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Adrian M. Tompkins

Abstract

A modeling study is conducted to gain insight into the factors that control the intensity and organization of tropical convection, and in particular to examine if organization occurs in the absence of factors such as vertical wind shear or underlying sea surface temperature (SST) gradient. The control experiment integrates a cloud-resolving model for 15 days using a 3D domain exceeding 1000 km in length, with no imposed winds, and horizontally uniform SST and forcing for convection. After 2 days of random activity, the convection organizes into clusters with dimensions of approximately 200 km. Convective systems propagate through the clusters at speeds of 2–3 m s−1, while the clusters themselves propagate at minimal speeds of around 0.5 m s−1.

Examining the thermodynamic structure of the model domain, it is found that the convective free bands separating the clusters are very dry throughout the troposphere, and due to virtual temperature effects, are correspondingly warmer in the lower troposphere and boundary layer. This suggests a positive feedback between convection and water vapor, where convective moistening of the local atmosphere renders it more favorable to future convection. The existence of this feedback is demonstrated by experiments in which the free-tropospheric water vapor is perturbed in convective regions, and it is found that the lower-atmospheric water vapor is most critical in controlling convection, most likely through the role of downdrafts. Examination of the boundary layer in the control experiment also indicated that convectively generated cold pools also play a key role in the organization of convection, possibly by their influence on the boundary layer water vapor field.

In order to see how the water vapor feedback modifies established convective organization, a further experiment was conducted with an SST gradient imposed, which established a mock Walker cell type circulation, with ascending motion over the warmest SSTs. After 5 days, the SST gradient is reversed to see how the convection would establish itself over the new SST maximum. This highly idealized experiment therefore represents a surrogate for the atmospheric response to SST “hotspots,” that observations have shown to form under the descending branch of large-scale tropical circulations such as the Madden–Jullian oscillation, due to increased incident solar radiation and decreased latent heat fluxes at the surface. It is found that the convection does not spontaneously initiate over the new SST maximum, but instead must propagate toward it. After a further 5 days, much longer than the boundary layer adjustment timescale, the warmest SSTs are still completely free from convection. This is directly due to the dryness of the atmosphere caused by the initial period of subsidence.

A further set of experiments examines the robustness of the feedback in cases of imposed vertical wind shear. It is found that strong wind shears prevent the feedback by effectively mixing water vapor. However, the feedback is still very important in cases of weak wind shears.

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Adrian M. Tompkins

Abstract

An investigation is conducted to document the role convectively generated cold pools play in determining the spatial organization of tropical deep convection. Using a high-resolution cloud-resolving model, the evolution of cold pools produced by deep convection is examined, in the situation of limited large-scale wind shear, and a homogeneous underlying sea surface temperature. Ignoring the cold pools resulting from multiple deep convective events, the mean model cold pool attained a minimum temperature and water vapor mixing ratio depression of 1 K and 1.5 g kg−1, respectively; a horizontal velocity increase of 4.8 m s−1; and the latent and sensible heat fluxes are increased by a factor of 1.9 and 2.6, respectively. The cold pools had a mean lifetime of approximately 2.5 h and attained maximum radii ranging from 3 to 18 km, with a mean of 8.6 km. Taking the organization of convection into account, these figures are consistent with observational studies of convective wakes.

The composite cold pool showed that development occurred in three distinct stages. As seen in observations, the air in the vicinity of deep convection has a higher equivalent potential energy than average. In the first stage, before the downdraft develops and reaches the subcloud layer, the area below the convection is cooled and moistened by the evaporation of rainfall. The downdraft then injects cold and dry air into the boundary layer, and the spreading cold pool is consequentially moister than average just inside the gust front but drier in the central regions. Finally, mass conservation requires that air from above the boundary layer be entrained into the wake of the expiring downdraft—thus causing the central regions of the cold pool to recover very quickly in temperature—but increases further the moisture perturbation. These features are confirmed by a number of observational studies.

The key to the triggering of new deep convective cells lies with the band of high equivalent potential temperature, but negatively buoyant air, situated inside the boundary of the spreading cold pools. It is this air that forms the new convective cells. The radius at which this occurs is determined by the time taken for surface fluxes to remove the negative temperature perturbation, thereby reducing convective inhibition energy. In summary, the primary mechanism by which cold pools organize tropical deep convection in low wind shear conditions is principally thermodynamical, and not dynamical as previously assumed.

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Adrian M. Tompkins

Abstract

Tropical observations show convective activity increasing sharply above sea surface temperatures (SSTs) of around 26°C and then decreasing as the SST exceeds 30°C, with maximum observed SSTs of around 32°C.Although some aspects of this relationship are reasonably well understood, as of yet no theory has explained the decrease in convective activity above 30°C. Here it is suggested that this aspect of the relationship could result from a organizational positive feedback, sometimes termed “self aggregation,” whereby the occurrence of convection makes future convection more likely to occur in the same location. Using cloud-resolving simulations, it is shown that the feedback between convection and the water vapor field is a good candidate for this role.

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Adrian M. Tompkins

Abstract

A parameterization for the horizontal subgrid-scale variability of water vapor and cloud condensate is introduced, which is used to diagnose cloud fraction in the spirit of statistically based cloud cover parameterizations. High-resolution cloud-resolving model data from tropical deep convective scenarios were used to justify the choice of probability density function (PDF). The PDF selected has the advantage of being bounded above and below, avoiding the complications of negative or infinite water mixing ratios, and can give both negatively and positively skewed functions as well as symmetric Gaussian-like bell-shaped curves, without discrete transitions, and is mathematically straightforward to implement.

A development from previous statistical parameterizations is that the new scheme is prognostic, with processes such as deep convection, turbulence, and microphysics directly affecting the distribution of higher-order moments of variance and skewness. The scheme is able to represent the growth and decay of cirrus cloud decks and also the creation of cloud in clear sky or breakup of an overcast cloud deck by boundary layer turbulence. After introducing the mathematical framework, results using the parameterization in a climate model are shown to illustrate its behavior. The parameterization is shown to reduce cloud cover biases almost globally, with a marked improvement in the stratocumulus regions in the eastern Pacific and Atlantic Oceans.

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Adrian M. Tompkins and Laura Feudale

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The West Africa monsoon precipitation of the ECMWF operational Seasonal Forecast System (SYS3) is evaluated at a lead time of 2–4 months in a 49-yr hindcast dataset, with special attention paid to the African Monsoon Multidisciplinary Analysis (AMMA) special observation period during 2006. In both the climatology and the year 2006 the SYS3 reproduces the progression of the West Africa monsoon but with a number of differences, most notably a southerly shift of the precipitation in the main monsoon months of July and August and the lack of preonset rainfall suppression and sudden onset jump. The model skill at predicting summer monsoon rainfall anomalies has increased in recent years indicating improvements in the ocean analysis since the 1990s.

Examination of other model fields shows a widespread warm sea surface temperature (SST) bias exceeding 1.5 K in the Gulf of Guinea throughout the monsoon months in addition to a cold bias in the North Atlantic, which would both tend to enhance rainfall over the Gulf of Guinea coast at the expense of the monsoon rainfall over the Sahel. Seasonal forecasts were repeated for 2006 using the same release of the atmospheric forecast model forced by observed SSTs, and the monsoon rainfall reverts to its observed position, indicating the importance of the SST biases.

A lack of stratocumulus off the west coast of Africa in SYS3 was hypothesized as a possible cause of the systematic rain and SST biases. Two more sets of ensembles were thus conducted with atmospheric model upgrades designed to tackle radiation, deep convection, and turbulence deficiencies. While these enhancements improve the simulation of stratocumulus significantly, it is found that the improvement in the warm SST bias is limited in scope to the southern cold tongue region. In contrast, the changes to the representation of convection cause an increase in surface downwelling shortwave radiation that, combined with latent heat flux changes associated with the wind stress field, increases the SST warm bias on and to the north of the equator. Thus, while the precipitation shortfall in the Sahel is reduced with the new physics, the overestimated rainfall of SYS3 in the coastal region is further enhanced, degrading the model systematic errors overall in the West Africa region. Finally, the difference in the systematic biases between the coupled and uncoupled systems was noted to be an impediment to the development of seamless forecasting systems.

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Adrian M. Tompkins and Adeyemi A. Adebiyi

Abstract

Daily precipitation retrievals from three algorithms [the Tropical Rainfall Measuring Mission 3B42 rain product (TRMM-3B42), the Climate Prediction Center morphing technique (CMORPH), and the second version (RFEv2) of the Famine Early Warning System (FEWS)] and CloudSat retrievals of cloud liquid water, ice amount, and cloud fraction are used to document the cloud structures associated with rainfall location and intensity in the West African monsoon. The different rainfall retrieval approaches lead to contrasting cloud sensitivities between all three algorithms most apparent in the onset period of June and July. During the monsoon preonset phase, CMORPH produces a precipitation peak at around 12°N associated with upper-level cirrus clouds, while FEWS and TRMM both produce rainfall maxima collocated with the tropospheric–deep convective cloud structures at 4°–6°N. In July similar relative displacements of the rainfall maxima are observed. Conditional sampling of several hundred convection systems proves that, while upper-level cirrus is advected northward relative to the motion of the convective system cores, the reduced cover and water content of lower-tropospheric clouds in the northern zone could be due to signal attenuation as the systems there appear to be more intense, producing higher ice water contents. Thus, while CMORPH may overestimate rainfall in the northern zone due to its reliance on cloud ice, TRMM and FEWS are likely underestimating precipitation in this zone, potentially due to the use of infrared based products in TRMM and FEWS when microwave is not available. Mapping the CloudSat retrievals as a function of rain rate confirms the greater sensitivity of CMORPH to ice cloud and indicates that high-intensity rainfall events are associated with systems that are deeper and of a greater spatial scale.

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Adrian M. Tompkins and Francesca Di Giuseppe

Abstract

Shortwave radiative transfer depends on the cloud field geometry as viewed from the direction of the sun. To date, the radiation schemes of large-scale models only consider a zenith view of the cloud field, and the apparent change in the cloud geometry with decreasing solar zenith angle is neglected. A simple extension to an existing cloud overlap scheme is suggested to account for this for the first time. It is based on the assumption that at low sun angles, the overlap between cloud elements is random for an unscattered photon. Using cloud scenes derived from radar retrievals at two European sites, it is shown that the increase of the apparent cloud cover with a descending sun is reproduced very well with the new scheme. Associated with this, there is a marked reduction in the mean radiative biases averaged across all solar zenith angles with respect to benchmark calculations. The scheme is implemented into the ECMWF global forecast model using imposed sea surface temperatures, and while the impact on the radiative statistics is significant, the feedback on the large-scale dynamics is minimal.

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Thomas Jung, Laura Ferranti, and Adrian M. Tompkins

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The sensitivity of the atmospheric circulation to the warm Mediterranean sea surface temperature (SST) anomalies observed during the summer of 2003 (July and August) is studied using the European Centre for Medium-Range Weather Forecasts (ECMWF) model. A control integration imposes climatological Mediterranean SSTs as a lower boundary condition. The first sensitivity experiment uniformly increases these Mediterranean SSTs by 2 K, the approximate mean observed in the 2003 summer season. A second experiment then investigates the additional impact of the SST distribution by imposing the observed SST summer anomaly.

The response of the atmospheric circulation in the European area shows some resemblance to the observed anomaly. The weakness of this response suggests, however, that the warm Mediterranean played a minor role, if any, in maintaining the anomalous atmospheric circulation as observed in the summer of 2003. Increasing SST in the Mediterranean locally leads to an increase in precipitation, particularly in the western Mediterranean. Furthermore, significantly increased Sahelian rainfall is simulated, deriving from enhanced evaporation in the Mediterranean Sea. In the ECMWF model the anomalously high moisture is advected by the climatological Harmattan and Etesian winds, where enhanced moisture flux convergence leads to more precipitation. The associated diabatic heating leads to a reduction of the African easterly jet strength. A similar Sahelian response has been previously documented using a different atmospheric model, increasing confidence in the robustness of the result. Finally, the results are discussed in the context of the seasonal predictability of European and African climate.

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Adrian M. Tompkins and George C. Craig

Abstract

The response of convection to changing sea surface temperature (SST) in the absence of large-scale flow is examined, using a three-dimensional cloud resolving model. The model includes a five-category bulk microphysical scheme representing snow, ice, graupel, rain, and cloud amounts in addition to an interactive radiation scheme for the shortwave and infrared. Long integrations are made to achieve a radiative–convective equilibrium state for SSTs of 298, 300, and 302 K, for which cloud and convection statistics are analyzed.

The main conclusion of the paper is that, despite significant temperature sensitivities in many of the conversion terms between bulk water categories, convection is very insensitive to changing SST in the absence of large-scale flow. This is a result of the moist adiabatic temperature profile that the tropical atmosphere is constrained to take. A parcel of air rising through a deep convective cloud experiences approximately the same range of temperatures but at higher altitudes as SST increases. Thus the vertical profiles of cloud fraction and other cloud-related statistics are simply shifted in height, but not changed in overall magnitude.

The small changes in cloud properties that do occur lead to a small reduction in cloud fraction as SST increases. This appears to be due to an increase in graupel amounts with respect to snow, giving smaller cloud fractions since graupel has a higher fall velocity. The radiative effects of the changes in atmospheric properties are examined and it is found that the model atmosphere exhibits no supergreenhouse effect since atmospheric relative humidity is not altered significantly by the SST changes. The water vapor feedback effect is largely canceled by the change in temperature. Clouds have a negligibly small, but highly nonlinear, feedback in the model climate, in the absence of large-scale flow.

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Francesca Di Giuseppe and Adrian M. Tompkins

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Six months of CloudSat and CALIPSO observations have been divided into over 8 million cloud scenes and collocated with ECMWF wind analyses to identify an empirical relationship between cloud overlap and wind shear for use in atmospheric models. For vertically continuous cloudy layers, cloud decorrelates from maximum toward random overlap as the layer separation distance increases, and the authors demonstrate a systematic impact of wind shear on the resulting decorrelation length scale. As expected, cloud decorrelates over smaller distances as wind shear increases. A simple, empirical linear fit parameterization is suggested that is straightforward to add to existing radiation schemes, although it is shown that the parameters are quite sensitive to the processing details of the cloud mask data and also to the fitting method used. The wind shear–overlap dependency is implemented in the radiation scheme of the ECMWF Integrated Forecast System. It has a similar-magnitude impact on the radiative budget as that of switching from a fixed decorrelation length scale to the latitude-dependent length scale presently used in the operational model, altering the zonal-mean, top-of-atmosphere, equator-to-midlatitude gradient of shortwave radiation by approximately 2 W m−2.

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