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Frontiers of Earth Science

Front. Earth Sci.    2019, Vol. 13 Issue (2) : 277-289
Bias characterization of ATMS low-level channels under clear-sky and cloudy conditions
Qi LI1, Xiaolei ZOU2()
1. Joint Center of Data Assimilation for Research and Application, College of Atmospheric Science, Nanjing University of Information Science and Technology (NUIST), Nanjing 210044, China
2. Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, College Park, MD 20740-3823, USA
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The Advanced Technology Microwave Sounder (ATMS) onboard the Suomi National Polar-Orbiting Partnership satellite is a cross-track scanning instrument containing 22 sounding channels in total. In this study, the bias characteristics of channels 1–6, which could have significant cloud contamination in heavy precipitation, are first analyzed based on the differences between ATMS observations (O) and model simulations (B) under clear-sky conditions over oceans. Latitudinal dependencies of the biases of window channels 1–3 are greater than those of channels 4–6. Biases of all nadir-only observations examined in different latitudinal bands [μ1(ϕ)] are positive and no more than 7.0 K. Biases at higher latitudes are larger. Channels 1–5 have a generally symmetric scan bias pattern [μ2(α)]. The global distributions of brightness temperature differences after subtracting the biases, i.e., O-B-m1(ϕ)-μ2(α), for channels 3–6 spatially match the liquid water path distributions. Excluding ice-affected observations, channel 3–6 O-B differences systematically increase as the liquid water path increases under cloudy conditions. Further investigation is needed to apply these findings for ATMS data assimilation under both clear-sky and cloudy conditions.

Keywords ATMS      O-B      clear-sky bias characteristics      impact of clouds on biases     
Corresponding Authors: Xiaolei ZOU   
Just Accepted Date: 20 March 2019   Online First Date: 17 April 2019    Issue Date: 16 May 2019
 Cite this article:   
Qi LI,Xiaolei ZOU. Bias characterization of ATMS low-level channels under clear-sky and cloudy conditions[J]. Front. Earth Sci., 2019, 13(2): 277-289.
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Fig.1  Weighting functions of the 22 ATMS channels.
Channel Center frequency/GHz Beam width/deg Weighting function peak/hPa
1 23.8 5.2 Window
2 31.4 5.2 Window
3 50.3 2.2 Window
4 51.76 2.2 950
5 52.8 2.2 850
6 53.596±0.115 2.2 700
7 54.40 2.2 400
8 54.94 2.2 250
9 55.50 2.2 200
10 57.29 2.2 100
11 57.29±0.217 2.2 50
12 57.29±0.322±0.048 2.2 25
13 57.29±0.322±0.022 2.2 10
14 57.29±0.322±0.010 2.2 5
15 57.29±0.322±0.0045 2.2 2
16 88.2 2.2 Window
17 165.5 1.1 Window
18 183.31±7 1.1 800
19 183.31±4.5 1.1 700
20 183.31±3 1.1 500
21 183.31±1.8 1.1 400
22 183.31±1.0 1.1 300
Tab.1  Instrument characteristics of the ATMS
Fig.2  Global distributions of (a) channel 1 and (b) channel 2 brightness temperatures (unit: K), as well as (c) LWP retrievals over oceans (unit: kg·m2) for ATMS ascending nodes on 17 July 2015.
Fig.3  (a) Spatial distributions of ATMS clear-sky data counts within 5°×5° grid boxes over oceans and (b) the number of data samples with respect to scan position and latitude. Data are from 1–30 January, 1–30 April, 1–30 July, and 1–30 October 2015.
Fig.4  Latitudinal dependencies of (a) O-B biases and (b) standard deviations for ATMS channels 1–6 obtained within 5° latitudinal bands from 55°S and 55°N under clear-sky nadir-only conditions over oceans. Data are from 1–30 January, 1–30 April, 1–30 July, and 1–30 October 2015.
Fig.5  Scan-dependent biases of ATMS channels (a) 1–3 and (b) 4–6 for all data within 55°S and 55°N under clear-sky conditions over oceans from 1–30 January, 1–30 April, 1–30 July, and 1–30 October 2015. The nadir bias has been subtracted.
Fig.6  (a) Nadir locations of ATMS ascending nodes from 1–16 July 2015. (b) Magnification of the area to the southwest of South Africa in (a) on 17 July 2015. The black circles show the nadir locations of the ATMS ascending nodes. Each color in the legend represents a date in the range of 1–16 July 2015.
Fig.7  Spatial distributions of the differences between observed brightness temperatures and model-simulated brightness temperatures (unit: K) without scan- and latitude-dependent biases for ATMS channels 1–6 [panels (a) to (f), respectively] from ascending nodes within 55°S and 55°N over oceans on 17 July 2015.
Fig.8  Spatial distributions of IWP retrievals (unit: kg·m2) from the ATMS over oceans from (a) ascending and (b) descending nodes within 60°S and 60°N on 17 July 2015.
Fig.9  IWP (coloured dots; unit: kg·m2) as a function of LWP and the corrected O-B bias for ATMS channels 3–6 within 55°S and 55°N under cloudy conditions over oceans on 17 July 2015. Black dots show data with IWP values less than 0.01 kg·m2.
Fig.10  Same as Fig. 9 except that data with IWP values greater than or equal to 0.01 kg·m?2 are excluded. Mean biases and standard deviations in each LWP bin are shown in green.
Fig.11  (a) The mean biases and (b) standard deviations of O-B before (dashed curves) and after (solid curves) the bias correction with respect to LWP for ATMS channels 3–6. Data falling within 55°S and 55°N over oceans on 17 July 2015 are shown.
Fig.12  Same as Fig. 11 but for data after the bias correction on 17 July of the years 2012–2017. The light gray shading shows the total data counts (right ordinate).
Fig.13  Spatial distributions of the differences in brightness temperature (unit: K) between observations and CRTM model simulations without scan-, latitude-, and LWP-dependent biases for ATMS channels 3–6 [panels (a) to (d), respectively] from ascending nodes within 55°S and 55°N over oceans on 17 July 2015.
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