Evaluation of radar reflectivity factor simulations of ice crystal populations from in situ observations for the retrieval of condensed water content in tropical mesoscale convective systems

Emmanuel Fontaine, Delphine Leroy, Alfons Schwarzenboeck, Julien Delanoë, Alain Protat, Fabien Dezitter, Alice Grandin, John Walter Strapp, Lyle Edward Lilie
2017 Atmospheric Measurement Techniques  
<p><strong>Abstract.</strong> This study presents the evaluation of a technique to estimate cloud condensed water content (CWC) in tropical convection from airborne cloud radar reflectivity factors at 94<span class="thinspace"></span>GHz and in situ measurements of particle size distributions (PSDs) and aspect ratios of ice crystal populations. The approach is to calculate from each 5<span class="thinspace"></span>s mean PSD and flight-level reflectivity the variability of all possible
more » ... l possible solutions of <i>m</i>(<i>D</i>) relationships fulfilling the condition that the simulated radar reflectivity factor (<b>T</b>-matrix method) matches the measured radar reflectivity factor. For the reflectivity simulations, ice crystals were approximated as oblate spheroids, without using a priori assumptions on the mass–size relationship of ice crystals. The CWC calculations demonstrate that individual CWC values are in the range ±32<span class="thinspace"></span>% of the retrieved average <span style="text-decoration: overline;">CWC</span> value over all CWC solutions for the chosen 5<span class="thinspace"></span>s time intervals. In addition, during the airborne field campaign performed out of Darwin in 2014, as part of the international High Altitude Ice Crystals/High Ice Water Content (HAIC/HIWC) projects, CWCs were measured independently with the new IKP-2 (isokinetic evaporator probe) instrument along with simultaneous particle imagery and radar reflectivity. Retrieved <span style="text-decoration: overline;">CWC</span>s from the <b>T</b>-matrix radar reflectivity simulations are on average 16<span class="thinspace"></span>% higher than the direct CWC<sub>IKP</sub> measurements. The differences between the CWC<sub>IKP</sub> and averaged retrieved <span style="text-decoration: overline;">CWC</span>s are found to be primarily a function of the total number concentration of ice crystals. Consequently, a correction term is applied (as a function of total number concentration) that significantly improves the retrieved CWC. After correction, the retrieved <span style="text-decoration: overline;">CWC</span>s have a median relative error with respect to measured values of only −1<span class="thinspace"></span>%. Uncertainties in the measurements of total concentration of hydrometeors are investigated in order to calculate their contribution to the relative error of calculated <span style="text-decoration: overline;">CWC</span> with respect to measured CWC<sub>IKP</sub>. It is shown that an overestimation of the concentration by about +50<span class="thinspace"></span>% increases the relative errors of retrieved <span style="text-decoration: overline;">CWC</span>s by only +29<span class="thinspace"></span>%, while possible shattering, which impacts only the concentration of small hydrometeors, increases the relative error by about +4<span class="thinspace"></span>%. Moreover, all cloud events with encountered graupel particles were studied and compared to events without observed graupel particles. Overall, graupel particles seem to have the largest impact on high crystal number-concentration conditions and show relative errors in retrieved <span style="text-decoration: overline;">CWC</span>s that are higher than for events without graupel particles.</p>
doi:10.5194/amt-10-2239-2017 fatcat:d4oqlg2dhrch5ju6pfe5z4jf5a