Infrared-Microwave synergy to improve low values Liquid Water Path retrievals in fog conditions.&#160;
Authors: Gallucci Donatello , Martinet Pauline , Cimini Domenico 
Published: June 2021

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&lt;p&gt;&lt;span&gt;Accurate measurement of&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;physically relevant parameters through cloudy atmospheres is&lt;/span&gt;&lt;span&gt;&amp;#160;crucial&amp;#160;&lt;/span&gt;&lt;span&gt;for climate studies and radiative transfer modelling.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;In particular, &lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;mall&lt;/span&gt;&lt;span&gt;&amp;#160;perturbations in&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;Liquid Water Path (LWP) retrieval&amp;#160;&lt;/span&gt;&lt;span&gt;within&lt;/span&gt;&lt;span&gt;&amp;#160;liquid water clouds&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;greatly&amp;#160;&lt;/span&gt;&lt;span&gt;affect longwave and shortwave radiative flux&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;&amp;#160;when LWP is lower than &lt;/span&gt;&lt;span&gt;10&lt;/span&gt;&lt;span&gt;0&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;g/m&lt;sup&gt;2&lt;/sup&gt;&lt;/span&gt;&lt;span&gt;. Since a large fraction of liquid water clouds in all climate regimes feature such&amp;#160;&lt;/span&gt;&lt;span&gt;a&amp;#160;&lt;/span&gt;&lt;span&gt;condition, improving the retrieval of&amp;#160;&lt;/span&gt;&lt;span&gt;low&amp;#160;&lt;/span&gt;&lt;span&gt;LWP&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;amounts&amp;#160;&lt;/span&gt;&lt;span&gt;from ground-based &lt;/span&gt;&lt;span&gt;observations is&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;high&lt;/span&gt;&lt;span&gt;ly desirable&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt;&amp;#160;In fact, uncertainties&amp;#160;&lt;/span&gt;&lt;span&gt;for LWP retrievals from Microwave radiometer (MWR)&amp;#160;&lt;/span&gt;&lt;span&gt;are&amp;#160;&lt;/span&gt;&lt;span&gt;generally&amp;#160;&lt;/span&gt;&lt;span&gt;acceptable&amp;#160;&lt;/span&gt;&lt;span&gt;for the &lt;/span&gt;&lt;span&gt;high&lt;/span&gt;&lt;span&gt;&amp;#160;LWP&amp;#160;values,&amp;#160;&lt;/span&gt;&lt;span&gt;but&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;are&amp;#160;&lt;/span&gt;&lt;span&gt;relatively&amp;#160;&lt;/span&gt;&lt;span&gt;large for lowe&lt;/span&gt;&lt;span&gt;r&amp;#160;&lt;/span&gt;&lt;span&gt;values typically&amp;#160;&lt;/span&gt;&lt;span&gt;observed&amp;#160;&lt;/span&gt;&lt;span&gt;in&amp;#160;&lt;/span&gt;&lt;span&gt;fog&amp;#160;&lt;/span&gt;&lt;span&gt;conditions. Further work is therefore required to achieve the accurac&lt;/span&gt;&lt;span&gt;y&lt;/span&gt;&lt;span&gt;&amp;#160;needed when the &lt;/span&gt;&lt;span&gt;LWP is small.&amp;#160;&lt;/span&gt;&lt;span&gt;In this work, w&lt;/span&gt;&lt;span&gt;e&amp;#160;&lt;/span&gt;&lt;span&gt;show the benefit of a combined&amp;#160;&lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;nfra&lt;/span&gt;&lt;span&gt;r&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt;&lt;span&gt;&amp;#160;(IR)&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;and&amp;#160;&lt;/span&gt;&lt;span&gt;m&lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;cro&lt;/span&gt;&lt;span&gt;w&lt;/span&gt;&lt;span&gt;ave&lt;/span&gt;&lt;span&gt;&amp;#160;(MW)&lt;/span&gt;&lt;span&gt;&amp;#160;approach, to improve ground-based liquid water path retr&lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;evals&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;during&lt;/span&gt;&lt;span&gt;&amp;#160;fog&lt;/span&gt;&lt;span&gt;.&amp;#160;&lt;/span&gt;&lt;span&gt;We consider a 14 MWR profiler operating in the 22-58 GHz frequency range and&amp;#160;&lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;&amp;#160;IR radiometer operating at 10.5um.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;The research is conducted over a simulated dataset of&amp;#160;&lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;tmospheric profiles from a&amp;#160;&lt;/span&gt;&lt;span&gt;numerical weather prediction model (AROME)&lt;/span&gt;&lt;span&gt;&amp;#160;and&lt;/span&gt;&lt;span&gt;&amp;#160;radiative transfer calculations&amp;#160;&lt;/span&gt;&lt;span&gt;from&amp;#160;&lt;/span&gt;&lt;span&gt;MonoRTM&amp;#160;and RTTOV-gb&amp;#160;r&lt;/span&gt;&lt;span&gt;adiative transfer models&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;The IR signal is&lt;/span&gt;&lt;span&gt;&amp;#160;shown to be&lt;/span&gt;&lt;span&gt;&amp;#160;very sensitive for LWP values lower than 100 g/m&lt;sup&gt;2&lt;/sup&gt;, spanning a temperature range&amp;#160;&lt;/span&gt;&lt;span&gt;as high as 8&lt;/span&gt;&lt;span&gt;0&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;K&lt;/span&gt;&lt;span&gt;. T&lt;/span&gt;&lt;span&gt;o that end, statistical&lt;/span&gt;&lt;span&gt;&amp;#160;(linear and quadratic)&lt;/span&gt;&lt;span&gt;&amp;#160;regressions&amp;#160;&lt;/span&gt;&lt;span&gt;are&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;first&amp;#160;&lt;/span&gt;&lt;span&gt;trained with synthetic observations&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;We&amp;#160;&lt;/span&gt;&lt;span&gt;analyse&amp;#160;the differences in LWP retrieval with and without the IR contribution, in order to evaluate its impact.&lt;/span&gt;&lt;span&gt;&amp;#160;As a future&amp;#160;&lt;/span&gt;&lt;span&gt;step, these coefficients&amp;#160;&lt;/span&gt;&lt;span&gt;will be&amp;#160;&lt;/span&gt;&lt;span&gt;applied&amp;#160;&lt;/span&gt;&lt;span&gt;to&lt;/span&gt;&lt;span&gt;&amp;#160;real measurements from the&amp;#160;&lt;/span&gt;&lt;span&gt;M&amp;#233;t&amp;#233;o&lt;/span&gt;&lt;span&gt;-France SOFOG3D&lt;/span&gt;&lt;span&gt;&amp;#160;experiment&amp;#160;&lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;Burnet et al., 2020;&amp;#160;&lt;/span&gt;&lt;span&gt;Martinet et al., 2020&lt;/span&gt;&lt;span&gt;)&amp;#160;&lt;/span&gt;&lt;span&gt;for evaluation.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;This work is conducted in&amp;#160;&lt;/span&gt;&lt;span&gt;the framework of&amp;#160;&lt;/span&gt;&lt;span&gt;COST Action&lt;/span&gt;&lt;span&gt;&amp;#160;PROBE (CA18235, Profiling the atmospheric Boundary layer at European scale&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;http://probe-cost.eu/&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;&amp;#160;(Cimini et al., 2020).&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;span&gt;Burnet et al., &lt;/span&gt;&lt;span&gt;https://doi.org/10.5194/egusphere-egu2020-17836&lt;/span&gt;&lt;span&gt;, 2020.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;Cimini et al.,&amp;#160;&lt;/span&gt;&lt;span&gt;https://doi.org/10.1007/s42865-020-00003-8&lt;/span&gt;&lt;span&gt;, 2020.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;Martinet et al.,&amp;#160;&lt;/span&gt;&lt;span&gt;https://doi.org/10.5194/amt-13-6593-2020&lt;/span&gt;&lt;span&gt;, 2020.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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