Abstract
Abstract. Uptake and removal of soluble trace gases and aerosols by precipitation represents a major uncertainty in the processes that control the vertical distribution of atmospheric trace species. Model representations of precipitation scavenging vary greatly in their complexity, and most are divorced from the physics of precipitation formation and transformation. Here, we describe a new large-scale precipitation scavenging algorithm, developed for the UCI chemistry-transport model (UCI-CTM), that represents a step toward a more physical treatment of scavenging through improvements in the formulation of the removal in sub-gridscale cloudy and ambient environments and their overlap within the column as well as ice phase uptake of soluble species. The UCI algorithm doubles the lifetime of HNO3 in the upper troposphere relative to a scheme with commonly used fractional cloud cover assumptions and ice uptake determined by Henry's Law and provides better agreement with HNO3 observations. We find that the process of ice phase scavenging of HNO3 is a critical component of the tropospheric O3 budget, but that NOx and O3 mixing ratios are relatively insensitive to large differences in the removal rate. Ozone abundances are much more sensitive to the lifetime of HNO4, highlighting the need for better understanding of its interactions with ice and for additional observational constraints.
Highlights
We describe a new treatment for large-scale precipitation scavenging that was developed with a focus on improving two aspects of the parameterization: (1) the overlap between cloud condensate and precipitation, which determines the partitioning between in-cloud and below-cloud scavenging and (2) the scavenging of soluble gases, in particular HNO3, by frozen precipitation
Scavenging is a highly complex process that depends on the spatial distribution of clouds, the overlap between condensate and precipitation, and the exchange of mass between clouds and the environment, as well as on the details of the microphysical processes of precipitation formation, growth, and evaporation
The partitioning of the precipitation into subgrid fractions is based on reasonable microphysical assumptions and is generally consistent with the JK00 precipitation parameterization in the ECMWF Integrated Forecast System, and it can be greatly improved if additional microphysical output is available from the underlying meteorological model
Summary
We describe a new treatment for large-scale precipitation scavenging that was developed with a focus on improving two aspects of the parameterization: (1) the overlap between cloud condensate and precipitation, which determines the partitioning between in-cloud and below-cloud scavenging and (2) the scavenging of soluble gases, in particular HNO3, by frozen precipitation Together, these processes constitute the largest inter-model differences in the representation of gas phase precipitation scavenging in modern global chemistry-transport models (CTMs). Prather: Cloud overlap and ice physics and their impact on tropospheric ozone some way, but here we define these volumes and their associated microphysical processes consistently based on a vertical overlap model in which cloudy layers connected by precipitation are maximally overlapped We use this model to assess the importance of vertical cloud structure in scavenging by large-scale precipitation. We assume that new precipitation forms from aggregation of these HNO3-containing ice crystals, which are in equilibrium with conditions where they originate, and calculate additional uptake of ice and HNO3 by riming as the precipitation falls
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