Ice supersaturation in the ECMWF integrated forecast system

Abstract

ABSTRACT:A parametrization for ice supersaturation is introduced into the ECMWF Integrated Forecast System (IFS), compatible with the cloud scheme that allows partial cloud coverage. It is based on the simple, but often justifiable, diagnostic assumption that the ice nucleation and subsequent depositional growth time-scales are short compared to the model time step, thus supersaturation is only permitted in the clear-sky portion of the grid cell.Results from model integrations using the new scheme are presented, which is demonstrated to increase uppertropospheric humidity, decrease high-level cloud cover and, to a much lesser extent, cloud ice amounts, all as expected from simple arguments. Evaluation of the relative distribution of supersaturated humidity amounts shows good agreement with the observed climatology derived from in situ aircraft observations. With the new scheme, the global distribution of frequency of occurrence of supersaturated regions compares well with remotely sensed microwave limb sounder (MLS) data, with the most marked errors of underprediction occurring in regions where the model is known to underpredict deep convection. Finally, it is also demonstrated that the new scheme leads to improved predictions of permanent contrail cloud over southern England, which indirectly implies upper-tropospheric humidity fields are better represented for this region.

Water vapor transport and dehydration above convective outflow during Asian monsoon

“…[12] We find best agreement with observations for K = 0.87, which is slightly higher than the value of K = 0.8 reported by Tompkins et al [2007]. Since RH crit depends mainly on the temperature, moisture is controlled by the minimum temperature along the trajectory.…”

“…Moistening can be obtained by other microphysical processes than supersaturation, like for example evaporation of ice crystals, not included in our model. However the results of Bonazzola and Haynes [2004] based on the model of Gettelman et al [2002a] indicate that these processes are secondary and supersaturation, on the other hand, is an important process that must be included in large-scale models [Tompkins et al, 2007].…”

Evaluation of the updated regional climate model RACMO2.3: summer snowfall impact on the Greenland Ice Sheet

“…The new cloud scheme includes an ice supersaturation parameterisation, which prolongs the vapour phase at low temperatures (Tompkins et al, 2007). The auto-conversion coefficient, controlling the conversion rate of water-vapour into precipitation in convective clouds, has been defined individually for liquid and ice water clouds, following Sundqvist (1978).…”

Global Modeling of the Contrail and Contrail Cirrus Climate Impact

“…Climate models exhibit commonly significant moisture biases in the upper troposphere/lower stratosphere and do not explicitly allow for ice supersaturation, which is essential in simulating formation, persistence, and optical properties of ice clouds. Ice supersaturation has been either explicitly represented in a numerical weather prediction model prescribing an empirical ice nucleation threshold, while retaining simplified ice microphysics (Tompkins et al 2007), or its fractional area has been parameterized in a global climate model (Burkhardt et al 2008b). Using an estimate of the ice-supersaturated area, the life cycle of contrail cirrus can be simulated.…”