A boundary layer leak slot is devised to improve flow quality in an open return wind tunnel and is tested in the large meteorological wind tunnel of the Meteorological Research Institute. The feature of the slot is that the forcible exhaust, as adopted in many automobile wind tunnels, is not used. The most effective width for improvement is determined to be 40 cm, by an experiment in which the slot width defined as the streamwise opening length is varied from 0 cm to 100 cm. Installing the boundary layer leak slot upstream of the test section remarkably improved the wind velocity uniformity in the cross-direction to the centerline of the wind tunnel in the upper reaches of the test section. The improvement is not clear in the lower reaches, however. The deterioration of uniformity downstream was caused by irregular floor-surface roughness resulting from long-time use. It can be concluded that the boundary layer leak slot is an effective low-cost method of improving flow quality in an open return wind tunnel.
Experiments related to twentieth century historical climate changes and the twenty-first century scenario are performed with the latest version of the MRI climate model, MRI-CGCM2.3. The model reproduces globally averaged surface air temperature (SAT) variations in the twentieth century with satisfactory agreement with interdecadal changes of the observed trend, as well as with the overall SAT increase of 0.5°C at the present-day compared to the pre-industrial level. The globally averaged SAT rises 2.4°C in the late twenty-first century in the experiment for scenario A1B of the Special Report on Emissions Scenarios (SRES) by the Intergovernmental Panel on Climate Change (IPCC). The spatial structures of the simulated trends for the late twentieth century are validated in various atmospheric fields through comparisons with observed data, which indicate that the model demonstrates reasonable agreement with the observed trend in each field. Most of the simulated changes for the twenty-first century reveal spatial patterns similar to those in the trends that appeared in the late twentieth century. The simulated trend patterns of the sea-level pressure (SLP) in both hemispheres bear a resemblance to the observed trends, with each spatial structure reminiscent of the annular modes in the northern and southern hemispheres (NAM and SAM). These SLP trend patterns are consistent with the trends in the SAT, precipitation, and zonal mean zonal wind fields, as in the NAM and SAM. The coherent trend structures of these fields are projected to be enhanced in twenty-first century climate changes. Changes of ocean and sea-ice in association with these atmospheric changes are also described.
This study addresses the changes in the probability density function (PDF) of the 500-hPa geopotential heights during El Niño and La Niña events using the dynamical seasonal hindcasts made at the National Center for Environmental Prediction. First, two-way layout analysis of variance (ANOVA) is applied to the hindcasts to assess the effects of lead time on the simulated climatological mean and variance. Results demonstrate that there is no statistically significant effect of lead time in DJF. However, there is a significant effect of lead time on the climatological mean for JJA over northeastern China, due to the initial soil wetness. This effect on anomalies can be eliminated by using the climatological mean with different lead times. Based on this analysis, we combined four 10-member ensembles with different lead times to increase the ensemble members to 40, which allows us to examine the statistical significance of the change in the PDF. We applied non-parametric Kolmogorov-Smirnov tests to the changes for the El Niño and La Niña events. Results indicate that the changes in the PDF are significant during these events over most of the globe and have unique features for each event, but the major factor is the change in the mean.