceeded their inter-annual standard deviation of the PW. Therefore, the inter-reanalyses dispersion in the tropical PW is significantly large.

We also conducted the downscaling experiments, which were forced by the different four reanalyses. The spatial and temporal variations of atmospheric circulation, including monsoon westerlies and various disturbances, were very similar among the reanalyses. However, the simulated precipitation was 40% less than the observed precipitation amounts, although the dry bias in the boundary conditions was only 6%, and the simulated atmospheric circulation was also basically the same. This result indicates that the dry bias has large effects on precipitation in downscaling experiments over the tropics even if atmospheric circulation is well simulated. Downscaled models can provide realistic simulations of regional tropical climates only if the boundary conditions include realistic absolute amounts of PW. Use of boundary conditions that include realistic absolute amounts of PW in downscaling in the tropics is imperative at the present time.