development of a hybrid probabilistic forecast scheme for ... fileintroduction procedure results...

Introduction Procedure Results Summary

Development of a hybrid probabilistic forecastscheme for post-frontal shower precipitation

Tanja Weusthoff

Institute for Meteorology and Climatology,Leibniz Universitat Hannover

PhD-Students and Post-Docs Meeting on QPF 06.03.2007

Tanja Weusthoff Post-frontal shower precipitation


Contents

IntroductionSynoptic situationObjectivesDefinitions

ProcedureData materialCluster tracking

Some resultsGeneral observationsGeometrical structureInvestigation of rain rates

Summary and Outlook


Introduction Procedure Results Summary Situation Objectives Definitions

Contents




Summary and Outlook



Post-frontal situation

I Rear side of coldfront

I Trough over NorthSea or AtlanticOcean

Berliner Wetterkarte.






I Cold, wet air aboverelatively warmsurface

Berliner Wetterkarte.







I Unstablestratification up to3 to 7 km








I Typical convectivestructure

DLR.Tanja Weusthoff Post-frontal shower precipitation







I Typical convectivestructure



Objectives

I Analysis of post-frontal shower precipitation and derivation ofprinciples.

I Improvement of understanding of structure and developmentof post-frontal shower cells.

I Validation of model forecast.

I Development of a hybrid probabilistic forecast scheme forpost-frontal shower precipitation:



Planned forecast scheme

Hybrid forecasting scheme for convective shower precipitationbehind cold fronts over Germany:





Step 1 Deterministic forecast of cold fronts, related post-frontalconvective precipitation fields and mean rain area motionfield based on NWP model output (LM of the DWD).






Step 2 Probabilistic forecast of

(i) number of rain areas,(ii) total rain area,(iii) areal precipitation,(iv) local precipitation likelihood,(v) mean maximum local precipitation,(vi) mean rain duration and other quantities.






Step 2 Probabilistic forecast of

(i) number of rain areas,(ii) total rain area,(iii) areal precipitation,(iv) local precipitation likelihood,(v) mean maximum local precipitation,(vi) mean rain duration and other quantities.

Result Hybrid probabilistic forecast scheme.



Some Definitions

Rain areaArea in radar data whererain rate R > threshold(here: 0.05 mm / 5 minwhich is about 19 dBz).

Maximum

Single cell, cluster

Maximum number



Some Definitions

Rain area

MaximumPixel of 1 km2 areawhich is a localmaximum of rain rate,also: Peak.


Maximum number



Some Definitions

Rain area

Maximum


Single cell Rain areawith onemaximum.

Cluster Rain areawith morethan onemaximum.

Maximum number



Some Definitions

Rain area

Maximum


Maximum numberNumber of maximainside a cluster.


Introduction Procedure Results Summary Data material Cluster tracking

Contents




Summary and Outlook



The RZ composite of the DWD

I Composite of 16 local radarstations distributed overGermany: DWD Radarverbund.

I Horizontal resolution: 1 km.

I Temporal resolution: 5 min.

I Rain rates with0.01 mm / 5 min.



Preparation of data

I Identifying days with post-frontal showers (17 so far processedin 2004 and 2006),

I extracting area of interest,

I label rain areas,

I determination of characteristics:



Preparation of data

I Identifying days with post-frontal showers (17 so far processedin 2004 and 2006),

I extracting area of interest,I label rain areas,I determination of characteristics:

0 time

1 label number

2 area

3 maxima number

4 mean rain rate

5 rain sum

6 maximum rain rate

7 x-value

8 y-valueTanja Weusthoff Post-frontal shower precipitation


Cluster tracking: searching box



Example data set

18 precursor

19-22 successor



Transitions



Transitions

Five different life stages could be identified:

(i) genesis



Transitions


(i) genesis

(ii) growth(including merging)



Transitions


(i) genesis


(iii) stagnation



Transitions


(i) genesis


(iii) stagnation

(iv) decrease(including splitting)



Transitions


(i) genesis


(iii) stagnation

(iv) decrease(including splitting)

(v) dissolving


Introduction Procedure Results Summary General observations Geometrical structure Rain rates

Contents




Summary and Outlook



Diurnal cycle

... of number of rain areas



Diurnal cycle

... of number of rain areas and number of maxima per rain area.



Diurnal cycle

... of precipitation covered area



Diurnal cycle

... of precipitation covered area and area per maximum.



Frequency distribution of maximum number: PND

N(p) = N0 · pc0 N0 = 0.72 c0 = −2.21



PND variation from day to day

(17 days)



Diurnal variation of PNDfor all 17 days, intervall length = 3h

I the variance from day to day is slightly larger than the diurnalvariance.



Spatial variation of PND

Theusner, 2007



Spatial variation of PND

Theusner, 2007

I The same slope of PND is observed in all four domains.



Area size distribution

Theusner, 2007

I ... can be fitted by lognormal funcions for each maximumnumber.



Distribution of rain rates- Euler analysis -

I Analysing actual rain rates foreach time step.



Distribution of rain rates- Euler analysis -

I Analysing actual rain rates foreach time step.

I Determination of rain sum andarea of clusters with respect totheir maximum number.



Rain rate per maximum for cluster of different maximumnumber



Rain rate per maximum for cluster of different maximumnumber

p < 10 rain sum ∝ p2

p > 10 rain sum ∝ p



Area per maximum for cluster of different maximumnumber



Area per maximum for cluster of different maximumnumber

p < 10 area ∝ p2

p > 10 area ∝ p



Development of individual rain areas- Lagrangian analysis -

I Cluster development is verycomplex→ combination of individualdevelopment of embedded cellsand interactions with other cellsand clusters.



Development of individual rain areas- Lagrangian analysis -

I Cluster development is verycomplex→ combination of individualdevelopment of embedded cellsand interactions with other cellsand clusters.

I Differentiation between singlecells and single clusters andclusters that are involved inmerging and splitting processes.



Role of single cells



Transition likelihoods (n → m)




(i) genesis

(ii) growth (including merging)

(iii) stagnation

(iv) decrease (including splitting)

(v) dissolving



Simple growth model



Rain sum over whole life time of single cells

I Maximum rain amount is observed within first half of life time.



Rain sum over whole life time of single cells

I rain sum ∝ life span2



Contents




Summary and Outlook



Summary

I Geometrical structure of the post-frontal precipitation fieldcan be described by simple analytical functions anddistributions⇒ Dissertation Michael Theusner (Leibniz UniversitatHannover, 2007)

I Development of convective rain areas is in principleunderstood.

I Further research concentrates on:I Rain amount of clusters with complex life cycle.I Analytical description of development processes.I Development of forecast scheme.



Applications

I Development and test of forecast scheme.

I Verification of model output with respect to the precipitationstructure ⇒ LMK.

I Nowcasting for smaller catchment areas.

I ...



Thank you!


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