Tropical Cyclone Prediction on Subseasonal Time-Scales

Here we discuss recent progress in understanding tropical cyclone (TC) subseasonal variability and its prediction. There has been a concerted effort to understand the sources of predictability at subseasonal time-scales, and this effort has continued to make progress in recent years. Besides the Madden-Julian Oscillation (MJO), other modes of variability affect TCs at these time-scales, in particular various equatorial waves. Additionally, TC activity is also modulated by extratropical processes via Rossby wave breaking. There has also been progress in the ability of models to simulate the MJO and its modulation of TC activity. Community efforts have created multi-model ensemble datasets, which have made it possible to evaluate the forecast skill of the MJO and TCs on subseasonal time-scales in multiple forecasting systems. While there is positive skill in some cases, there is strong dependence on the ensemble system considered, the basin examined, and whether the storms have extratropical influences or not. Furthermore, the definition of skill differs among studies. Forecasting centers are currently issuing subseasonal TC forecasts using various techniques (statistical, statistical-dynamical and dynamical). There is also a strong interest in the private sector for forecasts with 3-4 weeks lead time

Convectively-coupled Kelvin waves over the tropical Atlantic and African regions and their influence on Atlantic tropical cyclogenesis

High-amplitude convectively coupled atmospheric Kelvin waves (CCKWs) are explored over the tropical Atlantic during the boreal summer. Atlantic tropical cyclogenesis is found to be more frequent during the passage of the convectively active phase of the CCKW, and most frequent two days after its passage. CCKWs impact convection within the mean latitude of the inter-tropical convergence zone over the northern tropical Atlantic. In addition to convection, CCKWs also impact the large scale environment that favors Atlantic tropical cyclogenesis (i.e., deep vertical wind shear, moisture, and low-level relative vorticity). African easterly waves (AEWs) are known to be the main precursors for Atlantic tropical cyclones. Therefore, the relationship between CCKWs and AEW activity during boreal summer is explored. AEW activity is found to increase over the Guinea Highlands and Darfur Mountains during and after the passage of the convectively active phase of the CCKW. First, CCKWs increase the number of convective triggers for AEW genesis. Secondly, the associated zonal wind structure of the CCKW is found to affect the horizontal shear on the equatorward side of the African easterly jet (AEJ), such that the jet becomes more unstable during and after the passage of the convectively active phase of the CCKW. The more unstable AEJ is assumed to play a role with increased AEW growth. Through the increased number of AEWs propagating over the tropical Atlantic, as well as from the direct impact on convection and the large-scale environment over the tropical Atlantic, CCKWs are recommended to be used as a means for medium-range predictability of Atlantic tropical cyclones. In addition to modulating tropical cyclone activity over the tropical Atlantic, CCKWs might impact the intensification processes of tropical cyclones. A case study highlighting two August 2010 tropical cyclones (Danielle and Earl) is explored for potential CCKW-tropical cyclone interactions. While predicted to intensify by most model guidance, both Danielle and Earl struggled to do so. It is shown that Danielle and Earl interacted with the convectively suppressed phase of an eastward propagating CCKW during the time they were predicted to intensify. Composite analysis shows that during and after the passage of the convectively suppressed phase of the CCKW over the Atlantic, large-scale vertical wind shear increases as a result of anomalous upper-level westerlies collocated with anomalous lower-level easterlies. Large-scale subsidence associated with the convectively suppressed phase of the CCKW causes the atmosphere to dry. Further, when the upper-level westerly wind anomalies associated with the CCKW are located over the equatorial Atlantic, a tropical upper-tropospheric trough (TUTT) develops over the northern tropical Atlantic. TUTTs are upper-level disturbances known to negatively impact the intensity of tropical cyclones. CCKWs over the tropical Atlantic tend to occur during preferable locations of the Madden-Julian Oscillation (MJO). Results show that the MJO significantly modulates Atlantic tropical cyclogenesis using real-time multivariate MJO indices. Like CCKWs, AEW activity is found to vary coherently with MJO passages. Furthermore, the MJO also impacts the large-scale environment that favors for Atlantic tropical cyclogenesis. Therefore in addition to CCKWs, the state of the MJO should be used for Atlantic tropical cyclogenesis medium-range predictability

Advances in tropical cyclone prediction on subseasonal time scales during 2019–2022

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

Containment measures

OBSOLETE (project finished) - Description of containment measures during COVID'19 lockdown, in the context of SIlent Cities project. Please request access to Silent Cities if neede

Archived - General Information (DO NOT USE)

DO NOT USE - The goal of this component was to document the data collection process of the Silent Cities Dataset. This component is just left for archive