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Shannon Mason
,
Christian Jakob
,
Alain Protat
, and
Julien Delanoë

Abstract

Clouds strongly affect the absorption and reflection of shortwave and longwave radiation in the atmosphere. A key bias in climate models is related to excess absorbed shortwave radiation in the high-latitude Southern Ocean. Model evaluation studies attribute these biases in part to midtopped clouds, and observations confirm significant midtopped clouds in the zone of interest. However, it is not yet clear what cloud properties can be attributed to the deficit in modeled clouds. Present approaches using observed cloud regimes do not sufficiently differentiate between potentially distinct types of midtopped clouds and their meteorological contexts.

This study presents a refined set of midtopped cloud subregimes for the high-latitude Southern Ocean, which are distinct in their dynamical and thermodynamic background states. Active satellite observations from CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) are used to study the macrophysical structure and microphysical properties of the new cloud regimes. The subgrid-scale variability of cloud structure and microphysics is quantified within the cloud regimes by identifying representative physical cloud profiles at high resolution from the radar–lidar (DARDAR) cloud classification mask.

The midtopped cloud subregimes distinguish between stratiform clouds under a high inversion and moderate subsidence; an optically thin cold-air advection cloud regime occurring under weak subsidence and including altostratus over low clouds; optically thick clouds with frequent deep structures under weak ascent and warm midlevel anomalies; and a midlevel convective cloud regime associated with strong ascent and warm advection. The new midtopped cloud regimes for the high-latitude Southern Ocean will provide a refined tool for model evaluation and the attribution of shortwave radiation biases to distinct cloud processes and properties.

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Mick Pope
,
Christian Jakob
, and
Michael J. Reeder

Abstract

The variability of the north Australian wet season is examined by performing cluster analysis on the wind and thermodynamic information contained in the 2300 UTC radiosonde data at Darwin for 49 wet seasons (September–April) from 1957/58 to 2005/06. Five objectively derived regimes of the wet season are obtained and are found to differ significantly in their synoptic environment, cloud patterns, and rainfall distributions. One regime is primarily associated with the trade wind regime. Two regimes are associated with the lead up to and break periods of the monsoon at Darwin. A fourth regime is clearly identified with the active monsoon at Darwin and is offered as a definition of monsoon onset. This regime captures the active monsoon environment associated with significant widespread rainfall. The fifth regime is a mixed regime, with some days associated with the inactive monsoon, a period of westerly zonal winds at Darwin associated with relatively suppressed convection compared with the active monsoon. Other days for this regime are break period conditions with a low-level westerly flow below 900 hPa.

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Mick Pope
,
Christian Jakob
, and
Michael J. Reeder

Abstract

A cluster analysis is applied to the mesoscale convective systems (MCSs) that developed in northern Australia and the surrounding oceans during six wet seasons (September–April) from 1995/96 to 2000/01. During this period, 13 585 MCSs were identified and tracked using an infrared channel (IR1) on the Japanese Meteorological Agency Geostationary Meteorological Satellite 5 (GMS5). Based on the lifetimes of the MCSs, the area covered by cloud, the expansion rate of the cloud, the minimum cloud-top temperature, and their zonal direction of propagation, the MCSs are grouped objectively into four classes. One of the strengths of the analysis is that it objectively condenses a large dataset into a small number of classes, each with its own physical characteristics.

MCSs in class 1 (short) are relatively short lived, with 95% having lifetimes less than 5 h, and they are found most frequently over the oceans during the early and late parts of the wet season. MCSs in classes 2 and 3 [long and intermediate west (Int-West)] are longer lived and propagate to the west, developing over continental northwest Australia in deep easterly flow during breaks in the monsoon. These two classes are distinguished principally by their lifetime, with 95% of MCSs in the long class having lifetimes exceeding 4 h. Class 4 (Int-East) comprises MCSs that form over the subtropical latitudes of eastern Australia and in the deep westerly flow over northern parts of the continent during the monsoon and active phases of the MJO.

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Jackson Tan
,
Christian Jakob
, and
Todd P. Lane

Abstract

The use of cloud regimes in identifying tropical convection and the associated large-scale atmospheric properties is investigated. The regimes are derived by applying cluster analysis to satellite retrievals of daytime-averaged frequency distributions of cloud-top pressure and optical thickness within grids of 280 km by 280 km resolution from the International Satellite Cloud Climatology Project between 1983 and 2008. An investigation of atmospheric state variables as a function of cloud regime reveals that the regimes are useful indicators of the archetypal states of the tropical atmosphere ranging from a strongly convecting regime with large stratiform cloudiness to strongly suppressed conditions showing a large coverage with stratocumulus clouds. The convectively active regimes are shown to be moist and unstable with large-scale ascending motion, while convectively suppressed regimes are dry and stable with large-scale descending winds. Importantly, the cloud regimes also represent several transitional states. In particular, the cloud regime approach allows for the identification of the “building blocks” of tropical convection, namely, the regimes dominated by stratiform, deep, and congestus convection. The availability of the daily distribution of these building blocks for more than 20 years opens new avenues for the diagnosis of convective behavior as well as the evaluation of the representation of convection in global and regional models.

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Robin J. Hogan
,
Christian Jakob
, and
Anthony J. Illingworth

Abstract

Of great importance for the simulation of climate using general circulation models is their ability to represent accurately the vertical distribution of fractional cloud amount. In this paper, a technique to derive cloud fraction as a function of height using ground-based radar and lidar is described. The relatively unattenuated radar detects clouds and precipitation throughout the whole depth of the troposphere, whereas the lidar is able to locate cloud base accurately in the presence of rain or drizzle. From a direct comparison of 3 months of cloud fraction observed at Chilbolton, England, with the values held at the nearest grid box of the European Centre for Medium-Range Forecasts (ECMWF) model it is found that, on average, the model tends to underpredict cloud fraction below 7 km and considerably overpredict it above. The difference below 7 km can in large part be explained by the fact that the model treats snow and ice cloud separately, with snow not contributing to cloud fraction. Modifying the model cloud fraction to include the contribution from snow (already present in the form of fluxes between levels) results in much better agreement in mean cloud fraction, frequency of occurrence, and amount when present between 1 and 7 km. This, together with the fact that both the lidar and the radar echoes tend to be stronger in the regions of ice clouds that the model regards as snow, indicates that snow should not be treated as radiatively inert by the model radiation scheme. Above 7 km, the difference between the model and the observations is partly due to some of the high clouds in the model being associated with very low values of ice water content that one would not expect the radar to detect. However, removal of these from the model still leaves an apparent overestimate of cloud fraction by up to a factor of 2. A tendency in the lowest kilometer for the model to simulate cloud features up to 3 h before they are observed is also found. Overall, this study demonstrates the considerable potential of active instruments for validating the representation of clouds in models.

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Karsten Peters
,
Christian Jakob
,
Laura Davies
,
Boualem Khouider
, and
Andrew J. Majda

Abstract

The aim for a more accurate representation of tropical convection in global circulation models is a long-standing issue. Here, the relationships between large and convective scales in observations and a stochastic multicloud model (SMCM) to ultimately support the design of a novel convection parameterization with stochastic elements are investigated. Observations of tropical convection obtained at Darwin and Kwajalein are used here. It is found that the variability of observed tropical convection generally decreases with increasing large-scale forcing, implying a transition from stochastic to more deterministic behavior with increasing forcing. Convection shows a more systematic relationship with measures related to large-scale convergence compared to measures related to energetics (e.g., CAPE). Using the observations, the parameters in the SMCM are adjusted. Then, the SMCM is forced with the time series of the observed large-scale state and the simulated convective behavior is compared to that observed. It is found that the SMCM cloud fields compare better with observations when using predictors related to convergence rather than energetics. Furthermore, the underlying framework of the SMCM is able to reproduce the observed functional dependencies of convective variability on the imposed large-scale state—an encouraging result on the road toward a novel convection parameterization approach. However, establishing sound cause-and-effect relationships between tropical convection and the large-scale environment remains problematic and warrants further research.

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Vickal V. Kumar
,
Alain Protat
,
Christian Jakob
, and
Peter T. May

Abstract

Some cumulus clouds with tops between 3 and 7 km (Cu3km–7km) remain in this height region throughout their lifetime (congestus) while others develop into deeper clouds (cumulonimbus). This study describes two techniques to identify the congestus and cumulonimbus cloud types using data from scanning weather radar and identifies the atmospheric conditions that regulate these two modes. A two-wet-season cumulus cloud database of the Darwin C-band polarimetric radar is analyzed and the two modes are identified by examining the 0-dBZ cloud-top height (CTH) of the Cu3km–7km cells over a sequence of radar scans. It is found that ~26% of the classified Cu3km–7km population grow into cumulonimbus clouds. The cumulonimbus cells exhibit reflectivities, rain rates, and drop sizes larger than the congestus cells. The occurrence frequency of cumulonimbus cells peak in the afternoon at ~1500 local time—a few hours after the peak in congestus cells. The analysis of Darwin International Airport radiosonde profiles associated with the two types of cells shows no noticeable difference in the thermal stability rates, but a significant difference in midtropospheric (5–10 km) relative humidity. Moister conditions are found in the hours preceding the cumulonimbus cells when compared with the congestus cells. Using a moisture budget dataset derived for the Darwin region, it is shown that the existence of cumulonimbus cells, and hence deep convection, is mainly determined by the presence of the midtroposphere large-scale upward motion and not merely by the presence of congestus clouds prior to deep convection. This contradicts the thermodynamic viewpoint that the midtroposphere moistening prior to deep convection is solely due to the preceding cumulus congestus cells.

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Bhupendra A. Raut
,
Michael J. Reeder
, and
Christian Jakob

Abstract

Previous work has shown that the sharp fall in winter rainfall over coastal southwestern Australia in the 1970s was mainly due to a fall in the frequency of fronts; the gradual reduction in rainfall since the late 1990s was due to a reduction in the number of light-rain days; and the increased inland summer rainfall in the 1970s was due to an increased number of easterly troughs. The current paper extends this earlier work by identifying the rainfall patterns in the region in 14 CMIP5 models for the period 1980–2005 and by calculating how these patterns are projected to change in the twenty-first century. The patterns are identified using k-means clustering of the rainfall, which are validated against observed rainfall clusters. Although the agreement between the models and the observation is generally good, the models underestimate the frequency of raining fronts. In both representative concentration pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) scenarios the number of dry days increases significantly at the expense of light-rain days and frontal rainfall. However, these trends are twice as large in the RCP8.5 scenario as in the RCP4.5 scenario. The reduction in the rainfall from the historical period to the second half of the twenty-first century is produced mainly by a reduction in both the frequency and intensity of light rain and a reduction in the frequency of fronts in the westerlies.

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Sugata Narsey
,
Michael J. Reeder
,
Duncan Ackerley
, and
Christian Jakob

Abstract

The initiation of northern Australian monsoon rainfall bursts is accompanied by an increase in cyclonic circulation in the monsoon region. This study shows that the change in circulation at the start of the composite rainfall burst is predominantly influenced by midlatitude frontlike features. By exploiting the relationship between circulation tendency and the convergence of absolute vorticity flux, the circulation changes accompanying the initiation of Australian monsoon bursts is investigated. Moisture flux convergence is found to be proportional to the circulation changes in the monsoon region. Using a composite analysis it is shown that absolute vorticity fluxes through the southern boundary are by far the most important influence on monsoon burst circulation changes, with only one-third of events more closely related to other influences including the Madden–Julian oscillation. This is shown to be true throughout the wet season.

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Evan Weller
,
Kay Shelton
,
Michael J. Reeder
, and
Christian Jakob

Abstract

Precipitation is often organized along coherent lines of low-level convergence, which at longer time and space scales form well-known convergence zones over the world’s oceans. Here, an automated, objective method is used to identify instantaneous low-level convergence lines in reanalysis data and calculate their frequency for the period 1979–2013. Identified convergence lines are combined with precipitation observations to assess the extent to which precipitation around the globe is associated with convergence lines in the lower troposphere. It is shown that a large percentage of precipitation (between 65% and 90%) over the tropical oceans is associated with such convergence lines, with large regional variations of up to 30% throughout the year, especially in the eastern Pacific and Atlantic Oceans. Over land, the annual-mean proportion of precipitation associated with convergence lines ranges between 30% and 60%, and the lowest proportions (less than 15%) associated with convergence lines occur on the eastern flank of the subtropical highs. Overall, much greater precipitation is associated with long coherent lines (greater than 300 km in length) than with shorter fragmented lines (less than 300 km), and the majority of precipitation associated with shorter lines occurs over land. The proportion of precipitation not associated with any convergence line primarily occurs where both precipitation and frequency of convergence lines are low. The high temporal and spatial resolution of the climatology constructed also enables an examination of the diurnal cycle in the relationship between convergence lines and precipitation. Here an example is provided over the tropical Maritime Continent region.

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