The Impact of Two Coupled Cirrus Microphysics–Radiation Parameterizations on the Temperature and Specific Humidity Biases in the Tropical Tropopause Layer in a Climate Model

Abstract

The impact of two different coupled cirrus microphysics-radiation parameterizations on the zonally averaged temperature and humidity biases in the tropical tropopause layer (TTL) of a Met Office climate model configuration is assessed. One parameterization is based on a linear coupling between a model prognostic variable, the ice mass mixing ratio q i , and the integral optical properties. The second is based on the integral optical properties being parameterized as functions of q i and temperature, T c , where the mass coefficients (i.e., scattering and extinction) are parameterized as nonlinear functions of the ratio between q i and T c . The cirrus microphysics parameterization is based on a moment estimation parameterization of the particle size distribution (PSD), which relates the mass moment (i.e., second moment if mass is proportional to size raised to the power of 2) of the PSD to all other PSD moments through the magnitude of the second moment and T c . This same microphysics PSD parameterization is applied to calculate the integral optical properties used in both radiation parameterizations and, thus, ensures PSD and mass consistency between the cirrus microphysics and radiation schemes. In this paper, the temperature-nondependent and temperature-dependent parameterizations are shown to increase and decrease the zonally averaged temperature biases in the TTL by about 1 K, respectively. The temperature-dependent radiation parameterization is further demonstrated to have a positive impact on the specific humidity biases in the TTL, as well as decreasing the shortwave and longwave biases in the cloudy radiative effect. The temperature-dependent radiation parameterization is shown to be more consistent with TTL and global radiation observations.

Comparison of using distribution‐specific versus effective radius methods for hydrometeor single‐scattering properties for all‐sky microwave satellite radiance simulations with different microphysics parameterization schemes

“…Consider the results of CRTM simulations in Table in which different size distributions of ice spheres of identical bulk density with identical total water content and effective radius produce significantly different scattering optical depths and some differences in brightness temperature. Similar circumstances exist for ice cloud particles and infrared radiation; Baran et al [, ] have demonstrated improvement in modeled shortwave and longwave fluxes by directly coupling particle size distributions to scattering properties instead of parameterizing this relationship via effective radius.…”

The Met Office Unified Model Global Atmosphere 7.0/7.1 and JULES Global Land 7.0 configurations

“…In GA7, we parametrise the scalar optical properties of ice crystals using the scheme described in Baran et al (2016). This is based on an ensemble model of ice crystals developed by Baran and Labonnote (2007), where the bulk ice optical properties are derived by averaging habit-dependent scalar optical properties over an assumed particle size distribution function (PSD).…”

“…This approach has the advantage that it is possible to generate ice optical properties from PSDs with the same microphysical 20 assumptions used in the model's microphysics scheme; the same mass of ice is passed into each scheme and the bulk scalar ice optical properties are parametrised as a function of ice mass and temperature as described in Baran et al (2016). This improves the self-consistency within the model in a way that is generally not achieved with scalar optical properties determined from an ice crystal effective dimension as was done in GA6 and in most other atmospheric models; as a result, those models usually assume inconsistent PSDs and mass-diameter relations in the microphysics and radiation schemes.…”

“…The difference between the Baran et al (2016) scheme implemented in GA7 and the Baran et al (2014) scheme that was originally proposed is that in the original scheme, the derived optical properties are fitted to be functions of the spectral band (see Table 1) and the model's prognostic ice water mass mixing ratio only, whilst in GA7, and Baran et al (2016), there is an additional functional relationship to the atmospheric temperature (fitted to data sampled between -80…”

“…Hardiman et al (2015) discuss the importance of this bias and the role of various physical and numerical processes in its development. In developing GA7, the introduction of consistent ice optical and microphysical properties (GA ticket #17) was originally proposed via the scheme described in Baran et al (2014) and the detrimental impact on the TTL temperature bias led to the additional developments made in Baran et al (2016). The inclusion of cubic Hermite interpolation for moist prognostic variables (as part of GA ticket #135) was motivated primarily by its reduction of moisture biases above the tropical tropopause.…”