| dc.description.abstract | GPS radio occultation (RO) refractivity was retrieved with the assumption of spherical symmetry, possessing representative errors for the true state. Sokolovskiy et al. thus suggested a nonlocal observable (excess phase, defined as the integrated refractivity along a fixed raypath) to account for the effect of horizontal gradients existing in the true state, and this forward model was called the nonlocal operator. One of the main goals of this study is to develop the nonlocal operator for implementation into WRF 3DVAR. In addition, we estimated the observational errors for both excess phase and refractivity data during two months in 2007, one in summer and the other in winter. The observational errors of excess phase are roughly one half of the refractivity errors, regardless of the winter or summer month. Both observational errors are dependent on latitude, but such dependence is stronger at the lower levels in the winter month and is less pronounced in the summer month.
This study investigates the performance of the nonlocal operator in typhoon forecast for Shanshan (2006) using WRF. The model initial increments from the nonlocal operator are slightly larger and are somewhat extended along the raypath in elliptical contours as compared to those from the local operator. Typhoon track and recipitation forecasts show slight improvement for the first day when 27 refractivity soundings are assimilated into WRF. For local rainfall forecast at later time, the performance of the nonlocal operator is slightly better than the local operator. Verified against GPS RO observations for the environmental prediction, assimilation of GPS RO data shows improvement of early prediction (especially for the levels below 5 km) as the conventional GTS data are also assimilated; however, no significant improvement was found at later prediction for both nonlocal operator and local operator. As a bogus vortex is also combined into assimilation, initial typhoon intensity is enhanced and thus results in better track and rainfall prediction. The impact of bogus vortex overwhelms the impacts of all other data in this study. The cycling run shows that typhoon intensities and tracks can be further improved when more observational data can be assimilated at different times. In general, the impact of GPS RO data assimilation increases the skill score of prediction for local intense daily rainfall.
Observing Systems Simulation Experiments (OSSE) were designed to explore the impact of GPS RO data assimilation on weather prediction using high-resolution MM5 model for the modeled atmosphere providing the true state. Vertical soundings of nonlocal refractivity as modeled by a 2-D raytracing operator under the assumption of spherical symmetry then were assimilated into OSSE. The nonlocal refractivity soundings were produced for three assimilated areas, AREA1 to the northeast of the typhoon over the edge of Pacific high, AREA2 to the east of the typhoon over the environmental flow, and AREA3 over the region of typhoon circulation, with 30, 96 and 16 soundings, respectively. Verified against the true state, the initial analysis errors in AREA1 and AREA3 are quite different but are similar in AREA2 for different operators, possibly due to weaker horizontal gradients existing in the latter. Slight improvement in track prediction is found for all the operators when the 30 soundings in AREA1 were ingested. The initial condition and the ensuing tracks were more improved for the nonlocal operator than local operator as the 16 soundings in AREA3 were assimilated. The track prediction with assimilation in AREA 2 becomes worse compared to that without assimilation. It appears that typhoon track is sensitive to initial tiny discrepancies in this upstream steering environment, which can be produced by the refractivity soundings as simulated by the model. | en_US |