Precipitation fine structure

Transcription

1 Precipitation fine structure University of Ferrara Department of Physics and Earth Sciences Leo Pio D'Adderio PhD Student 27 th cycle - Ferrara, December 3 rd, 2013

2 Work done Study of precipitation characteristics over Tibetan plateau during monsoon season with particular interest on variation of drop size distribution (DSD) with respect to altitude and latitude. Detection of breakup occurrence studying DSD shape to recognize the process regardless the physical principle of the instrument. Study of the vertical variability of DSD. Start of a cooperation with NASA to deepen these studies in order to find new DSD parameterization in presence of breakup situations, for a direct application in rain retrieval from DPR (to be launched next year).

3 Precipitation properties over Tibetan Plateau To completion of CEOP-AEGIS Project an overall study on the precipitation properties over Tibetan Plateau has been carried out. 37 rain events during monsoon season were collected from 2 measurement sites: Linzhi 3300 m a.s.l. and Lhasa 3600 m a.s.l. Five case studies (from stratiform to mixed stratiform/convective to deep convective) have been analyzed in terms of rainfall rate and DSD evolution. All minutes with rainfall rate higher than 0.1 mm/h have been analyzed in terms of DSD trend as function of rainfall rate intensity and in terms of Z-R relationship.

4 Two case studies 5 th September 2010, Linzhi mixed Long-lasting precipitation event with two intensity peaks at the beginning and at the end of event (data are averaged over 5 minutes); The central part of event has statiform characteristics with exponential DSD; The peaks reveal different cloud structure, with breakup evident for the ending peak while is absent for the beginning peak.

5 Two case studies 28 th July 2010, Linzhi convective Short convective precipitation event (data are averaged over 2 minutes); For the higher intensity minutes the presence of breakup is evident with an increase of drop number around 2.5 mm; For both cases the maximum dimension reached by drop does not exceed 4 mm of diameter.

6 Integral parameters Change DSD shape and concavity moving from light rain to heavy rain Very low b term in Z = AR b relationship it means limited presence of large drops From case studies: confirmation that drops reach smaller dimension with respect to sea level From case studies: easier reaching of breakup situation due to the higher drops kinetic energy

7 Automatic recognizing of breakup occurrence in DSD Numerical simulations Experimental data Willis and Tattelman, 1988 Pratt and Barros, 2007 Agreement between numerical and experimental results; DSD shows two or three peaks depending on breakup kernel used; The peak around 2.5 mm is a strong feature that is absent when only coalescence contribute in DSD formation; The more recent parameterizations show two peak only (McFarquhar, 2004).

8 How to recognize breakup situations from DSD spectra DSD technique (DS) minutes with RR higher than 8 mmh -1 are selected; the point by point DSD spectrum ascending normalized derivative between 1.55 mm and 2.45 mm diameter is calculated (see figure below); the point by point DSD spectrum descending normalized derivative for diameters higher than 2.45 mm is calculated; the ten minutes with the highest sum of renormalized ascending and descending derivative value are considered as breakup minutes; the central diameter of diametral class with the maximum descending derivative is considered as breakup diameter; finally the mean breakup diameter, with its standard deviation, is estimated for each measuring site.

9 Breakup: power spectra vs DSD spectra The selected Pludix power spectra and the corresponding DSDs for breakup occurrence. The selected DSDs by the DS technique for breakup occurrence for the same site.

10 Breakup recognizing Two main aims: Distinguish breakup situation from no-breakup situation; Estimate the breakup diameter. The algorithm recognizes the breakup situations, but is not yet able to determine automatically which DSD shape marks the transition from breakup to no-breakup and vice versa future studies; The estimated breakup diameter on the whole dataset shows a decrease with altitude.

11 NASA collaboration A recent collaboration with NASA gives me the possibility to access to a very extended disdrometric database; Different measuring sites: Iowa (Ifloods, ~41000 mins), Finland (Lpvex, ~8100 mins), Italy/France (Hymex ~10300 mins), Oklahoma (Mc3e, ~10000 mins); The aim is to study a different parameterization for breakup DSD, with respect to that used at the moment, for radar/satellite retrieval. These studies and collaboration are in the frame of Global Precipitation Measurement (GPM) mission. The new parameterization can be used to analyze data from the first space-borne Ku/Ka-band Dual-frequency Precipitation Radar (DPR).

12 First results The developed algorithm applied to the whole dataset well identifies the breakup situations (reddish lines). The correlation coefficient between measured DSD and Gamma distribution assumes very low values for breakup minutes. Does a different distribution fit better these situations?

13 MRR The Micro Rain Radar (MRR) is a vertical radar Doppler that investigates the atmosphere up to 6000 m above it. It allows to have instantaneous vertical reconstruction of rain properties (DSD, rainfall rate, reflectivity, liquid water content, etc.). Data from 2 MRR installed at La Sapienza (Rome) and Trafoi (1570m a.s.l., Trentino Alto Adige) they investigated 1000 m of atmosphere. We are able to cover about 3000 m of atmosphere. The MRR measurement can be used to check the results found be take into account the differences due to climatology, season, etc. NB: it has to

14 Vertical structure Better spatial resolution than the others operational radars; Possibility to study the variations of rain properties in a limited portion of atmosphere; Possibility to study sudden variations especially during more intense precipitation (left figure).

15 Integral parameters Measurement with rainfall rate higher than 0.2 mm/h are considered for both sites; A and b terms of Z-R relationship are calculated as function of altitude; The trend shows a strong decrease of A term with altitude, with values lower for Tafoi site: The same trend is found for b term, with the highest elevation in strong agreement with results found.

16 Runoff studies Conclusion of collaboration with Engineering Department of University of Cagliari; The fall velocity measurement of drops in accelerations were used to rescale the rain simulator measurement. The DSD obtained by rain simulator are similar to those of natural rain. The rain simulator has been used to study the runoff as function of soil moisture and rainfall intensity. High impact of initial soil moisture on the surface runoff process; Lower rainfall rate increase the ponding time and decrease the surface runoff.

17 Summary and perspectives To conclusion of CEOP-AEGIS project microphysics precipitation properties over Tibetan Plateau during monsoon season have been analyzed; Different datasets are used to study the DSD properties during breakup situations; An automatic algorithm to recognize these situations and estimate the breakup diameter analyzing DSD directly is under developing; A collaboration with NASA, in the frame of GPM mission, is started; the aim is study the possibility of a new parameterization of breakup DSD to apply to radar/satellite retrieval (in particular for the first dual frequency radar); Conclusion of collaboration with Engineering Department of University of Cagliari published paper On the estimation of surface runoff through a new plot scale rainfall simulator in Sardinia, Italy.