A microphysics guide to cirrus – Part 2: Climatologies of clouds and humidity from observations

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.