Sensitivity to large-scale environmental fields of the relaxed Arakawa-Schubert parameterization in the NASA GEOS-1 GCM

An adjoint sensitivity analysis of the relaxed Arakawa-Schubert scheme in t he National Aeronautics and Space Administration GEOS-1 GCM with respect to perturbations in large-scale environmental fields was conducted. The respo nse functions were defined as measures of the strength of convective cloud precipitation, the cloud-induced heating and drying (moistening) in both th e instantaneous and time-integrated sense. The roles of different variables in producing variations on the response functions were evaluated and the m ost sensitive vertical levels of the perturbations were identified with the gradient provided by the adjoint model. It was found that the potential temperature perturbations had significant i mpact on ail the response functionals analyzed, especially on the convectiv e precipitation. The perturbations at subcloud layers and at midtroposphere from 500 to 600 hPa were found to be the most influential. The impact from the moisture fields was most significant on cloud heating and drying effec ts and the strongest influence came from the subcloud layers. The moisture perturbations at midtroposphere also significantly influenced the cloud dry ing (moistening) effect. On the other hand, the cloud-induced heating and d rying at levels between 400 and 600 hPa felt the strongest impact from pert urbations in large-scale fields. The influence of the perturbations in the wind field was weaker but still provided reasonable sensitivity patterns. T he time-integrated and instantaneous sensitivities for the same response di ffer only in magnitude but not in the general distributions. The impact of large-scale condensation and reevaporation on the sensitivity was also evaluated. Their effect was significant at the midtropospheric le vel and they enhanced the model sensitivity to perturbations in temperature and moisture fields. The sensitivity analysis results obtained indicated that accurate gridscale vertical profile of temperature and moisture, especially at subcloud layer s and midtroposphere between 500 and 600 hPa were essential for the accurat e evaluation of the cumulus cloud effects. The implications of the results of this work for variational data assimilation were also discussed.