A microphysics guide to cirrus – Part 2: Climatologies of clouds and humidity from observations
Authors: Krämer Martina , Rolf Christian , Spelten Nicole , Afchine Armin , Fahey David , Jensen Eric , Khaykin Sergey , Kuhn Thomas , Lawson Paul , Lykov Alexey , Pan Laura L. , Riese Martin , Rollins Andrew , Stroh Fred , Thornberry Troy , Wolf Veronika , Woods Sarah , Spichtinger Peter , Quaas Johannes , Sourdeval Odran 
Published: November 2020

Abstract. This study presents airborne in situ and satellite remote sensing
climatologies of cirrus clouds and humidity. The climatologies serve
as a guide to the properties of cirrus clouds, with the new in situ
database providing detailed insights into boreal midlatitudes and
the tropics, while the satellite-borne data set offers a global
overview. To this end, an extensive, quality-checked data archive, the Cirrus
Guide II in situ database, is created from airborne in situ
measurements during 150 flights in 24 campaigns. The archive contains
meteorological parameters, ice water content (IWC), ice crystal number
concentration (Nice), ice crystal mean mass radius
(Rice), relative
humidity with respect to ice (RHice), and water vapor mixing ratio
(H2O) for each of the flights.
Depending on the parameter, the database has been extended by about a
factor of 5–10 compared to earlier studies. As one result of our investigation, we show that the medians of Nice,
Rice, and RHice have distinct patterns in the IWC–T parameter space.
Lookup tables of these variables as functions of IWC and T can be used
to improve global model cirrus representation and remote
sensing retrieval methods.
Another outcome of our investigation is that across all latitudes,
the thicker liquid-origin cirrus predominate at lower altitudes, while
at higher altitudes the thinner in situ-origin cirrus prevail. Further,
examination of the radiative characteristics of in situ-origin
and liquid-origin cirrus shows that the in situ-origin cirrus only
slightly warm the atmosphere, while liquid-origin cirrus have a strong
cooling effect. An important step in completing the Cirrus Guide II is the provision
of the global cirrus Nice climatology, derived by means of the
retrieval algorithm DARDAR-Nice from 10 years of cirrus remote
sensing observations from satellite. The in situ measurement database
has been used to evaluate and improve the satellite observations. We
found that the global median Nice from satellite observations is
almost 2 times higher than the in situ median and increases slightly
with decreasing temperature. Nice medians of the most frequently
occurring cirrus sorted by geographical regions are highest in the
tropics, followed by austral and boreal midlatitudes, Antarctica, and the
Arctic. Since the satellite climatologies enclose the entire spatial
and temporal Nice occurrence, we could deduce that half of the
cirrus are located in the lowest, warmest (224–242 K) cirrus layer
and contain a significant amount of liquid-origin cirrus. A specific highlight of the study is the in situ observations of
cirrus and humidity in the Asian monsoon anticyclone and the
comparison to the surrounding tropics. In the convectively very active
Asian monsoon, peak values of Nice and IWC of 30 cm−3 and 1000 ppmv are detected around the cold point tropopause (CPT). Above the
CPT, ice particles that are convectively injected can locally add a
significant amount of water available for exchange with the
stratosphere. We found IWCs of up to 8 ppmv in the Asian monsoon in
comparison to only 2 ppmv in the surrounding tropics.
Also, the highest RHice values (120 %–150 %) inside of clouds and in clear
sky are observed around and above the CPT. We attribute this to the
high H2O mixing ratios (typically 3–5 ppmv) observed in
the Asian monsoon compared to 1.5 to 3 ppmv found in the
tropics.
Above the CPT, supersaturations of 10 %–20 % are observed in regions of
weak convective activity and up to about 50 % in the Asian
monsoon. This implies that the water available for transport into the
stratosphere might be higher than the expected saturation value.