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 (
As one result of our investigation, we show that the medians of
An important step in completing the Cirrus Guide II is the provision
of the global 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
In part 1 of the study
Since then, a number of studies have been published that shed further
light on the exploration of the high ice clouds.
For example, some new studies, mostly based on aircraft or lidar
observations, provide overviews and climatologies of cirrus cloud
properties
The wealth of earlier
In this study, we approach these requirements as follows: we first
compile a data archive of airborne in situ observations which is
extended with respect to earlier versions
Mean mass radius
The article is structured as follows:
the Cirrus Guide II in situ databases and the methods used are
described in Sect.
In Sect.
The last part of the study (Sect.
The observations presented here include the ice water content IWC, the
ice crystal number concentration
A map of flights during the various campaigns (extended map of Cirrus
Guide: Part 1) is shown in Fig.
Aircraft flight paths during the 24 campaigns listed
in Table
The climatologies are advanced in several aspects in comparison to the
compilations of IWC by The number of flights and total time in cirrus increased from
104 flights for 94 h For IWC, a new data analysis method has been developed that
increases the observed data volume (Appendix For The geographical spread of the observations has broadened, so
that a portrayal of cirrus and humidity with respect to the midlatitude and tropical
geographical regions, and also with respect to
latitude and altitude, seems worthwhile. As in the earlier climatologies, all data underwent
strict quality control.
DARDAR-Nice provides observation-based estimates of
This evaluation of DARDAR-Nice is here repeated on the basis of five in situ
campaigns archived in the Cirrus Guide II in situ database:
COALESC2011, ACRIDICON2014, ATTREX2014, MLCIRRUS2014,
STRATOCLIM2017. Based on the agreement between DARDAR-Nice and the
in situ observations, a global
As an introduction, atmospheric temperature profiles in the Arctic,
at midlatitudes, and in the tropics are shown in
Fig.
Left panel: temperature vs. altitude inside of
cirrus clouds
As a first application of the Cirrus Guide II data set the
distribution of cirrus clouds and humidity is shown with respect to
latitude and altitude in Fig.
Cirrus cloud distribution with latitude and altitude of
(left column, panels
Cirrus clouds are found at lower altitudes at midlatitudes and reach
higher levels in the tropical region (Fig.
For a more detailed description of the freezing mechanisms see
Typical characteristics of cirrus types in the initial stage.
Slow updraft:
Slow updrafts: Fast updrafts: 10 cm s
The meteorological situations where slow updraft in situ-origin cirrus
frequently occurs
As outlined in the Cirrus Guide I
Cirrus types with the most striking features (high IWC and/or
Relation between ice crystal concentration
In Fig.
Most of the highest IWCs (
Generally, the IWC roughly shows a vertical structure of decreasing
IWC with increasing altitude. This is caused on the one hand by the
amount of available water that decreases with decreasing temperature
and on the other hand because cirrus of liquid origin predominate in lower layers,
whereas cirrus with in situ origin become more abundant at higher
altitudes, i.e., colder temperatures. This is in accordance with the
findings of
While IWC is an indication for the cirrus type,
Simulated radiative forcing versus optical depth
for exemplary in situ-origin
slow- and fast-updraft cirrus (light and dark green dots) as well as liquid-origin cirrus
(blue dots); the idealized scenarios are summarized in
the table.
Description of the idealized scenarios: a temperature profile is
prescribed representative for midlatitude conditions; the surface
temperature is set to 288 K (at
Another source of midlatitude high
A more detailed discussion of the microphysical properties of cirrus
including frequencies of occurrence of specific signatures which cannot be seen from Fig.
In the left panel of Fig.
In more detail, the slow in situ-origin cirrus have only small optical
depth (
Thus, from these first very idealized simulations, we can conclude that in situ-formed cirrus clouds are most likely to warm the atmosphere, whereas liquid-origin ice clouds have the potential for strong cooling. Note here that we only investigate local time 12 h, where the cooling is probably most pronounced. For lower sun position (i.e., larger zenith angle) the cooling is probably reduced, and during nighttime cirrus clouds can only warm the atmosphere (due to the thermal greenhouse effect). Thus, the net effect of cirrus clouds averaged over the whole daily cycle is not yet clear. Such investigations go beyond the scope of this study and are the subject of future work.
The distribution of in-cloud and clear-sky RH
With the term climatologies we refer here to statistical evaluations of the available variables with regard to temperature or potential temperature.
In the Cirrus Guide I
With the extended Cirrus Guide II data set, we further investigate the
statistics of
Median
Observational evidence for the correlation between IWC and
The ice crystal sizes also form colored bands in the IWC–
One could have expected to find distinct patterns for in situ-origin
and liquid-origin cirrus in this type of analysis to quantify the
characteristics of the cirrus types shown in
Table
In the bottom panel of Fig.
The Cirrus Guide II data set shown in Fig.
Frequencies of occurrence in dependence on temperature, binned in 1 K
intervals of ice water content (IWC;
Figure
The number of hours spent sampling in cirrus clouds increased from 27 h in
However, the median IWC and the core IWC
band – decreasing with temperature as described by
About 90 h of Note that for
A total of 90 h of aircraft
It is an open question why the
A further consideration of
The ice crystal mean size is calculated as mean mass radius
A total of 84 h of observations are compiled in Fig.
The 10th, 25th, 50th, 75th, and 90th percentiles of the data set are plotted as
thin, dotted, and solid red lines. The black lines represent the
minimum, middle, and maximum
Remarkable is the drop of the most frequent
Same as Fig.
The new RH
For the new data set, in addition to the clear-sky RH
The data sets of midlatitude and tropical cirrus consist of 67 and 101 h
of IWC, 29 and 61 h of
Same as Fig.
The midlatitude and tropical humidity data sets include 24 and 72 h
of in-cloud RH
In clear sky at temperatures higher than about 200 K, RH
Below about 200 K, i.e., in the TTL (see also
Sect.
Clear-sky RH
Inside of cirrus, the peak of the RH
The tropical tropopause layer is the region above
the upper level of main convective outflow, where the transition from the
troposphere to stratosphere occurs. It is placed at temperatures
Cirrus clouds and humidity in the TTL deserve a special consideration,
because this region represents the main pathway by which water vapor enters
the upper troposphere and lower stratosphere (UT–LS) where it is further
distributed over long distances (e.g.,
This section specifically reports cirrus clouds together with humidity
(from the climatologies presented in Sect.
The airborne measurements in the Asian monsoon (see Fig. The additional
Most of the measurements during StratoClim are performed at
temperatures
The surrounding, typically calmer tropical TTL
regions are represented by observations during the campaigns shown in
Fig.
In the tropics outside of the Asian monsoon (Fig. It should be noted that concentrations
The clouds observed in the Asian monsoon include in situ-formed cirrus as
well as cirrus clouds from overshooting deep convection.
In the much more convectively unstable Asian monsoon conditions, IWC
and
The distribution of Asian monsoon cirrus clouds in the
From Fig.
In the Asian monsoon, the maximum
These overshooting cirrus around and below the CPT are also seen in
high ice crystal numbers
Above 390 K, at the transition to the stratosphere,
In the TTL outside of the Asian monsoon, the two branches of more frequent
Considering the in-cloud and clear-sky RH
Same as Fig.
Climatology of cirrus clouds as a function of potential temperature
Climatology of humidity as a function of potential temperature
The high RH
In the Asian monsoon, on the other hand, the in-cloud RH
The
However, e.g.,
High supersaturations at the coldest points of the TTL are
also discussed in Sect.
As mentioned earlier, the question of how much
From our measurements in and above the Asian monsoon anticyclone, it
is difficult to estimate to what extent cirrus cloud particles
contribute to the amount of
The question remains of whether the ice particles found in the upper TTL will
further grow and sediment out in supersaturated conditions
(dehydration) or evaporate and add
Above the CPT in the surrounding tropics outside of the Asian monsoon,
the in-cloud RH
Comparing gas phase
Though the new in situ cirrus climatologies presented in
Sect.
In this section, we show a global climatology of
As a base for reliable
The black solid and dotted lines of the in situ climatology indicate
the 50th, 25th, and 75th percentiles, which agree well with
those of the entire in situ climatology (see Fig.
A detailed comparison of the DARDAR-Nice percentiles (black solid and
dashed lines) with those of the in situ observations, however, yields a
small offset by a factor of 1.73, which is visualized in Fig.
Other assumptions on the PSD shape by the satellite remote sensing
method might also contribute to this bias. The PSD shape indeed is
provided by four parameters, two of which are fixed and two are retrieved
(see Sect.
A further expected difference between the data sets arises from their
detection limits. The lowest
Overall, following the abovementioned arguments, an overestimation
within a factor of 2 in DARDAR-Nice by comparison to the in situ data
set is expected, although there is still some uncertainty as to how
this overestimation will propagate globally and over various ice cloud
regimes. The ratio of 1.73 in
The global frequency distribution of
From the median (solid black line), a slight increase in
The slightly increasing median
The thinnest cirrus with low
DARDAR-Nice and in situ observation
DARDAR-Nice median
The median
Altogether, both data sets have advantages and disadvantages:
DARDAR-Nice has the advantage of the long, global time series where
all atmospheric situations above the detection limit showing up at the
times of observations are sampled. However, the thinnest cirrus at
cold temperatures are not detected. Further, an offset is found
between DARDAR-Nice and the in situ observations. On the other hand,
the in situ
For comparisons with other
The DARDAR-Nice data set not only provides the
From this portrayal it can be seen that the cirrus clouds do not
spread evenly across all temperatures, but that about 50 % of the cirrus
appear at temperatures above about 225 K (see also
Table
The analysis presented here provides the information that globally half of
the cirrus are in the lowest, warmest cirrus layer between about 220 and
240 K, where, as mentioned above, about half of the cirrus are of liquid origin.
This is of importance because of the differing radiative properties of
in situ-origin and liquid-origin cirrus shown in
Sect.
Note that an analysis of
Lastly, we provide an impression of the regional variations in
Comparing the three median
The global most frequent median
Tropical cirrus clouds are different from those in the other
regions. The most frequent cirrus have the highest median (0.074
For all regions, except the tropics, more than half of the cirrus clouds are found at temperatures warmer than about 225 K and contain a considerable amount of liquid-origin cirrus, as discussed in the previous subsection. The same is true in the tropics, but for colder temperatures.
The Cirrus Guide II aims to represent cirrus clouds and their environment
from the perspective of in situ and satellite remote sensing observations.
To this end, an in situ database is created, which is based on measurements
with state-of-the-art instrumentation and extended in comparison to earlier
studies
From the extended Cirrus Guide II in situ data set, we generally In situ-origin and liquid-origin cirrus consist of two
subclasses determined by the updraft
(1: slow updrafts – few large ice crystals form from
heterogeneous freezing; 2: fast updrafts – many small ice crystals nucleate
homogeneously). New in this concept is that liquid-origin
cirrus also differ in the two updraft regimes. Often, the two
subclasses occur in succession and form a bimodal size
distribution. Liquid-origin cirrus are thicker (higher IWC) than in situ-origin
cirrus and are usually characterized by larger ice
crystals. The differences between the cirrus types are most pronounced
in the formation phase of the clouds and are blurred with
increasing lifetime due to ice crystal growth and sedimentation or additional ice formation.
In addition, we present a Across all latitudes, the thicker liquid-origin cirrus
predominate at lower altitudes, while at higher altitudes the
thinner in situ-origin cirrus prevail. In between, the two cirrus types
overlap. This finding is in agreement with
Finally, a Slow in situ-origin cirrus have a small optical depth
( The optical depth of fast in situ-origin cirrus is larger
( Liquid-origin cirrus have large optical depths (
The new in situ data set is extended by observations in the tropical
TTL outside, but also, for the first time, inside of the Asian monsoon
anticyclone. Therefore, we put special emphasis on the analysis of
the TTL environment.
Two types of most likely in situ-formed cirrus are identified
in slow large-scale updrafts at low temperatures ( The highest
In the Asian monsoon, in-cloud and clear-sky RH Taking saturation at the stratospheric entry of an air mass as
set point for water vapor transport to the stratosphere, the
transport is underestimated by Convectively injected ice over the Asian monsoon CPT
(
Median Distinct patterns are found: The entire extended data set (Fig. The median IWC and the core IWC band are the same in both data sets,
showing that the in situ IWC measurement techniques are robust and that
the IWC is a stable parameter describing cirrus clouds. The overall picture of the in-cloud and clear-sky
RH An extended view is presented for The in situ data set is subdivided into midlatitude and
tropical climatologies (Figs.
(Figs. A global climatology of The The global median In the satellite climatologies of Regarding the frequent appearance of liquid-origin cirrus
together with strong cooling effect is a motivation to investigate their
influence on the overall cirrus radiative feedback on climate in
future studies.
The field campaigns, locations, and deployed instruments are listed in
Table
Overview of campaigns and instruments.
IWC: ice water content (uncertainties
Cirrus Guide II database for the IWC,
The data evaluation methods, detection ranges, and data quality criteria of
the measurements of IWC,
Data coverage of the Cirrus Guide II in situ database:
IWC,
In the earlier studies of
Unlike in the earlier studies, we here combine the two methods,
IWC(
As discussed by
From the campaigns where cloud particle size distributions (PSDs) are
measured, we chose those where cloud particles between 3 and
Two instruments are needed to be deployed to cover the complete cirrus ice
particle size range, one for smaller and the other for larger cloud
particles. The PSDs from NIXE-CAPS (CAS-Depol
Figure
The effect of this correction on the occurrence frequencies can be seen in
Fig.
Single-particle events are less common if the measurements are averaged over larger time periods. The considered time interval can be adapted according to the required spatial resolution of the measurements. However, as outlined above, for this study we choose to show the lowest time interval (1 s) to achieve the highest possible spatial resolution.
Example of
Example of biased
Fragmentation of large ice crystals at the cloud probe housings distorted a
correct recording of the ice crystal concentrations in earlier
times. Today, new inlets and also postprocessing algorithms based on the
interarrival times of the crystals in the sampling volume
Water vapor measurements where, as ice crystal concentrations, have been under
discussion in the last decade
Nevertheless, we check the
The Cirrus Guide II in-situ data set is accessible under
The supplement related to this article is available online at:
The authors declare that they have no conflict of interest.
This paper is dedicated to our colleague Cornelius Schiller, who passed
away much too early in 2012. He was the initiator of the StratoClim
project, including the tropical aircraft campaign. Through this
campaign, he intended to complement his work on hydration and
dehydration in the tropical tropopause layer, based on observations
over Brazil, Australia, and Africa
The StratoClim project is funded by the European Commission within Framework program 7 under ENV.2013.6.1-2, grant agreement no. 603557. Funding for Cirrus Guide II is partly provided by the DFG HALO-SPP project ACIS (KR 2957/1-1).
The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
This paper was edited by Heini Wernli and reviewed by two anonymous referees.