Leon D. Rotstayn

Tropical Rainfall Trends and the Indirect Aerosol Effect

An atmospheric global climate model coupled to a mixed layer ocean model is used to study changes in tropical rainfall due to the indirect effects of anthropogenic sulfate aerosol. The model is run to equilibrium for present-day (PD) and preindustrial (PI) sulfur emission scenarios. As in two other recent studies, the model generally gives a southward shift of tropical rainfall in the PD run relative to the PI run. This is largely due to a hemispheric asymmetry in the reduction of sea surface temperature (SST) induced by the perturbation of cloud albedo and lifetime. Observed precipitation trends over land for the period 1900‐98 show a complex pattern in the Tropics, but when zonally averaged, a southward shift similar to (but weaker than) the modeled shift is clearly evident. The zonally averaged tropical trends are significant at the 5% level in several latitude bands. The modeled presentday hemispheric contrast in cloud droplet effective radius (which affects cloud albedo) is well supported by one long-term satellite retrieval, but not by another. A third satellite retrieval, which only covers an 8-month period, does show a marked hemispheric contrast in effective radius. Both in the modeled changes and the observed trends, a prominent feature is the drying of the Sahel in North Africa. Modeled dynamical changes in this region are similar to observed changes that have been associated with Sahelian drought. Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The present results, combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. More generally, it is concluded that spatially varying aerosol-related forcing (both direct and indirect) can substantially alter low-latitude circulation and rainfall.

A Scheme for Calculation of the Liquid Fraction in Mixed-Phase Stratiform Clouds in Large-Scale Models

A scheme for calculation of the liquid fraction f l in mixed-phase stratiform clouds has been developed for use in large-scale models. An advantage of the scheme, compared to the interpolation in temperature that is typically used, is that it makes it possible to simulate the life cycles of mixed-phase clouds, and the differences between deep and shallow clouds. The central part of the scheme is a physically based calculation of the growth of cloud ice crystals by vapor deposition at the expense of coexisting cloud liquid water, the so-called Bergeron‐ Findeisen mechanism. Versions of this calculation have been derived for three different ice-crystal habits (spheres, hexagonal plates, or columns) and for two different assumed spatial relationships of the coexisting cloud ice and liquid water (horizontally adjacent or uniformly mixed). The scheme also requires a parameterization of the ice crystal number concentration Ni. The variation with temperature of f l looks broadly realistic compared to aircraft observations taken in the vicinity of the British Isles when the scheme is used in the CSIRO GCM, if Ni is parameterized using a supersaturation-dependent expression due to Meyers et al. If Fletcher’s earlier temperature-dependent expression for Ni is used, the scheme generates liquid fractions that are too large. There is also considerable sensitivity to the ice-crystal habit, and some sensitivity to model horizontal resolution and to the assumed spatial relationship of the liquid water and ice. A further test shows that the liquid fractions are lower in cloud layers that are seeded from above by falling ice, than in layers that are not seeded in this way. The scheme has also been tested in limited-area model simulations of a frontal system over southeastern Australia. Supercooled liquid water forms initially in the updraft, but mature parts of the cloud are mostly glaciated down to the melting level. This behavior, which is considered to be realistic based on observations of similar cloud systems, is not captured by a conventional temperature-dependent parameterization of f l. The variation with temperature of f l shows a marked sensitivity to the assumed spatial relationship of the liquid water and ice. The results obtained using the uniformly mixed assumption are considered to be more realistic than those obtained using the horizontally adjacent assumption. There is also much less sensitivity to the time step when the former assumption is used.

Intercomparison and evaluation of cumulus parametrizations under summertime midlatitude continental conditions

This study reports the Single-Column Model (SCM) part of the Atmospheric Radiation Measurement (ARM)/the Global Energy and Water Cycle Experiment (GEWEX) Cloud System Study (GCSS) joint SCM and Cloud-Resolving Model (CRM) Case 3 intercomparison study, with a focus on evaluation of cumulus parametrizations used in SCMs. Fifteen SCMs are evaluated under summertime midlatitude continental conditions using data collected at the ARM Southern Great Plains site during the summer 1997 Intensive Observing Period. Results from ten CRMs are also used to diagnose problems in the SCMs. It is shown that most SCMs can generally capture well the convective events that were well-developed within the SCM domain, while most of them have difficulties in simulating the occurrence of those convective events that only occurred within a small part of the domain. All models significantly underestimate the surface stratiform precipitation. A third of them produce large errors in surface precipitation and thermodynamic structures. Deficiencies in convective triggering mechanisms are thought to be one of the major reasons. Using a triggering mechanism that is based on the vertical integral of parcel buoyant energy without additional appropriate constraints results in overactive convection, which in turn leads to large systematic warm/dry biases in the troposphere. It is also shown that a non-penetrative convection scheme can underestimate the depth of instability for midlatitude convection, which leads to large systematic cold/moist biases in the troposphere. SCMs agree well quantitatively with CRMs in the updraught mass fluxes, while most models significantly underestimate the downdraught mass fluxes. Neglect of mesoscale updraught and downdraught mass fluxes in the SCMs contributes considerably to the discrepancies between the SCMs and the CRMs. In addition, uncertainties in the diagnosed mass fluxes in the CRMs and deficiencies with cumulus parametrizations are not negligible. Similar results are obtained in the sensitivity tests when different forcing approaches are used. Finally, sensitivity tests from an SCM indicate that its simulations can be greatly improved when its triggering mechanism and closure assumption are improved.

On the “tuning” of autoconversion parameterizations in climate models

Autoconversion is a highly nonlinear process, which is usually evaluated in global climate models (GCMs) from the mean in-cloud value of the liquid-water mixing ratio q′l. This biases the calculated autoconversion rate, and may explain why it usually seems to be necessary to reduce the autoconversion threshold to an unrealistically low value to obtain a realistic simulation in a GCM. Two versions of a threshold-dependent autoconversion parameterization are compared in the CSIRO GCM. In the standard (“OLD”) treatment, autoconversion occurs in a grid box whenever the mean in-cloud q′l exceeds the threshold qcrit, which is derived from a prescribed threshold cloud-droplet radius rcrit. In the modified (“NEW”) version, the assumed subgrid moisture distribution from the models condensation scheme is applied in each grid box to determine the fraction of the cloudy area in which q′l > qcrit, and autoconversion occurs in this fraction only. Simulations are performed using both treatments, for present-day and preindustrial distributions of cloud-droplet concentration, and using different values for rcrit. Changing from the OLD to the NEW treatment means that rcrit can be increased from 7.5 μm to a more realistic 9.3 μm, while maintaining the global-mean liquid-water path at about the same value. Simulations for preindustrial and present-day conditions are compared, to see whether the change of scheme alters the modeled cloud-lifetime effect. It is found that the NEW scheme with rcrit = 9.3 μm gives a 0.5 W m−2 (62%) stronger cloud-lifetime effect than the OLD scheme with rcrit = 7.5 μm.

A comparison of single column model simulations of summertime midlatitude continental convection

Eleven different single-column models (SCMs) and one cloud ensemble model (CEM) are driven by boundary conditions observed at the Atmospheric Radiation Measurement (ARM) program southern Great Plains site for a 17 day period during the summer of 1995. Comparison of the model simulations reveals common signatures identifiable as products of errors in the boundary conditions. Intermodel differences in the simulated temperature, humidity, cloud, precipitation, and radiative fluxes reflect differences in model resolution or physical parameterizations, although sensitive dependence on initial conditions can also contribute to intermodel differences. All models perform well at times but poorly at others. Although none of the SCM simulations stands out as superior to the others, the simulation by the CEM is in several respects in better agreement with the observations than the simulations by the SCMs. Nudging of the simulated temperature and humidity toward observations generally improves the simulated cloud and radiation fields as well as the simulated temperature and humidity but degrades the precipitation simulation for models with large temperature and humidity biases without nudging. Although some of the intermodel differences have not been explained, others have been identified as model problems that can be or have been corrected as a result of the comparison.

Indirect Aerosol Forcing, Quasi Forcing, and Climate Response

Abstract The component of the indirect aerosol effect related to changes in precipitation efficiency (the second indirect or Albrecht effect) is presently evaluated in climate models by taking the difference in net irradiance between a present-day and a preindustrial simulation using fixed sea surface temperatures (SSTs). This approach gives a “quasi forcing,” which differs from a pure forcing in that fields other than the initially perturbed quantity have been allowed to vary. It is routinely used because, in contrast to the first indirect (Twomey) effect, there is no straightforward method of calculating a pure forcing for the second indirect effect. This raises the question of whether evaluation of the second indirect effect in this manner is adequate as an indication of the likely effect of this perturbation on the global-mean surface temperature. An atmospheric global climate model (AGCM) is used to compare the evaluation of different radiative perturbations as both pure forcings (when available) and...

Indirect forcing by anthropogenic aerosols : A global climate model calculation of the effective-radius and cloud-lifetime effects

A global climate model (GCM) that includes a physically based cloud scheme is used to calculate the indirect radiative forcing due to the modification of liquid-water cloud properties by anthropogenic aerosols. The distribution of cloud-droplet number concentration Nd required by the cloud scheme is estimated empirically from monthly mean fields of sulfate mass generated by a chemical transport model. The effects of anthropogenic changes in Nd are considered in the calculation of precipitation (the “cloud-lifetime” effect) and of the droplet effective radius used in the shortwave and longwave radiation schemes (the “effective-radius” effect). The modeled cloud-droplet effective radii for present-day conditions agree quite well with satellite-retrieved values, although the land-ocean and hemispheric contrasts are weaker in the model than in the observations. The total indirect forcing is −2.1 W m−2, including a small longwave forcing of +0.1 W m−2. The forcing results from a 1% increase in cloudiness, a 6% increase in liquid water path, and a 7% decrease in droplet effective radius. The breakdown of the total indirect forcing into the effective-radius and cloud-lifetime effects is estimated by performing separate GCM experiments in which each effect is included individually. The estimated forcings due to the effective-radius and cloud-lifetime effects are −1.2 and −1.0 W m−2, respectively. The calculated forcings show some sensitivity to the autoconversion threshold, the sulfate-Nd relation, and the vertical distribution of sulfate, but in each case the cloud-lifetime forcing is at least 25% of the total indirect forcing. These results suggest that the cloud-lifetime effect should not be ignored in future calculations of the indirect forcing due to anthropogenic aerosols.

Sensitivity of the First Indirect Aerosol Effect to an Increase of Cloud Droplet Spectral Dispersion with Droplet Number Concentration

Observations show that an increase in anthropogenic aerosols leads to concurrent increases in the cloud droplet concentration and the relative dispersion of the cloud droplet spectrum, other factors being equal. It has been suggested that the increase in effective radius resulting from increased relative dispersion may substantially negate the indirect aerosol effect, but this is usually not parameterized in global climate models (GCMs). Empirical parameterizations, designed to represent the average of this effect, as well as its lower and upper bounds, are tested in the CSIRO GCM. Compared to a control simulation, in which the relative dispersion of the cloud droplet spectrum is prescribed separately over land and ocean, inclusion of this effect reduces the magnitude of the first indirect aerosol effect by between 12% and 35%.

Variability and Trend of North West Australia Rainfall: Observations and Coupled Climate Modeling

Abstract Since 1950, there has been an increase in rainfall over North West Australia (NWA), occurring mainly during the Southern Hemisphere (SH) summer season. A recent study using twentieth-century multimember ensemble simulations in a global climate model forced with and without increasing anthropogenic aerosols suggests that the rainfall increase is attributable to increasing Northern Hemisphere aerosols. The present study investigates the dynamics of the observed trend toward increased rainfall and compares the observed trend with that generated in the model forced with increasing aerosols. It is found that the observed positive trend in rainfall is projected onto two modes of variability. The first mode is associated with an anomalously low mean sea level pressure (MSLP) off NWA instigated by the enhanced sea surface temperature (SST) gradients toward the coast. The associated cyclonic flows bring high-moisture air to northern Australia, leading to an increase in rainfall. The second mode is associa...

Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols

An atmospheric GCM coupled to a mixed-layer ocean model is used to study changes in rainfall due to the indirect effects of anthropogenic aerosols. The model in- cludes treatments of both the first and second indirect ef- fects. The most striking feature of the equilibrium rainfall response to the indirect effects of anthropogenic aerosols is a southward shift of the equatorial rainfall. We hypothesize that this is caused by a hemispheric asymmetry in the cool- ing of the sea surface. This is supported by another pair of experiments, in which the model is run with prescribed present-day SSTs. In one experiment the standard model is used, and in the other the SSTs in the Northern Hemisphere are increased by i K in the calculation of the surface fluxes, to mimic the effect of a return to pre-industrial conditions. Compared to the run with increased NH SST, the standard run shows a southward shift of equatorial rainfall, similar to that obtained as a response to anthropogenic aerosols.