Estimation of light precipitation parameters by combining observations from C-, K-band radarand the passive radiometer ADMIRARI during the LPVEx campaignPablo Saavedra Garfias1 ([email protected]),L. Baldini2, A. Battaglia3, N. Roberto2, A. Tokay5, D. Moissev4 and C. Simmer11Meteorological Institute, University of Bonn, Germany- 2Istituto di Scienze dell’ Atmosfera e del Clima, Italy; 3University of Leicester, UK; 4University of Helsinki, Finland; 5NASA GSFC, USA
LPVEX CAMPAIGN
From Setpember to October 2010, the Light Precipitationand Verification Experiment (LPVEx) took place in Finland aspart of one of the Ground Validation GPM experimetns in or-der to study light precipitation and snow.The microwave radiometer ADMIRARI from University ofBonn was installed at the Emäsalo site (among several instru-ments e.g. 2DVD and Parsivel disdrometer, Pluvio, POSS),36.5 km away from Helsinki downtown.
The pictures above shows the location (left) of the instrumentsused in this work, mainly the dual-pol C-band KUMPULAradar from University of Helsinki, and the radiometer AD-MIRARI (right) which comprise of three dual-pol frequencies10.7, 21.0 and 36.5 GHz, a Micro Rain Radar (24.1GHz) and aCeilometer. All these instrumentation measured in synergyat the same slant angle.
CASE STUDY 20TH OCTOBER 2010
The objective of this work is to put into consideration amulti-sensor approach to retrieve atmospheric precipitationparameters. ADMIRARI is able to retrieve cloud and rainLWP, additionally a method to isolate the extinction dueto melting layer by combining radiometer-radar observa-tion is presented. The case of October 20th is selected asone of LPVEx light precipitation events. In that case, AD-MIRARI was measuring at slant 30◦ elevation and towardthe KUMPULA radar, while the radar was performing RHIscans over ADMIRARI every approx. 6 minutes.As an example of the measurements, the two picturesshows a typical set of observations collected by theKUMPULA radar and ADMIRARI.
The radiometer was located 36.5 km distance and sensingthe rain scenario within its 6◦ beam-width. This particularevent was characterized by a light rain with a freezing levelbelow 1 km altitude.
The radiometer collects the integrated effect due to rain,cloud, ice phase and melting layer (ML). By extracting theADMIRARI observation volume from the KUMPULA RHI,the ML can be identified and isolated to have informationonly due to the rain layer. Thus a method to detect the melt-ing layer becomes crucial, and can be achieved by taking ad-vantage of the dual-pol capabilities of the KUMPULA radar.
EXTRACTING THE ADMIRARI FOV FROM KUMPULA RHI
In order to extract the ADMIRARI FOV from theKUMPULA RHIs, some considerations must bedone, according with the experiment set up:
The KUMPULA radar is located approx. 50 mabove ground level and due to the distance tothe radiometer, the Earth curvature has alreadya slight effect on the geometry showed in thecartoon.Even with the lowest RHI elevation angle, atthe radiometer side the lowest 450 meters rangecannot be observed. Therefore there is no datafrom KUMPULA radar at that low range. This,however can be filled with the co-located MRRand corrected by attenuation computed fromthe 2DVD disdrometer.
The method to detect the high of the ML fromthe KUMPULA RHI, follows the criteria:
• ZH > 10dBz
• σ(Zdr) > 1.5
• σ(φdp) > 3.5
• elevation angles > 0.5◦
Based on this ML detection method, the bot-tom height of bright-band is correctly identifiedthe most of the cases. Where no results wasobtained (i.e. the ML signature is not strongenough), ML height was obtained by interpolat-ing from nearby elevations.
The picture below shows the result after extracting the ADMIRARI FOV from theKUMPULA RHI: in the Top panel the co-located MRR reflectivity profiles is shownafter correcting a bias by using the estimated reflectivity from the 2DVD disdrometer.The middle panel, depicts the extracted KUMPULA reflectivity profiles from the Zhproduct. The Zh is the result after applying a clutter filter, which has also an effecton the reflectivity profiles. The second panel from the bottom, it is the same as beforebut using the non filtered KUMPULA reflectivity (i.e. total reflectivity TP) which is inmore agreement with the direct measured by the MRR. The advantage here is that theextracted reflectivity profile has a better range resolution than the MRR (300 meters).
RETRIEVED PARAMETERS
From the extracted KUMPULA re-flectivity, the following parame-ters have been computed:
• LWC(r) = aZh(r)b
• specific attenuationαf (r)hv = chvf Zh(r)d
hvf
with f ranging for the ADMI-RARI frequencies (10.7, 21.0 and36.5 GHz). Two set of parame-ters a, b, chvf and dhvf where com-puting using T-matrix simulationsover a widely varying DSD pa-rameters Nw, D0, µ (Bringi andChandrasekar, 2001). A second setof parameters was obtained fromthose collected over four years us-ing a Joss-Walvogel disdrometerlocated in Järvenpää, Finland.
ADMIRARI Bayesian retrievals:
Ppost(x|yO) =pf (yO|x) ppr(x)∫pf (yO|x) ppr(x) dx
,
Rain_LWP cab be also derived from KUMPULAradar as well τrain =
∫αf (r)dr for the layer below
Melting.
τADMIRARI = τrain + τcloud + τML + τgases
τcloud ≈ 6π νIm{εw(rlidar)}|εw(rlidar) + 2|2
C_LWP
RESULTS
∆τ = τrain+ML − τrain can be interpreted as the increase due to ML:Increase on ∆τ at 10GHz is found when rainrate increase (see plot at the left). This casestudy present a very light precipitation, withmax. rain rates ∼ 1.8mmr−1, and the only10GHz shows a systematic increase due to thepresence of ML. For 21GHz the τgases mustbe taken into account. While for 36GHz theresults depends strongly on the DSD used tocompute the parameters for α = cZd
h relation-ship.As future work, more cases will be ana-lyzed in order to have a statistical signif-icant result for the behavior of estimated∆τ using this approach.
REFERENCES
[ADMIRARI web site] http://www2.meteo.uni-bonn.de/admirari/LWPVEx
[ADMIRARI publications] http://www2.meteo.uni-bonn.de/admirari/admirari_publications.html
[LPVEx web site] http://lpvex.atmos.colostate.edu/
[Saaveda(2011)] Saavedra P., Battaglia A., Simmer C., 2011, Partitioning of cloud water and rain water content by ground-based observations with the Advanced Microwave Radiometer for Rain Identification (ADMIRARI) in synergy with amicro rain radar. J. Geo. Research., 117,D05203,doi:10.1029/2011JD016382.
[Baldini(2006)] Baldini L., et al., Identification of the Melting Layer through Dual-Polarized Radar measurements at Verti-cal Incidence, J. Atmos. Oceanic Technol.,23,829-839.
[Leinonen(2012)] Leinonen J., et al., A climatology of disdrometer measurements of rainfall in Finland over five years withimplications for global radar observations, J. Appl. Meteor. Climatol.,51,392-404.
ACKNOWLEDGES
The author wants to thank VAISALA for funding the ADMIRARI transportation from Germany to Finland and
participation of the author to the LPVEx campaign, as well as the staff from FMI and University of Helsinki. ADMIRARI is
a project funded by the Deutche Forschungsgemeinschaft (DFG).
