Development and recent evaluation of the MT_CKD model of continuum absorption

Abstract

Water vapour continuum absorption is an important contributor to the Earth's radiative cooling and energy balance. Here, we describe the development and status of the MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) water vapour continuum absorption model. The perspective adopted in developing the MT_CKD model has been to constrain the model so that it is consistent with quality analyses of spectral atmospheric and laboratory measurements of the foreign and self continuum. For field measurements, only cases for which the characterization of the atmospheric state has been highly scrutinized have been used. Continuum coefficients in spectral regions that have not been subject to compelling analyses are determined by a mathematical formulation of the spectral shape associated with each water vapour monomer line. This formulation, which is based on continuum values in spectral regions in which the coefficients are well constrained by measurements, is applied consistently to all water vapour monomer lines from the microwave to the visible. The results are summed-up (separately for the foreign and self) to obtain continuum coefficients from 0 to 20 000 cm −1 . For each water vapour line, the MT_CKD line shape formulation consists of two components: exponentially decaying far wings of the line plus a contribution from a water vapour molecule undergoing a weak interaction with a second molecule. In the MT_CKD model, the first component is the primary agent for the continuum between water vapour bands, while the second component is responsible for the majority of the continuum within water vapour bands. The MT_CKD model should be regarded as a semi-empirical model with strong constraints provided by the known physics. Keeping the MT_CKD continuum consistent with current observational studies necessitates periodic updates to the water vapour continuum coefficients. In addition to providing details on the MT_CKD line shape formulation, we describe the most recent update to the model, MT_CKD_2.5, which is based on an analysis of satellite-and ground-based observations from 2385 to 2600 cm −1 (approx. 4 mm).

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

“…As was stressed earlier, the contribution from the water vapour self-continuum (H 2 O−H 2 O) and from N 2 −N 2 and O 2 −O 2 CIA was excluded from C f . The MTCKD-2.5 model [9,10] and the far-wings model of Tipping & Ma [8] are presented for comparison. Figure 5 shows in more detail the 4 mm (2500 cm −1 ) window.…”

“…The sample measurements (step 3) were typically made in a mixture of 300-600 mbar water vapour, depending on temperature, and 4400-4700 mbar of air (table 1). The background measurements with an empty cell and with air were carried out, first, to ensure that any possible cell distortion due to pressure was taken into account and, [20], light grey lines) calculated up to 25 cm −1 from the line centres without a plinth; experimental RAL (dark grey lines) spectrum; MTCKD-2.5 (dashed lines) total continuum [9,10]; optical depth of the total (i.e. self plus foreign) continuum (dots) derived as a spectrally smoothed difference between RAL and UCL08 in microwindows between lines; contribution from the self-continuum (circles) derived using data [16] for the same temperature.…”

“…An example of the consecutive stages of the foreign-continuum retrieval (stars) in a small spectral interval: (i) the total continuum (black squares) is derived as a spectrally smoothed and filtered difference (grey squares) between the experimental RAL (solid black line) spectrum and the line-by-line simulation using UCL08 (grey line) data [20] in the microwindows between lines; (ii) the foreign continuum is derived as a spectrally smoothed difference between the total and self-continuum (circles) [16]. The total MTCKD-2.5 continuum (dashed line) [9,10] is shown for comparison. The data correspond to: T = 402 K, P H 2 O = 600 mbar, P air = 4400 mbar, L = 17.7 m.…”

Airborne and satellite remote sensing of the mid-infrared water vapour continuum

“…One spectral region subject to large changes recently in v. 2.5 is in the range 3-5 mm (2000-3200 cm −1 ) based on analysis of satellite-and ground-based infrared measurements [22]. Other work has previously suggested that past versions of MT_CKD underestimated the magnitude of the self continuum in this region [9].…”

ARTS, the Atmospheric Radiative Transfer Simulator – version 2.2, the planetary toolbox edition

“…This particularly covers models for the micro-and millimeter-wave region (e.g., Liebe et al, 1993;Rosenkranz, 1993Rosenkranz, , 1998 but also several editions of the Clough-Kneizys-Davies continuum model (CKD; Clough et al, 1989Clough et al, , 2005, later enhanced by Mlawer and Tobin (MT_CKD;Mlawer et al, 2012), models that cover the entire millimeter to infrared spectral range. However, all of these continua have been developed with a focus on Earth observations.…”