by Judith Curry

So, what’s going on in the world of research on the climate dynamics of clouds?

In two words: not much.

In the past decade, cloud-climate research has been focused on the aerosol indirect effect (aerosol-cloud interactions), improving climate model parameterizations of clouds, and evaluating new satellite cloud observing systems. What exactly do I mean about the climate dynamics of clouds? Here are some key questions that I see:

Could changes in cloud distribution or optical properties contribute to the global surface temperature hiatus?

How do cloud patterns (and TOA and surface radiative fluxes) change with shifts in in atmospheric circulation and teleconnection regimes (e.g. AO, NAO, PDO)?

How do feedbacks between clouds, surface temperature, and atmospheric thermodynamics/circulations interact with global warming and the atmospheric circulation and teleconnection regimes?

This particular post is triggered by the following paper that was recently published:

Late 20th century warming and variations in cloud cover

John Lean

Abstract. From 1950 to 1987 a strong relationship existed between the El Nino-Southern Oscillation (ENSO) and HadCRUT4 global average temperature anomaly, interrupted occasionally by volcanic eruptions. After 1987 the relationship diverged, with temperature anomaly increasing more than expected, but was re-established after 1997 at an offset of ~0.48˚C higher. The period of increased warming from 1987 to 1997 loosely coincided with the divergence of the global average temperature anomalies over land, which are derived from observation station recordings, and the global average anomalies in sea surface temperatures. Land-based temperatures averaged 0.04˚C below sea temperatures for the period 1950 to 1987 but after 1997 averaged 0.41˚C above sea temperatures. The increase in the global average temperature anomaly and the divergence of land and sea surface temperatures also coincided with two significant changes in global average cloud cover. Total cloud cover decreased during the period from 1987 to 1997 and, for most of the remainder of the period from 1984 to 2009, decreases in low-level cloud were accompanied by increases in middle and upper level cloud. These changes can be found in both global average cloud cover and in each of the six 30˚-latitude bands. The impact of these changes in cloud cover can account for the variations in HadCRUT4 global average temperature anomalies and the divergence between land and sea temperatures.

The paper is published online in Atmospheric and Climate Sciences, full manuscript available [here].

The paper provides some interesting observational analyses. I’m not going to critique the paper in detail, although I will state that the last sentence in the abstract is very weakly supported in the paper, which states in the conclusion:

According to the energy balance described by Trenberth et al. (2009) [34], the reduction in total cloud cover accounts for the increase in temperature since 1987, leaving little, if any, of the temperature change to be attributed to other forcings.

You cannot simply infer changes to global surface temperature from changes in horizontal and vertical distribution of clouds using Trenberth et al’s global energy balance.

The paper is significant in that it attempts to address some big questions in the climate dynamics of clouds, a topic where there is somewhat of a research void.

IPCC AR5

Why the relative research void on the topic of the climate dynamics of clouds? What does the IPCC have to say? The AR5 has an entire chapter (Chapter 7) entitled Clouds and Aerosols, but doesn’t have much to say on the questions that I posed above and barely mentions the the satellite cloud datasets, other than to show a few pretty global images. However, there are some interesting insights from the AR4 section 3.4.3.2 Satellite Cloud Observations:

Since the TAR, there has been considerable effort in the development and analysis of satellite data sets for documenting changes in global cloud cover over the past few decades. The most comprehensive cloud climatology is that of the International Satellite Cloud Climatology Project (ISCCP), begun in July 1983. The ISCCP shows an increase in globally averaged total cloud cover of about 2% from 1983 to 1987, followed by a decline of about 4% from 1987 to 2001

In summary, while there is some consistency between ISCCP, ERBS, SAGE II and surface observations of a reduction in high cloud cover during the 1990s relative to the 1980s, there are substantial uncertainties in decadal trends in all data sets and at present there is no clear consensus on changes in total cloudiness over decadal time scales.

WCRP Assessment

In 2012, the World Climate Research Programme (WCRP) Global Energy and Water Experiment (GEWEX) prepared a comprehensive report Assessment of Global Cloud Data Sets from Satellites. A reader’s digest version of this report is published in the Bulletin of the American Meteorological Society by Stubenrauch et al. From the Executive Summary of the WCRP Report:

Only satellite observations provide a continuous survey of the state of the atmosphere over the whole globe, at space-time scales at which cloud processes occur. Satellite cloud data records now exceed more than 25 years in length. While not as long as records from human observers, satellites provide the only globally complete data record at spatial and temporal scales consistent with cloud processes (approximately 3 hr, 25 km). The International Satellite Cloud Climatology Project (ISCCP), which is the GEWEX cloud project, uses multi-spectral imager data from a combination of polar orbiting and geostationary weather satellites to achieve the necessary sampling. During the past decade, other global cloud data records have been established from various instruments, mostly onboard polar orbiting satellites. New sensors such as MODIS, POLDER, CALIPSO and CloudSat have expanded cloud measurement capabilities. It is imperative that the longer time series products such as ISCCP be compared to recent intruments to assess the accuracy and error sources relevant for climate studies and for evaluation of general circulation models (GCM). In 2005, the GEWEX Radiation Panel (GRP, now GEWEX Data and Assessments Panel) initiated the GEWEX Cloud Assessment to intercompare these cloud data with ISCCP.

Conclusions, Recommendations and Outlook: In addition to self-assessments (summarized in Annex I), which show the maturity of the various data sets, the analyses have shown how cloud properties are perceived by instruments measuring different parts of the electromagnetic spectrum and how their averages and distributions are affected by instrument choice as well as some methodological decisions. These satellite cloud products are very valuable for climate studies or model evaluation: even if absolute values, especially those of high-level cloud statistics depend on instrument (or retrieval) performance to detect and/or identify thin cirrus, relative geographical and seasonal variations in the cloud properties agree very well (with only a few exceptions like over deserts and snow-covered regions). The study of long-term variations with these data sets requires consideration of many factors, which have to be carefully investigated before attributing any detected trends to climate change. This database will facilitate future assessments, climate studies and the evaluation of climate models. ISCCP cloud properties have also been assessed during the GEWEX Assessment of Global Radiative Flux Data Sets, revealing excellent quantitative agreement between fluxes.

ISCCP is not mentioned in the AR5, although the Stubenrauch et al. (BAMS article) receives a cursory mention.

ISCCP – fit for purpose?

The ISCCP cloud data set is the only one that has the time/space coverage to be useful for climate dynamics analyses (and ISCCP was used in the McLean paper). So . . . is the ISCCP data set useful for climate studies?

The ISCCP project has an excellent and comprehensive website at [link]. ISCCP cloud properties are one of the most mature climate data records that we have – for a discussion of climate data record maturity, see this previous post Climate Data Records – Maturity Matrix. The WCRP and BAMS articles provide clear guidance as to uncertainties in the ISCCP data set.

Relative to ocean heat content, for example, the ISCCP cloud data set is very mature and fit for purpose of climate dynamics analysis. I find the IPCC neglect of this important data record and resource to be inexplicable.

The time is ripe for application of satellite cloud data sets (particularly ISCCP) to addressing the fundamental questions related to the climate dynamics of clouds.

I have invited the father of ISCCP, Bill Rossow, to do some guest posts on this topic, something to definitely look forward to.