Comments On The Nature Article “Afternoon Rain More Likely Over Drier Soils” By Taylor Et Al 2012 – More Rocking Of The IPCC Boat

I have posted on the documented (in peer reviewed papers) gross inadequacies of the global climate models to provide skillful predictions in a hindcast mode; e.g.. see

More CMIP5 Regional Model Shortcomings

CMIP5 Climate Model Runs – A Scientifically Flawed Approach

I have been alerted to yet another study that documents this shortcoming [h/t Marcel Crok]. The final sentence of this study has the blunt comment regarding climate model skill that

“….the erroneous sensitivity of convection schemes demonstrated here is likely to contribute to a tendency for large-scale models to `lock-in’ dry conditions, extending droughts unrealistically, and potentially exaggerating the role of soil moisture feedbacks in the climate system.”

This new paper is

Taylor et al, 2012: Afternoon rain more likely over drier soils. Nature. doi:10.1038/nature11377. Received 19 March 2012 Accepted 29 June 2012 Published online 12 September 2012

[as a side note, I published with the senior author in the paper Taylor, C.M., R.J. Harding, R.A. Pielke, Sr., P.L. Vidale, R.L. Walko,  and J.W. Pomeroy, 1998: Snow breezes in the boreal forest. J. Geophys.  Res., 103, 23087-23101.]

The abstract reads [highlight added]

Land surface properties, such as vegetation cover and soil moisture, influence the partitioning of radiative energy between latent and sensible heat fluxes in daytime hours.During dry periods, soil-water deficit can limit evapotranspiration, leading to warmer and drier conditions in the lower atmosphere. Soil moisture can influence the development of convective storms through such modifications of low-level atmospheric temperature and humidity, which in turn feeds back on soil moisture. Yet there is considerable uncertainty in how soil moisture affects convective storms across the world, owing to a lack of observational evidence and uncertainty in large-scale models. Here we present a global-scale observational analysis of the coupling between soil moisture and precipitation. We show that across all six continents studied, afternoon rain falls preferentially over soils that are relatively dry compared to the surrounding area. The signal emerges most clearly in the observations over semi-arid regions, where surface fluxes are sensitive to soil moisture, and convective events are frequent. Mechanistically, our results are consistent with enhanced afternoon moist convection driven by increased sensible heat flux over drier soils, and/or mesoscale variability in soil moisture. We find no evidence in our analysis of a positive feedback—that is, a preference for rain over wetter soils—at the spatial scale (50–100 kilometres) studied. In contrast, we find that a positive feedback of soil moisture on simulated precipitation does dominate in six state-of-the-art global weather and climate models— a difference that may contribute to excessive simulated droughts in large-scale models.

The conclusions include the text

Finally, we repeat our analysis using 3-hourly diagnostics from six global models, ranging in horizontal resolution from 0.5 to 2.0 (degrees). Our results (Fig. 3) indicate a strong preference for rain over wet soils for large parts of the world, in contrast to the observations. Only one model (Fig. 3e) produces more than the expected 10% of grid cells with P<10, largely due to contributions at mid-latitudes. The crossmodel signal favouring precipitation over wet soil, particularly across the tropics (Supplementary Table 3), demonstrates a fundamental failing in the ability of convective parameterizations to represent land feedbacks on daytime precipitation. This is likely to be linked to the oft-reported phase lag in the diurnal cycle of precipitation; that is, simulated rainfall tends to start several hours too early, and is possibly amplified by a lack of boundary-layer clouds in some models. This weakness has been related to the crude criteria used to trigger deep convection in large-scale models. The onset of convective precipitation is overly sensitive to the daytime increase of moist convective instability, which is typically faster over wetter soils3, favouring a positive feedback. Early initiation limits the effect of other daytime processes on triggering convection in the models. In contrast, our observational analysis points to the importance of dry boundary-layer dynamics for this phenomenon over land.

The observed preference for afternoon rain over locally drier soil on scales of 50–100 km is consistent with a number of regional studies based on remotely sensed data. Our failure to find areas of positive feedback may indicate the importance of surface-induced mesoscale flows in triggering convection, although the coarse spatial resolution of our data sets prevents us from drawing firm conclusions on this issue. Equally, mixing processes in the growth stage of convective clouds before precipitation, may play an important role. Neither of these processes is captured in existing one-dimensional analyses. Furthermore, our results raise questions about the ability of models reliant on convective parameterizations to represent these processes adequately. Although the coarser-resolution models analysed here (HadGEM2, CNRM-CM5 and INMCM4) cannot resolve mesoscale soil moisture structures, nor their potential impacts on convective triggering, all the models have a strong tendency towards rain over wetter soils, for which we find no observational support. Our study does not, however, imply that the soil moisture feedback is negative at temporal and spatial scales different from those analysed here. The multi-day accumulation of moisture in the lower atmosphere from a freely transpiring land surface may provide more favourable initial (dawn) conditions for daytime convection than the equivalent accumulation over a drought-affected region. Equally, the large-scale dynamical response to soil moisture may dominate in some regions. However, the erroneous sensitivity of convection schemes demonstrated here is likely to contribute to a tendency for large-scale models to `lock-in’ dry conditions, extending droughts unrealistically, and potentially exaggerating the role of soil moisture feedbacks in the climate system.

We have also looked at this issue, for example, in our paper

Pielke Sr., R.A., 2001: Influence of the spatial distribution of vegetation  and soils on the prediction of cumulus convective rainfall. Rev. Geophys.,  39, 151-177

where we reported that

Wetzel et al. [1996], in a study in the Oklahoma area, found that cumulus clouds form first over hotter, more sparsely vegetated areas. Over areas covered with deciduous forest, clouds were observed to form 1–2 hours later due to the suppression of vertical mixing. Rabin et al. [1990] also found from satellite images that cumulus clouds form earliest over regions of large sensible heat flux and are suppressed over regions with large latent heat flux during relatively dry atmospheric conditions.

While we also reported that

Clark and Arritt [1995], however, found while the cumulus cloud precipitation was delayed when the soil moisture was higher, the total accumulation of precipitation was greater. The largest rainfall was generally predicted to occur for moist, fully vegetated surfaces…..De Ridder [1998] found that dense vegetation produces a positive feedback to precipitation.

This indicates, however, that if the delay is long enough during the day over moist soils and/or transpiring vegetation, cumulus rainfall will not ever be able to develop.

The take-home message from the Taylor et al 2012 paper is yet another demonstration of the failure of the climate models as a tool to skillfully simulate the climate system. Their use by the IPCC and others to predict droughts decades from now for the impacts communties is misleading and erroneous abuse of the current state of climate science. 

source of image

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