Sensitivity Analysis of the Super Heavy Rainfall Event in Henan on 20 July(2021)Using ECMWF Ensemble Forecasts

An unprecedented heavy rainfall event occurred in Henan Province,China,during the period of 1200 UTC 19-1200 UTC 20 July 2021 with a record of 522 mm accumulated rainfall.Zhengzhou,the capital city of Henan,received 201.9 mm of rainfall in just one hour on the day.In the present study,the sensitivity of this event to atmospheric variables is investigated using the ECMWF ensemble forecasts.The sensitivity analysis first indicates that a local YellowHuai River low vortex(YHV)in the southern part of Henan played a crucial role in this extreme event.Meanwhile,the western Pacific subtropical high(WPSH)was stronger than the long-term average and to the west of its climatological position.Moreover,the existence of a tropical cyclone(TC)In-Fa pushed into the peripheral of the WPSH and brought an enhanced easterly flow between the TC and WPSH channeling abundant moisture to inland China and feeding into the YHV.Members of the ECMWF ensemble are selected and grouped into the GOOD and the POOR groups based on their predicted maximum rainfall accumulations during the event.Some good members of ECMWF ensemble Prediction System(ECMWF-EPS)are able to capture good spatial distribution of the heavy rainfall,but still underpredict its extremity.The better prediction ability of these members comes from the better prediction of the evolution characteristics(i.e.,intensity and location)of the YHV and TC In-Fa.When the YHV was moving westward to the south of Henan,a relatively strong southerly wind in the southwestern part of Henan converged with the easterly flow from the channel wind between In-Fa and WPSH.The convergence and accompanying ascending motion induced heavy precipitation.

Factors Controlling the Interannual Variations of MJO Intensity

The interannual variations of intensity of the Madden-Julian Oscillation（MJO） during boreal winter are investigated by using the observed outgoing longwave radiation（OLR） and the reanalysis data of ECMWF and NCEP.The standard deviation of 20-80-day filtered OLR anomaly is used to measure the MJO intensity.The dominant spatial structure of the interannual variability is revealed by an EOF analysis of the MJO intensity field.It is found that the leading mode is associated with eastern Pacific type ENSO,whereas the second mode is related to central Pacific type ENSO.A simple atmospheric model is used to investigate the relative roles of background moisture and wind changes in affecting the overall strength of MJO.The numerical experiments indicate that the background moisture effect is dominant while the background wind change has a minor effect.

What Controls Early or Late Onset of Tropical North Atlantic Hurricane Season？

The occurrence of first hurricane in early summer signifies the onset of an active Atlantic hurricane season.The interannual variation of this hurricane onset date is examined for the period 1979-2013.It is found that the onset date has a marked interannual variation.The standard deviation of the interannual variation of the onset day is 17.5 days,with the climatological mean onset happening on July 23.A diagnosis of tropical cyclone（TC） genesis potential index（GPI） indicates that the major difference between an early and a late onset group lies in the maximum potential intensity（MPI）.A further diagnosis of the MPI shows that it is primarily controlled by the local SST anomaly（SSTA）.Besides the SSTA,vertical shear and mid-tropospheric relative humidity anomalies also contribute significantly to the GPI difference between the early and late onset groups.It is found that the anomalous warm（cold） SST over the tropical Atlantic,while uncorrected with the Nino3 index,persists from the preceding winter to concurrent summer in the early（late） onset group.The net surface heat flux anomaly always tends to damp the SSTA,which suggests that ocean dynamics may play a role in maintaining the SSTA in the tropical Atlantic.The SSTA pattern with a maximum center in northeastern tropical Atlantic appears responsible for generating the observed wind and moisture anomalies over the main TC development region.A further study is needed to understand the initiation mechanism of the SSTA in the Atlantic.