Evaluation of radar polarimetric ice microphysical retrievals using in-situ aircraft measurements from the OLYMPEX campaign
Authors: Blanke Armin , Heymsfield Andrew , Moser Manuel , Voigt Christiane , Trömel Silke 
Published: June 2022

&lt;p&gt;&lt;span&gt;Polarimetric microphysical retrievals &lt;/span&gt;&lt;span&gt;are of &lt;/span&gt;&lt;span&gt;great &lt;/span&gt;&lt;span&gt;value&lt;/span&gt;&lt;span&gt; for data assimilation, numerical model evaluation and &lt;/span&gt;&lt;span&gt;improvement&lt;/span&gt;&lt;span&gt;. However, the accuracy of ice microphysical retrievals remains poorly explored. &lt;/span&gt;&lt;span&gt;In order to&lt;/span&gt;&lt;span&gt; evaluate these retrievals and assess their accuracy, polarimetric radar measurements are spatially and temporally collocated with in-situ aircraft measurements conducted during the Olympic Mountain Experiment (OLYMPEX) campaign. Retrievals &lt;/span&gt;&lt;span&gt;for&lt;/span&gt;&lt;span&gt; ice water content, total number concentration, and mea&lt;/span&gt;&lt;span&gt;n&lt;/span&gt;&lt;span&gt; volume diameter of ice particles &lt;/span&gt;&lt;span&gt;derived from the polarimetric &lt;/span&gt;&lt;span&gt;X-band Doppler on Wheels (DOW) &lt;/span&gt;&lt;span&gt;measurements &lt;/span&gt;&lt;span&gt;are evaluated utilizing an in-situ data set &lt;/span&gt;&lt;span&gt;of&lt;/span&gt;&lt;span&gt; the University of Dakota (UND) Citation aircraft. Vertical profiles of microphysical retrievals are derived from sector-averaged RHI scans. The comparison of these estimates with in-situ data &lt;/span&gt;&lt;span&gt;fr&lt;/span&gt;&lt;span&gt;om&lt;/span&gt; &lt;span&gt;ten&lt;/span&gt;&lt;span&gt; flight missions &lt;/span&gt;&lt;span&gt;provides insights into strengths, weaknesses, and accuracy of the retrievals, and quantifies the improvements of polarimetry-informed retrievals compared to non-polarimetric, conventional ones. Especially the recently introduced hybrid &lt;/span&gt;&lt;span&gt;IWC&lt;/span&gt;&lt;span&gt; retrieval exploiting reflectivity &lt;/span&gt;&lt;span&gt;ZH,&lt;/span&gt;&lt;span&gt; differential reflectivity &lt;/span&gt;&lt;span&gt;ZDR &lt;/span&gt;&lt;span&gt;and specific differential phase &lt;/span&gt;&lt;span&gt;KDP&lt;/span&gt;&lt;span&gt; outperforms other retrievals based on either (&lt;/span&gt;&lt;span&gt;ZH&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;ZDR&lt;/span&gt;&lt;span&gt;) or (&lt;/span&gt;&lt;span&gt;ZH&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;KDP&lt;/span&gt;&lt;span&gt;) or non-polarimetric retrievals in terms of root mean square error &lt;/span&gt;&lt;span&gt;and &lt;/span&gt;&lt;span&gt;correlations with in-situ measurements. &lt;/span&gt;&lt;span&gt;Only&lt;/span&gt;&lt;span&gt; IWC retrievals &lt;/span&gt;&lt;span&gt;derived&lt;/span&gt; &lt;span&gt;via&lt;/span&gt;&lt;span&gt; optimal fi&lt;/span&gt;&lt;span&gt;t&lt;/span&gt;&lt;span&gt;ting parameters from &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;High Altitude Ice Crystal &amp;#8211; High Ice Water Content (HAIC-HIWC) field campaign data using either KDP or ZDR and KDP achieve comparable correlations, but exhibit a higher root mean square error. ZH-based retrievals for &lt;/span&gt;&lt;span&gt;t&lt;/span&gt;&lt;span&gt;he &lt;/span&gt;&lt;span&gt;mean volume diameter partly exhibit significant deviations from airborne in-situ measurements, while polarimetric retrievals show good agreement. For the latter, &lt;/span&gt;&lt;span&gt;however&lt;/span&gt;&lt;span&gt;, a discrepancy can be observed for large particles at warmer temperatures near the &lt;/span&gt;&lt;span&gt;melting layer&lt;/span&gt;&lt;span&gt;. On the basis of&lt;/span&gt;&lt;span&gt; our evaluation study, the most accurate ice microphysical retri&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt;vals are now used to evaluate the ICON &lt;/span&gt;&lt;span&gt;numerical weather prediction &lt;/span&gt;&lt;span&gt;model to reveal potential biases and deficiencies as a first step towards model improvements&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;