6 research outputs found

    Orographic effects on convective precipitation and space-time rainfall variability: preliminary results

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    International audienceIn the EFFS Project, an attempt has been made to develop a general framework to study the predictability of severe convective rainfall events in the presence of orography. Convective activity is embedded in orographic rainfall and can be thought as the result of several physical mechanisms. Quantifying its variability on selected area and time scales requires choosing the best physical representation of the rainfall variability on these scales. The main goal was (i) to formulate a meaningful set of experiments to compute the oscillation of variance due to convection inside model forecasts in the presence of orography and (ii) to give a statistical measure of it that might be of value in the operational use of atmospheric data. The study has been limited to atmospheric scales that span the atmosphere from 2 to 200 km and has been focused on extreme events with deep convection. Suitable measures of the changing of convection in the presence of orography have been related to the physical properties of the rainfall environment. Preliminary results for the statistical variability of the convective field are presented

    An intercomparison between low-frequency variability indices

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    Possible connections between spatial patterns, of limited regional extent and identified in teleconnectionpatterns and in blocking climatology studies, with hemispheric planetary-wave activitymodes defined by the wave amplitude index (WAI) are investigated. The WAI probabilitydensity function (PDF) for the northern extratropics winter fields is estimated and the sensitivityof the WAI distribution to the presence of low-frequency variability modes is evaluated bystratifying the available dataset according to the sign of blocking and teleconnection indices.It is found that low-frequency variability modes affect both the mean and the variance of thewave amplitude index. Both the positive phase of the North Atlantic Oscillation (NAO) andthe negative phase of the Pacific North American pattern (PNA) are associated with anenhanced frequency of very large amplitude planetary waves. Furthermore, distributions characterisedby a maximum corresponding to high WAI values also exhibit a large variance. NegativeNAO and positive PNA influence the mean and the variance of WAI PDF in the oppositesense. Similar results are found when the blocking index is considered. WAI PDFs relative tohighly blocked months are broader with a secondary maximum corresponding to very highWAI values. DOI: 10.1034/j.1600-0870.1999.00016.

    Orographic effects on convective precipitation and space-time rainfall variability: preliminary results

    No full text
    In the EFFS Project, an attempt has been made to develop a general framework to study the predictability of severe convective rainfall events in the presence of orography. Convective activity is embedded in orographic rainfall and can be thought as the result of several physical mechanisms. Quantifying its variability on selected area and time scales requires choosing the best physical representation of the rainfall variability on these scales. The main goal was (i) to formulate a meaningful set of experiments to compute the oscillation of variance due to convection inside model forecasts in the presence of orography and (ii) to give a statistical measure of it that might be of value in the operational use of atmospheric data. The study has been limited to atmospheric scales that span the atmosphere from 2 to 200 km and has been focused on extreme events with deep convection. Suitable measures of the changing of convection in the presence of orography have been related to the physical properties of the rainfall environment. Preliminary results for the statistical variability of the convective field are presented

    Competing Effect of Radiative and Moisture Feedback in Convective Aggregation States in Two CRMs

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    Abstract The radiative‐convective equilibrium (RCE) of two models exhibiting convective aggregation has been compared. The goal of the work, following the suggestion from the RCE Model Intercomparison Project (RCEMIP), is to identify key parameters controlling self‐aggregation in RCE for both models, to discuss the processes controlled by these parameters and to underline the models similarities and differences. The two cloud resolving models studied, the SAM (System for Atmospheric Modeling) and the ARPS (Advanced Regional Prediction System), present similar statistics concerning precipitation, but different warming, and drying of the atmosphere, within the spread of the RCEMIP values. On the other hand, the two models show different strengths of the moisture feedback, due to the different saturation of the sub‐cloud layer. A saturated sub‐cloud layer in ARPS (which was not artificially imposed in the numerical setup) allows the localization of convection in moist regions, by weakening the negative influence of cold pools. Such a mechanism leads to a lower degree of aggregation (based on three organization metrics) and a weaker effect of the organized state on the average domain statistics in ARPS. Stronger cold pools in SAM, instead, help the creation of shallow clouds in dry regions, increasing the longwave feedback responsible for their expansion; while delocalizing convection in moist regions and therefore opposing high‐cloud radiative‐feedback. Further experiments are needed to generalize such findings to other RCEMIP models, also investigating the role of microphysics and turbulence schemes in regulating such mechanisms
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