Shuyun Zhao

Cut‐off low pressure systems over southern Australia: climatology and case study

Cut-off low-pressure systems are among the most important weather systems that affect southern Australia. They can bring moderate to heavy rainfall over large areas and as such are extremely important to the Australian agricultural industry. There are two components of this study. First, a statistical analysis is carried out on 14 years of archived data (1983–1996) to produce a climatology. The emphasis in the climatology is on the frequency, duration and tracks (location and movement) of cut-off lows in southern Australia. Second is a case study of the cut-off low of late August 1997, in which the distinctive features of cut-off lows are highlighted. The results from the climatology reveal the following: there is a maximum in the frequency of cut-off lows in southern Australia during May–October; the most active area for the genesis of cut-off lows in southern Australia is the southwestern region; and the vast majority of cut-off lows move either eastward or southeastward. The case study is aimed at revealing details of the formation and structure of one particular ‘classical’ example of a cut-off low. It is found that there is strong baroclinicity in the mid-upper troposphere, favourable for the intensification of the low. The synoptic analysis also shows that the moisture supply for the precipitation was transported from the tropical ocean (the Coral Sea) to higher latitude regions by a low level jet (LLJ), forming a narrow moist tongue which interacted with the cut-off low and its associated cold front. The cut-off low was located at the right-front quadrant of an upper level jet (ULJ), which is a favourable position for low system development. Copyright

A modeling study of effective radiative forcing and climate response due to tropospheric ozone

This study simulates the effective radiative forcing (ERF) of tropospheric ozone from 1850 to 2013 and its effects on global climate using an aerosol–climate coupled model, BCC AGCM2.0.1 CUACE/Aero, in combination with OMI (Ozone Monitoring Instrument) satellite ozone data. According to the OMI observations, the global annual mean tropospheric column ozone (TCO) was 33.9 DU in 2013, and the largest TCO was distributed in the belts between 30°N and 45°N and at approximately 30°S; the annual mean TCO was higher in the Northern Hemisphere than that in the Southern Hemisphere; and in boreal summer and autumn, the global mean TCO was higher than in winter and spring. The simulated ERF due to the change in tropospheric ozone concentration from 1850 to 2013 was 0.46 W m−2, thereby causing an increase in the global annual mean surface temperature by 0.36°C, and precipitation by 0.02 mm d−1 (the increase of surface temperature had a significance level above 95%). The surface temperature was increased more obviously over the high latitudes in both hemispheres, with the maximum exceeding 1.4°C in Siberia. There were opposite changes in precipitation near the equator, with an increase of 0.5 mm d−1 near the Hawaiian Islands and a decrease of about −0.6 mm d−1 near the middle of the Indian Ocean.