Analysis of the characteristics of gravity waves during a local rainstorm event in Foshan,China

A detailed analysis of the dynamic frequency spectrum characteristics of gravity waves(GWs)during a local heavy rainfall event on 20–21 November 2016 in Foshan,China,is presented.The results of this analysis,which was based on high-precision microbarograph data,indicate that GWs played a key role in generating the rainstorm.The GWs experienced two intermittent periods of amplitude enhancement and period widening.The largest amplitudes of the GWs were 80–160 Pa,with a corresponding period range of 140–270 min,which were approximately 4 h ahead of the rainstorm.The severe storms appeared to affect the GWs by augmenting the wave amplitudes with center amplitudes of approximately 80–100 Pa and periods ranging between 210 and 270 min;in particular,the amplitudes increased to approximately 10 Pa for GWs with shorter periods(less than 36 min).The pre-existing large-amplitude GWs may be precursors to severe storms;that is,these GWs occurred approximately 4 h earlier than the time radars and satellites identified convections.Thus,these results indicate that large-amplitude GWs constitute a possible mechanism for severe-storm warning.

Trends in day-to-day variability of surface air temperature in China during 1961-2012

Using daily mean, maximum, and minimum surface air temperature （abbreviated as Tmean, Tmax, and Tmin, respectively） data from CN05.2 and the Met Office Hadley Centre observation data-sets for 1961-2012, the trends in day-to-day variability of Tmean, Tmax, and Tmin （abbreviated as DVTFmean, DVTTmax, and DVTTmin, respectively） are examined. It is revealed that the annual trends of DVTTmean, DVTTmax, and DVTTmin are all negative in Northeast China （NEC）, and more obvious in North China than in South China. Seasonal trends of DVTTmean are also negative in NEC, except in summer. For DVTTmax, trends are generally more obvious than DVTTmin in all seasons in NEC; moreover, trends in spring are obvious in both the north and the south, but trends in summer and winter are more obvious in the southern region than in the northern part. As far as DVTT-Fmin is concerned, except in autumn, seasonal trends are more obvious in the north than in other regions of the country.

Interdecadal Variations of Cold Air Activities in Northeast China during Springtime

Based on the daily mean temperature data of CN05.2 from 1961 to 2012, cold events （CEs） are first divided into two categories according to their duration： strong cold events （SCEs） and weak cold events （WCEs）. Then, the characteristics of CEs, SCEs, and WCEs during springtime are investigated. The results indicate that in the pre-1990s epoch, ENSO and Arctic Oscillation events in the previous winter are closely related to SCEs in the following spring. The multidecadal variations of CEs, SCEs, and WCEs are obvious. The intensity trend for SCEs is significantly negative, but it seems less apparent for WCEs. Further analysis reveals that when both SCEs and WCEs occur, a typical East Asian trough in the 850- hPa wind field, whose northwesterly wind component invades Northeast China （NEC） and causes freezing days, can be found in every decade. For the SCEs, a cold vortex, with its center located over Okhotsk and northeasterly current affecting NEC, is found as an additional feature. For the WCEs, the cold vortex is located in Karafuto and its northwesterly airflow intrudes into NEC. As for the difference between SCEs and WCEs, the northwestern flow is weaker while the northeastern counterpart is stronger during the SCEs, in all decades. In the Takaya-Nakamura flux and divergence fields, for the SCEs, a divergence center exists over NEC; and over its downstream regions, a stronger divergence center appears, not like a wave train. However, the opposite is the case for the WCEs; moreover, the wave train appears clearly during the WCEs, which means that the wave energy can propagate and dissipate more easily during WCEs.