Improving Forecasts of Asian Monsoon Rainfall Using COSMIC Radio Occultation Refractivity Observations

Monsoon rainfall is very important for the economy of Asia but its

prediction is still not satisfactory. One reason is a lack of

good observations of the subtropical high over the

Western Pacific ocean.

The newly available COSMIC GPS radio occultation (RO) refractivity

soundings provide good coverage globally including the Western

Pacific ocean. The measurements contain valuable water vapor and

temperature information with high vertical resolution in the lower

troposphere and are not contaminated by clouds or precipitation.

The DART/WRF ensemble filter/prediction system is used to examine

the impact of COSMIC GPS refractivity

observations on forecasts of heavy monsoon rainfall during June

1-14, 2007. Radiosondes,

AIRS retrieved temperature, and satellite cloud/water vapor drift

winds are assimilated using WRF with a 36 km resolution for

the period of June 1-14, 2007 in a control case. A comparison is

made to an assimilation using all of these observations plus GPS

RO data. Assimilating the GPS

RO data leads to a better defined analysis of the subtropical high

and its circulation and significantly reduces the bias of the water vapor

analysis over eastern Asia. Forecasts of the heavy monsoon

rainfall events are significantly improved by adding the GPS RO

refractivity observations.