Radiative effects of CH_4, N_2O, halocarbons and the foreign-broadened H_2O continuum: A GCM experiment

M. Daniel Schwarzkopf; V. Ramaswamy

摘要

The simplified exchange approximation (SEA) method for calculation of infrared radiative transfer, used for general circulation model (GCM) climate simulations at the Geophysical Fluid Dynamics Laboratory (GFDL) and other institutions, has been updated to permit inclusion of the effects of methane (CH_4), nitrous oxide (N_2O), halocarbons, and water-vapor-air molecular broadening (foreign broadening). The effects of CH_4 and N_2O are incorporated by interpolation of line-by-line (LBL) transmissivity calculations evaluated at standard species concentrations; halocarbon effects are calculated from transmissivities computed using recently measured frequency-dependent absorption coefficients. The effects of foreign broadening are included by adoption of the "CKD" formalism for the water vapor continuum [Clough et al., 1989]. For a standard midlatitude summer profile, the change in the net infrared flux at the model tropopause due to the inclusion of present-day concentrations of CH_4 and N_2O is evaluated to within approx 5% of corresponding LBL results; the change in net flux at the tropopause upon inclusion of 1 ppbv of CFC-11, CFC-12, CFC-113, and HCFC-22 is within approx 10% of the LBL results. Tropospheric heating rate changes resulting from the introduction of trace species (CH_4, N_2O, and halocarbons) are calculated to within approx 0.03 k/d of the LBL results. Introduction of the CKD water vapor continuum causes LBL-computed heating rates to decrease by up to approx 0.4 K/d in the upper troposphere and to increase by up to approx 0.25 K/d in the midtroposphere; the SEA method gives changes within approx 0.05 K/d of the LBL values. The revised SEA formulation has been incorporated into the GFDL "SKYHI" GCM. Two simulations (using fixed sea surface temperatures and prescribed clouds) have been performed to determine the changes to the model climate from that of a control calculation upon inclusion of (1) the trace species and (2) the foreign-broadened water vapor continuum. When the trace species are added, statistically significant warming (approx 1 K) occurs in the annual-mean tropical upper troposphere, while cooling (approx 1.5 K) is noted in the upper stratosphere and stratopause region. The changes are generally similar to annual-mean equilibrium calculations made using a radiative-convective model assuming fixed dynamical heating. The effects of the CKD water vapor continuum include cooling (approx 1 K) in the annual-mean troposphere above approx 6 km, with significant warming in the lower troposphere. When effects of both trace gases and the CKD continuum are included, the annual-mean temperature increases below approx 5 km and cools between 5 and 10 km, indicating that continuum effects dominate in determining temperature changes in the lower and middle troposphere. Above, trace gas effects dominate, resulting in warming in the tropical upper troposphere and cooling in most of the middle atmosphere. Clear-sky outgoing longwave irradiances have been computed for observed European Centre for Medium-Range Weather Forecasting atmospheric profiles using three versions of the SEA formulation, including the effects of (1) water vapor, carbon dioxide, and ozone; (2) the above species plus present-day concentrations of the new trace species; (3) all of the above species plus the CKD H_2O continuum. Results for all three cases are within approx 10 W/m~2 of corresponding Earth Radiation Budget Experiment clear-sky irradiance measurements. The combined effect of trace gases and the CKD continuum result in a decrease of approx 8 W/m~2 in the computed irradiances.

机译：用于地球物理流体动力学实验室（GFDL）和其他机构的一般循环模型（GCM）气候模拟的用于红外辐射传递计算的简化交换近似（SEA）方法已更新，以允许包含甲烷的影响（ CH_4），一氧化二氮（N_2O），卤代烃和水蒸气-空气分子展宽（外国展宽）。 CH_4和N_2O的影响通过在标准物种浓度下评估的逐行（LBL）透射率计算的插值而合并;卤代烃效应是根据使用最近测得的频率相关吸收系数计算出的透射率计算得出的。通过采用“ CKD”形式表示水蒸气连续性，可以包括异物加宽的影响[Clough et al。，1989]。对于标准的中纬度夏季剖面，对对流层顶模型的净红外通量变化的估计是由于纳入了当前的CH_4和N_2O浓度，大约在相应的LBL结果的5％范围内;包含1 ppbv的CFC-11，CFC-12，CFC-113和HCFC-22时，对流层顶的净通量变化在LBL结果的约10％以内。由引入微量物质（CH_4，N_2O和卤代烃）引起的对流层加热速率变化经计算约为LBL结果的0.03 k / d。引入CKD水蒸气连续体会导致对流层上层对流层计算的加热速率在对流层上层降低约0.4 K / d，在对流层中层升高约0.25 K / d。 SEA方法得出的变化在LBL值的大约0.05 K / d之内。修订后的SEA公式已合并到GFDL“ SKYHI” GCM中。进行了两次模拟（使用固定的海面温度和规定的云层），从包含（1）痕量物种和（2）异化的水蒸气连续体在内的控制计算中确定模型气候的变化。当添加痕量物种时，在年平均热带对流层上发生统计上显着的变暖（约1 K），而在平流层上层和平流层顶区域则出现降温（约1.5 K）。这些变化通常类似于使用固定辐射动态对流的辐射对流模型进行的年均平衡计算。 CKD水汽连续体的影响包括年平均对流层约6 km以上的冷却（约1 K），而对流层下层则明显变暖。当同时包含痕量气体和CKD连续体的影响时，年平均温度升高到约5 km以下，并在5至10 km之间冷却，这表明连续体效应在确定对流层中低层温度变化方面起主要作用。在上方，微量气体效应占主导地位，导致热带对流层上部变暖，而大多数中层大气变冷。已经使用三种版本的SEA公式为观察到的欧洲中距离天气预报中心大气廓线计算了晴空外向长波辐照度，其中包括（1）水蒸气，二氧化碳和臭氧的影响; （2）上述物种加上新痕量物种的当今浓度; （3）以上所有物种加上CKD H_2O连续体。这三种情况的结果均在相应的“地球辐射预算”实验晴朗天空辐照度测量值的约10 W / m〜2之内。痕量气体和CKD连续体的综合作用导致计算出的辐照度降低约8 W / m〜2。

Radiative effects of CH_4, N_2O, halocarbons and the foreign-broadened H_2O continuum: A GCM experiment

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