Effect of Boundary Layer Latent Heating on MJO Simulations

A latent heating peak in the PBL was detected in a simulation by a global GCM that failed to reproduce Madden Julian Oscillation （MJO）. The latent heating peak in the PBL was generated by very shallow convection, which prevented moisture from being transported to the free troposphere. Large amount of moisture was therefore confined to the PBL, leading to a dry bias in the free atmosphere. Suffering from this dry bias, deep convection became lethargic, and MJO signals failed to occur. When the latent heating peak in the PBL was removed in another simulation, reasonable MJO signals, including the eastward propagation and the structure of its large-scale circulation, appeared. We therefore propose that the excessive latent heating peak in the PBL due to hyperactive shallow convection may be a reason for a lack of MJO signals in some simulations by other GCMs as well.

Advances in tropical cyclone prediction on subseasonal time scales

This review describes advances in understanding and forecasting tropical cyclone(TC)subseasonal variability during the past four years.A large effort by the scientific community has been in understanding the sources of predictability at subseasonal timescales beyond the well-known modulation of TC activity by the Madden-Julian Oscillation(MJO).In particular,the strong modulation of TC activity over the western North Pacific by the Boreal Summer Intra-Seasonal Oscillation(BSISO)has been documented.Progress has also been realized in understanding the role of tropical-extratropical interactions in improving subseasonal forecasts.In addition,several recent publications have shown that extratropical wave breaking may have a role in the genesis and development of TCs.Analyses of multi-model ensemble data sets such as the Subseasonal to Seasonal(S2S)and Subseasonal Experiment(SubX)have shown that the skill of S2S models in predicting the genesis of TCs varies strongly among models and regions but is often tied to their ability to simulate the MJO and its impacts.The skill in select models has led to an increase over the past four years in the number of forecasting centers issuing subseasonal TC forecasts using various techniques(statistical,statistical-dynamical and dynamical).More extensive verification studies have been published over the last four years,but often only for the North Atlantic and eastern North Pacific.