scholarly journals Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals

2018 ◽  
Author(s):  
Ivan Ortega ◽  
Rebecca Buchholz ◽  
Emrys Hall ◽  
Dale Hurst ◽  
Allen Jordan ◽  
...  
Keyword(s):  
Water Vapor ◽  
Temporal Variability ◽  
A Priori ◽  
Linear Regression Analysis ◽  
Ground Truth ◽  
Negative Bias ◽  
Vertical Profiles ◽  
Solar Absorption ◽  
Frost Point ◽  
The Impact

Abstract. Retrievals of vertical profiles of key atmospheric gases provide a critical long-term data record from ground-based Fourier Transform InfraRed (FTIR) solar absorption measurements. However, the characterization of the retrieved vertical profile structure can be difficult to validate, especially for gases with large vertical gradients and spatial-temporal variability such as water vapor. In this work, we evaluate the accuracy of the most common water vapor isotope (H216O, hereafter WV) FTIR retrievals in the lower and upper troposphere – lower stratosphere. Coincident high-quality vertically resolved WV profile measurements obtained from 2010 to 2016 with balloon-borne NOAA Frost Point Hygrometers (FPH) are used as reference to evaluate the performance of the retrieved profiles at two sites: Boulder, Colorado and in the mountain top observatory of Mauna Loa, Hawaii. For a meaningful comparison, the spatial-temporal variability has been investigated. Additionally, we evaluate the quantitative impact of different a priori profiles in the retrieval of WV vertical profiles using un-smoothed comparisons. An orthogonal linear regression analysis shows the best correlation among all layers using ERA-Interim (ERA-I) a priori profiles. In Boulder, we found a negative bias of 0.02 ± 1.9 % and precision of 3.7 % (r = 0.95) for the 1.5–3 km layer. A larger negative bias of 11.1 ± 3.5 % and precision of 7.0 % was found in the lower free troposphere layer of 3–5 km (r = 0.97) attributed to rapid vertical change of WV, which is not always captured by the retrievals. The bias improves in the 5–7.5 km layer (1.0 ± 5.3 %) and the precision worsens to about 10 %. The bias remains at about 13 % and the precision remains to about 10 % for layers above 7.5 km but below 13.5 km. At MLO the spatial mismatch is significantly larger due to the launch of the sonde being farther from the FTIR location. Nevertheless, we estimate a negative biases of 5.9 ± 4.6 % for the 3.5–5.5 km layer (r = 0.93) and 9.9 ± 3.7 % for the 5.5–7.5 km layer (r = 0.93), and positive biases of 6.2 ± 3.6 % for the 7.5–10 km layer (r = 0.95), and 12.6 % and greater values above 10 km. The agreement for the first layer is significantly better at BLD likely that the air masses are similar for both FTIR and FPH. Furthermore, for the first time we study the influence of different sources of WV profiles in the retrieval of selected gas profiles. Using NDACC standard retrievals we present results for hydrogen cyanide (HCN), carbon monoxide (CO), and ethane (C2H6) by taking NOAA FPH profiles as the ground-truth and evaluate the impact of other WV profile sources. We show that the effect is minor for C2H6 (bias

scholarly journals Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals

2019 ◽  
Vol 12 (2) ◽  
pp. 873-890
Author(s):  
Ivan Ortega ◽  
Rebecca R. Buchholz ◽  
Emrys G. Hall ◽  
Dale F. Hurst ◽  
Allen F. Jordan ◽  
...  
Keyword(s):  
Water Vapor ◽  
Temporal Variability ◽  
A Priori ◽  
Linear Regression Analysis ◽  
Ground Truth ◽  
Negative Bias ◽  
Upper Troposphere ◽  
Frost Point ◽  
The Impact ◽  
Vertical Gradients

Abstract. Retrievals of vertical profiles of key atmospheric gases provide a critical long-term record from ground-based Fourier transform infrared (FTIR) solar absorption measurements. However, the characterization of the retrieved vertical profile structure can be difficult to validate, especially for gases with large vertical gradients and spatial–temporal variability such as water vapor. In this work, we evaluate the accuracy of the most common water vapor isotope (H216O, hereafter WV) FTIR retrievals in the lower and upper troposphere–lower stratosphere. Coincident high-quality vertically resolved WV profile measurements obtained from 2010 to 2016 with balloon-borne NOAA frost point hygrometers (FPHs) are used as reference to evaluate the performance of the retrieved profiles at two sites: Boulder (BLD), Colorado, and at the mountaintop observatory of Mauna Loa (MLO), Hawaii. For a meaningful comparison, the spatial–temporal variability has been investigated. We present results of comparisons among FTIR retrievals with unsmoothed and smoothed FPH profiles to assess WV vertical gradients. Additionally, we evaluate the quantitative impact of different a priori profiles in the retrieval of WV. An orthogonal linear regression analysis shows the best correlation among tropospheric layers using ERA-Interim (ERA-I) a priori profiles and biases are lower for unsmoothed comparisons. In Boulder, we found a negative bias of 0.02±1.9 % (r=0.95) for the 1.5–3 km layer. A larger negative bias of 11.1±3.5 % (r=0.97) was found in the lower free troposphere layer of 3–5 km attributed to rapid vertical change of WV, which is not always captured by the retrievals. The bias improves in the 5–7.5 km layer (1.0±5.3 %, r=0.94). The bias remains at about 13 % for layers above 7.5 km but below 13.5 km. At MLO the spatial mismatch is significantly larger due to the launch of the sonde being farther from the FTIR location. Nevertheless, we estimate a negative bias of 5.9±4.6 % (r=0.93) for the 3.5–5.5 km layer and 9.9±3.7 % (r=0.93) for the 5.5–7.5 km layer, and we measure positive biases of 6.2±3.6 % (r=0.95) for the 7.5–10 km layer and 12.6 % and greater values above 10 km. The agreement for the first layer is significantly better at BLD because the air masses are similar for both FTIR and FPH. Furthermore, for the first time we study the influence of different WV a priori profiles in the retrieval of selected gas profiles. Using NDACC standard retrievals we present results for hydrogen cyanide (HCN), carbon monoxide (CO), and ethane (C2H6) by taking NOAA FPH profiles as the ground truth and evaluating the impact of other WV profiles. We show that the effect is minor for C2H6 (bias <0.5 % for all WV sources) among all vertical layers. However, for HCN we found significant biases between 6 % for layers close to the surface and 2 % for the upper troposphere depending on the WV profile source. The best results (reduced bias and precision and r values closer to unity) are always found for pre-retrieved WV. Therefore, we recommend first retrieving WV to use in subsequent retrieval of gases.

scholarly journals Retrieval of temperature and water vapor profiles from radio occultation refractivity and bending angle measurements using an Optimal Estimation approach: a simulation study

2005 ◽  
Vol 5 (6) ◽  
pp. 1665-1677 ◽  
Author(s):  
A. von Engeln ◽  
G. Nedoluha
Keyword(s):  
Water Vapor ◽  
Radio Occultation ◽  
A Priori ◽  
Estimation Method ◽  
Lower Troposphere ◽  
Optimal Estimation ◽  
Pronounced Difference ◽  
Bending Angle ◽  
Advantages And Disadvantages ◽  
The Impact

Abstract. The Optimal Estimation Method is used to retrieve temperature and water vapor profiles from simulated radio occultation measurements in order to assess how different retrieval schemes may affect the assimilation of this data. High resolution ECMWF global fields are used by a state-of-the-art radio occultation simulator to provide quasi-realistic bending angle and refractivity profiles. Both types of profiles are used in the retrieval process to assess their advantages and disadvantages. The impact of the GPS measurement is expressed as an improvement over the a priori knowledge (taken from a 24h old analysis). Large improvements are found for temperature in the upper troposphere and lower stratosphere. Only very small improvements are found in the lower troposphere, where water vapor is present. Water vapor improvements are only significant between about 1 km to 7 km. No pronounced difference is found between retrievals based upon bending angles or refractivity. Results are compared to idealized retrievals, where the atmosphere is spherically symmetric and instrument noise is not included. Comparing idealized to quasi-realistic calculations shows that the main impact of a ray tracing algorithm can be expected for low latitude water vapor, where the horizontal variability is high. We also address the effect of altitude correlations in the temperature and water vapor. Overall, we find that water vapor and temperature retrievals using bending angle profiles are more CPU intensive than refractivity profiles, but that they do not provide significantly better results.

Regional Assimilation System for Transformed Retrievals from Satellite High-Resolution Infrared Data

2020 ◽  
Vol 59 (7) ◽  
pp. 1171-1193
Author(s):  
Paolo Antonelli ◽  
Tiziana Cherubini ◽  
Steven Businger ◽  
Siebren de Haan ◽  
Paolo Scaccia ◽  
...  
Keyword(s):  
Temperature Field ◽  
Water Vapor ◽  
North Pacific Ocean ◽  
A Priori ◽  
Satellite Sensors ◽  
Forecasting System ◽  
Cycling System ◽  
Priori Information ◽  
The Impact ◽  
Vapor Distribution

AbstractSatellite retrievals strive to exploit the information contained in thousands of channels provided by hyperspectral sensors and show promise in providing a gain in computational efficiency over current radiance assimilation methods by transferring computationally expensive radiative transfer calculations to retrieval providers. This paper describes the implementation of a new approach based on the transformation proposed in 2008 by Migliorini et al., which reduces the impact of the a priori information in the retrievals and generates transformed retrievals (TRs) whose assimilation does not require knowledge of the hyperspectral instruments characteristics. Significantly, the results confirm both the viability of Migliorini’s approach and the possibility of assimilating data from different hyperspectral satellite sensors regardless of the instrument characteristics. The Weather Research and Forecasting (WRF) Model’s Data Assimilation (WRFDA) 3-h cycling system was tested over the central North Pacific Ocean, and the results show that the assimilation of TRs has a greater impact in the characterization of the water vapor distribution than on the temperature field. These results are consistent with the knowledge that temperature field is well constrained by the initial and boundary conditions of the Global Forecast System (GFS), whereas the water vapor distribution is less well constrained in the GFS. While some preliminary results on the comparison between the assimilation with and without TRs in the forecasting system are presented in this paper, additional work remains to explore the impact of the new assimilation approach on forecasts and will be provided in a follow-up publication.

scholarly journals Recent divergences in stratospheric water vapor measurements by frost point hygrometers and the Aura Microwave Limb Sounder

2016 ◽  
Author(s):  
Dale F. Hurst ◽  
William G. Read ◽  
Holger Vömel ◽  
Henry B. Selkirk ◽  
Karen H. Rosenlof ◽  
...  
Keyword(s):  
Water Vapor ◽  
Linear Regression Analysis ◽  
San Jose ◽  
Average Growth ◽  
Upper Troposphere ◽  
Stratospheric Water Vapor ◽  
Frost Point ◽  
Comparison Period ◽  
Better Than

Abstract. Balloon-borne frost point hygrometers (FPs) and the Aura Microwave Limb Sounder (MLS) provide high-quality vertical profile measurements of water vapor in the upper troposphere and lower stratosphere (UTLS). A previous comparison of stratospheric water vapor measurements by FPs and MLS over three FP sites, Boulder, Colorado (40.0° N), Hilo, Hawaii (19.7° N) and Lauder, New Zealand (45.0° S), from August 2004 through December 2012, demonstrated agreement better than 1 % between 68 and 26 hPa, but also exposed statistically significant biases of 2 to 10 % at 83 and 100 hPa (Hurst et al., 2014). A simple linear regression analysis of the FPH-MLS differences revealed no significant long-term drifts between the two instruments. Here we extend the drift comparison to mid-2015 and add two FP sites, Lindenberg, Germany (52.2° N) and San José, Costa Rica (10.0° N) that employ FPs of different manufacture and calibration for their water vapor soundings. The extended comparison period reveals that stratospheric FP and MLS measurements over 4 of the 5 sites have diverged at rates of 0.03 to 0.07 ppmv yr−1 (0.6 to 1.5 % yr−1) from ~2010 to mid-2015. These rates are similar in magnitude to the 30-year (1980–2010) average growth rate of stratospheric water vapor (~1 % yr−1) measured by FPs over Boulder (Hurst et al., 2011). By mid-2015, the FP-MLS differences at some sites were large enough to exceed the combined accuracy estimates of the FP and MLS measurements.

scholarly journals The Mainz profile algorithm (MAPA)

2019 ◽  
Vol 12 (3) ◽  
pp. 1785-1806 ◽  
Author(s):  
Steffen Beirle ◽  
Steffen Dörner ◽  
Sebastian Donner ◽  
Julia Remmers ◽  
Yang Wang ◽  
...  
Keyword(s):  
A Priori ◽  
Scaling Factor ◽  
Least Square ◽  
Trace Gas ◽  
Vertical Profiles ◽  
Mixing Ratios ◽  
Surface Mixing ◽  
Sensitivity Studies ◽  
The Impact ◽  
Good Agreement

Abstract. The Mainz profile algorithm (MAPA) derives vertical profiles of aerosol extinction and trace gas concentrations from MAX-DOAS measurements of slant column densities under multiple elevation angles. This paper presents (a) a detailed description of the MAPA (v0.98), (b) results for the CINDI-2 campaign, and (c) sensitivity studies on the impact of a priori assumptions such as flag thresholds. Like previous profile retrieval schemes developed at MPIC, MAPA is based on a profile parameterization combining box profiles, which also might be lifted, and exponential profiles. But in contrast to previous inversion schemes based on least-square fits, MAPA follows a Monte Carlo approach for deriving those profile parameters yielding best match to the MAX-DOAS observations. This is much faster and directly provides physically meaningful distributions of profile parameters. In addition, MAPA includes an elaborated flagging scheme for the identification of questionable or dubious results. The AODs derived with MAPA for the CINDI-2 campaign show good agreement with AERONET if a scaling factor of 0.8 is applied for O4, and the respective NO2 and HCHO surface mixing ratios match those derived from coincident long-path DOAS measurements. MAPA results are robust with respect to modifications of the a priori MAPA settings within plausible limits.

scholarly journals An update on the uncertainties of water vapor measurements using cryogenic frost point hygrometers

2016 ◽  
Vol 9 (8) ◽  
pp. 3755-3768 ◽  
Author(s):  
Holger Vömel ◽  
Tatjana Naebert ◽  
Ruud Dirksen ◽  
Michael Sommer
Keyword(s):  
Water Vapor ◽  
Lower Troposphere ◽  
Dew Point ◽  
Data Series ◽  
Mixing Ratio ◽  
Stratospheric Water Vapor ◽  
Frost Point ◽  
Tropical Tropopause ◽  
The Impact

Abstract. Long time series of observations of essential climate variables in the troposphere and stratosphere are often impacted by inconsistencies in instrumentation and ambiguities in the interpretation of the data. To reduce these problems of long-term data series, all measurements should include an estimate of their uncertainty and a description of their sources. Here we present an update of the uncertainties for tropospheric and stratospheric water vapor observations using the cryogenic frost point hygrometer (CFH). The largest source of measurement uncertainty is the controller stability, which is discussed here in detail. We describe a method to quantify this uncertainty for each profile based on the measurements. We also show the importance of a manufacturer-independent ground check, which is an essential tool to continuously monitor the uncertainty introduced by instrument variability. A small bias, which has previously been indicated in lower tropospheric measurements, is described here in detail and has been rectified. Under good conditions, the total from all sources of uncertainty of frost point or dew point measurements using the CFH can be better than 0.2 K. Systematic errors, which are most likely to impact long-term climate series, are verified to be less than 0.1 K. The impact of the radiosonde pressure uncertainty on the mixing ratio for properly processed radiosondes is considered small. The mixing ratio uncertainty may be as low as 2 to 3 %. The impact of the ambient temperature uncertainty on relative humidity (RH) is generally larger than that of the frost point uncertainty. The relative RH uncertainty may be as low as 2 % in the lower troposphere and 5 % in the tropical tropopause region.

Simulating Mars D/H and atmospheric chemistry during the 2018 Global Dust Storm and comparing with NOMAD observations

2020 ◽  
Author(s):  
Frank Daerden ◽  
Lori Neary ◽  
Geronimo Villanueva ◽  
Shohei Aoki ◽  
Sebastien Viscardy ◽  
...  
Keyword(s):  
Water Vapor ◽  
Atmospheric Chemistry ◽  
Dust Storm ◽  
General Circulation ◽  
Circulation Model ◽  
Cloud Formation ◽  
Vertical Profiles ◽  
Considerable Impact ◽  
Physical And Chemical ◽  
The Impact

&lt;p&gt;The NOMAD instrument suite on the ESA-Roskosmos ExoMars Trace Gas Orbiter (TGO) observes the physical and chemical composition of the Martian atmosphere with highly resolved vertical profiles and nadir sounding in the IR and UV-vis domains. Vertically resolved profiles of, amongst other species, water vapor, HDO, ozone, CO, CO&lt;sub&gt;2&lt;/sub&gt;, oxygen airglow, dust and clouds were obtained for more than one Martian year [1-5]. During its first year of operations, NOMAD witnessed the 2018 Global Dust Storm (GDS) during its onset, peak and decline. The redistribution of water vapor to high altitudes and latitudes observed during the GDS was explained using the GEM-Mars General Circulation Model (GCM) [6-8]. The GCM was driven by the dust optical depths for Mars Year 34 provided by [9]. The photolysis products of water vapor are a major driver for the atmospheric chemistry on Mars. As water vapor is redistributed over the atmosphere, it is expected to have considerable impact on many other species. GEM-Mars contains routines for atmospheric chemistry and here we present some results of the simulated impact of the GDS on atmospheric chemistry and on several of the observed species. GEM-Mars now also includes the simulation of HDO and the fractionation of water vapor upon cloud formation. The simulations will be compared with the vertical profiles of the D/H ratio obtained from NOMAD observations. The impact of the GDS on D/H can be estimated from these simulations.&lt;/p&gt;&lt;p&gt; &amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;[1] Vandaele, A. C. et al. (2019), Nature, 568, 7753, 521-525, doi: 10.1038/s41586-019-1097-3.&lt;/p&gt;&lt;p&gt;[2] Aoki, S. et al. (2019), J. Geophys. Res.: Planets, 124, 3482&amp;#8211;3497. https://doi.org/10.1029/2019JE006109&lt;/p&gt;&lt;p&gt;[3] G&amp;#233;rard et al. (2020), Nature Astronomy, https://doi.org/10.1038/s41550-020-1123-2&lt;/p&gt;&lt;p&gt;[4] Villanueva et al., submitted.&lt;/p&gt;&lt;p&gt;[5] Korablev et al., 2020, in rev.&lt;/p&gt;&lt;p&gt;[6] Neary, L. and F. Daerden (2018), Icarus, 300, 458&amp;#8211;476, https://doi.org/10.1016/j.icarus.2017.09.028&lt;/p&gt;&lt;p&gt;[7] Daerden, F. et al. (2019), Icarus, 326, 197-224, doi: 10.1016/j.icarus.2019.02.030.&lt;/p&gt;&lt;p&gt;[8] Neary, L. et al. (2020), Geophys. Res. Lett., 47, e2019GL084354. https://doi.org/10.1029/2019GL084354&lt;/p&gt;&lt;p&gt;[9] Montabone, L. et al. (2019), J. Geophys. Res.: Planets. doi: 10.1029/2019JE006111.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt; &lt;div&gt;2.11.0.0&lt;/div&gt;&lt;!-- COMO-HTML-CONTENT-END --&gt; &lt;div&gt; &lt;div&gt;BIRA-IASB NOMAD team (continued):&lt;/div&gt; &lt;p&gt;S. Robert (1), L. Trompet (1), A. Mahieux (1), C. Depiesse (1), E. Neefs (1) and B. Ristic (1).&lt;/p&gt; &lt;/div&gt; &lt;!-- COMO-HTML-CONTENT-END --&gt; &lt;div class=&quot;co_mto_htmlabstract-teamMembers d-none d-md-block&quot;&gt; &lt;div class=&quot;co_mto_htmlabstract-teamMembers-name h1-special journal-contentHeaderColor header-element color-primary&quot;&gt;BIRA-IASB NOMAD team (continued):&lt;/div&gt; &lt;p&gt;S. Robert (1), L. Trompet (1), A. Mahieux (1), C. Depiesse (1), E. Neefs (1) and B. Ristic (1).&lt;/p&gt; &lt;/div&gt; &lt;p class=&quot;co_mto_htmlabstract-citationHeader&quot;&gt; &lt;strong class=&quot;co_mto_htmlabstract-citationHeader-intro&quot;&gt;How to cite:&lt;/strong&gt; Daerden, F., Neary, L., Villanueva, G., Aoki, S., Viscardy, S., Thomas, I., Vandaele, A. C., Liuzzi, G., Crismani, M., Khayat, A., Smith, M. D., Clancy, R. T., Wolff, M. J., Sandor, B. J., Whiteway, J. A., Mumma, M. J., Erwin, J., Willame, Y., and Piccialli, A. and the BIRA-IASB NOMAD team (continued): Simulating Mars D/H and atmospheric chemistry during the 2018 Global Dust Storm and comparing with NOMAD observations , Europlanet Science Congress 2020, online, 21 September&amp;#8211;9 Oct 2020, EPSC2020-371, 2020 &lt;/p&gt;

scholarly journals Recent divergences in stratospheric water vapor measurements by frost point hygrometers and the Aura Microwave Limb Sounder

2016 ◽  
Vol 9 (9) ◽  
pp. 4447-4457 ◽  
Author(s):  
Dale F. Hurst ◽  
William G. Read ◽  
Holger Vömel ◽  
Henry B. Selkirk ◽  
Karen H. Rosenlof ◽  
...  
Keyword(s):  
Water Vapor ◽  
Linear Regression Analysis ◽  
San Jose ◽  
Average Growth ◽  
Upper Troposphere ◽  
Stratospheric Water Vapor ◽  
Frost Point ◽  
Comparison Period ◽  
Better Than

Abstract. Balloon-borne frost point hygrometers (FPs) and the Aura Microwave Limb Sounder (MLS) provide high-quality vertical profile measurements of water vapor in the upper troposphere and lower stratosphere (UTLS). A previous comparison of stratospheric water vapor measurements by FPs and MLS over three sites – Boulder, Colorado (40.0° N); Hilo, Hawaii (19.7° N); and Lauder, New Zealand (45.0° S) – from August 2004 through December 2012 not only demonstrated agreement better than 1 % between 68 and 26 hPa but also exposed statistically significant biases of 2 to 10 % at 83 and 100 hPa (Hurst et al., 2014). A simple linear regression analysis of the FP–MLS differences revealed no significant long-term drifts between the two instruments. Here we extend the drift comparison to mid-2015 and add two FP sites – Lindenberg, Germany (52.2° N), and San José, Costa Rica (10.0° N) – that employ FPs of different manufacture and calibration for their water vapor soundings. The extended comparison period reveals that stratospheric FP and MLS measurements over four of the five sites have diverged at rates of 0.03 to 0.07 ppmv year−1 (0.6 to 1.5 % year−1) from  ∼  2010 to mid-2015. These rates are similar in magnitude to the 30-year (1980–2010) average growth rate of stratospheric water vapor ( ∼  1 % year−1) measured by FPs over Boulder (Hurst et al., 2011). By mid-2015, the FP–MLS differences at some sites were large enough to exceed the combined accuracy estimates of the FP and MLS measurements.

scholarly journals Spatio-temporal variability of water vapor investigated by lidar and FTIR vertical soundings above Mt. Zugspitze

2014 ◽  
Vol 14 (20) ◽  
pp. 28231-28268 ◽  
Author(s):  
H. Vogelmann ◽  
R. Sussmann ◽  
T. Trickl ◽  
A. Reichert
Keyword(s):  
Water Vapor ◽  
Temporal Variability ◽  
Time Interval ◽  
Horizontal Distance ◽  
Free Troposphere ◽  
Time Intervals ◽  
Spatio Temporal ◽  
Matching Criteria ◽  
The Impact ◽  
Relative Variability

Abstract. Water vapor is the most important greenhouse gas and its spatio-temporal variability strongly exceeds that of all other greenhouse gases. However, this variability has hardly been studied quantitatively so far. We present an analysis of a five-year period of water vapor measurements in the free troposphere above Mt. Zugspitze (2962 m a.s.l., Germany). Our results are obtained from a combination of measurements of vertically integrated water vapor (IWV), recorded with a solar Fourier Transform InfraRed (FTIR) spectrometer on the summit of Mt. Zugspitze and of water vapor profiles recorded with the nearby differential absorption lidar (DIAL) at the Schneefernerhaus research station. The special geometrical arrangement of one zenith-viewing and one sun-pointing instrument and the temporal resolution of both instruments allow for an investigation of the spatio-temporal variability of IWV on a spatial scale of less than one kilometer and on a time scale of less than one hour. The SD of differences between both instruments σIWV calculated for varied subsets of data serves as a measure of variability. The different subsets are based on various spatial and temporal matching criteria. Within a time interval of 20 min, the spatial variability becomes significant for horizontal distances above 2 km, but only in the warm season (σIWV = 0.35 mm). However, it is not sensitive to the horizontal distance during the winter season. The variability of IWV within a time interval of 30 min peaks in July and August (σIWV > 0.55 mm, mean horizontal distance = 2.5 km and has its minimum around midwinter (σIWV < 0.2 mm, mean distance > 5 km). The temporal variability of IWV is derived by selecting subsets of data from both instruments with optimal volume matching. For a short time interval of 5 min, the variability is 0.05 mm and increases to more than 0.5 mm for a time interval of 15 h. The profile variability of water vapor is determined by analyzing subsets of water vapor profiles recorded by the DIAL within time intervals from 1 to 5 h. For all altitudes, the variability increases with widened time intervals. The lowest relative variability is observed in the lower free troposphere around an altitude of 4.5 km. Above 5 km, the relative variability increases continuously up to the tropopause by about a factor of 3. Analysis of the covariance of the vertical variability reveals an enhanced variability of water vapor in the upper troposphere above 6 km. It is attributed to a more coherent flow of heterogeneous air masses, while the variability at lower altitudes is also driven by local atmospheric dynamics. By studying the short-term variability of vertical water vapor profiles recorded within a day, we come to the conclusion that the contribution of long-range transport and the advection of heterogeneous layer structures may exceed the impact of local convection by one order of magnitude even in the altitude range between 3 and 5 km.

scholarly journals Retrieval of temperature and water vapor profiles from radio occultation refractivity and bending angle measurements using an optimal estimation approach: a simulation study

2005 ◽  
Vol 5 (2) ◽  
pp. 1585-1617 ◽  
Author(s):  
A. von Engeln ◽  
G. Nedoluha
Keyword(s):  
Water Vapor ◽  
Radio Occultation ◽  
A Priori ◽  
Estimation Method ◽  
Lower Troposphere ◽  
Optimal Estimation ◽  
Pronounced Difference ◽  
Bending Angle ◽  
Advantages And Disadvantages ◽  
The Impact

Abstract. The Optimal Estimation Method is used to retrieve temperature and water vapor profiles from simulated radio occultation measurements in order to assess possible assimilation impacts of this data. High resolution ECMWF global fields are used by 5 a state-of-the-art radio occultation simulator to provide quasi-realistic bending angle and refractivity profiles. Both types of profiles are used in the retrieval process to assess their advantages and disadvantages. The impact of the GPS measurement is expressed as an improvement over the a priori knowledge (taken from a 24 h old analysis). Large improvements are found for temperature in the upper troposphere and 10 lower stratosphere. Only very small improvements are found in the lower troposphere, where water vapor is present. Water vapor improvements are only significant between about 1 km to 7 km. No pronounced difference is found between retrievals based upon bending angles or refractivity. Results are compared to idealized retrievals, where the atmosphere is spherically symmetric and instrument noise is not included. Comparing 15 idealized to quasi-realistic calculations shows that the main impact of a ray tracing algorithm can be expected for low latitude water vapor, where the horizontal variability is high. We also address the effect of altitude correlations in the temperature and water vapor. Overall, we find that water vapor and temperature retrievals using bending angle profiles are significantly more CPU intensive than refractivity profiles, but that they do 20 not provide significantly better results.

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