scholarly journals Biases caused by the instrument bandwidth and beam width on simulated brightness temperature measurements from scanning microwave radiometers

2013 ◽  
Vol 6 (5) ◽  
pp. 1171-1187 ◽  
Author(s):  
V. Meunier ◽  
U. Löhnert ◽  
P. Kollias ◽  
S. Crewell
Keyword(s):  
Water Vapour ◽  
Liquid Water ◽  
Beam Width ◽  
Liquid Water Path ◽  
Water Path ◽  
Antenna Beam ◽  
W Band ◽  
Integrated Water Vapour ◽  
The Impact ◽  
Receiver Bandwidth

Abstract. More so than the traditional fixed radiometers, the scanning radiometer requires a careful design to ensure high quality measurements. Here the impact of the radiometer characteristics (e.g., antenna beam width and receiver bandwidth) and atmospheric propagation (e.g. curvature of the Earth and vertical gradient of refractive index) on scanning radiometer measurements are presented. A forward radiative transfer model that includes all these effects to represent the instrument measurements is used to estimate the biases. These biases are estimated using differences between the measurement with and without these characteristics for three commonly used frequency bands: K, V and W-band. The receiver channel bandwidth errors are less important in K-band and W-band. Thus, the use of a wider bandwidth to improve detection at low signal-to-noise conditions is acceptable at these frequencies. The biases caused by omitting the antenna beam width in measurement simulations are larger than those caused by omitting the receiver bandwidth, except for V-band where the bandwidth may be more important in the vicinity of absorption peaks. Using simple regression algorithms, the effects of the bandwidth and beam width biases in liquid water path, integrated water vapour, and temperature are also examined. The largest errors in liquid water path and integrated water vapour are associated with the beam width errors.

scholarly journals Biases caused by the instrument bandwidth and beam width on simulated brightness temperature measurements from scanning microwave radiometers

2012 ◽  
Vol 5 (6) ◽  
pp. 8085-8130
Author(s):  
V. Meunier ◽  
U. Löhnert ◽  
P. Kollias ◽  
S. Crewell
Keyword(s):  
Water Vapor ◽  
Liquid Water ◽  
Beam Width ◽  
Liquid Water Path ◽  
Water Path ◽  
Antenna Beam ◽  
Atmospheric Propagation ◽  
W Band ◽  
The Impact ◽  
Receiver Bandwidth

Abstract. More so than the traditional fixed radiometers, the scanning radiometer requires a careful design to ensure high quality measurements. Here the impact of the radiometer characteristics (e.g. antenna beam width, receiver bandwidth) and atmospheric propagation (e.g. curvature of the earth and refractivity) on the scanning radiometer measurements are presented. A forward radiative transfer model that includes all these effects to represent the instrument measurements is used to estimate the biases as differences between the measurement with and without these characteristics for three commonly used frequency bands: K, V and W-band. The receiver channel bandwidth errors are not so important in K-band and W-band. Thus, the use of a wider bandwidth to improve detection at low signal-to-noise conditions is acceptable. The impact of the antenna beam width is higher than the receiver bandwidth, but, for V-band where they are of similar importance. Using simple regression algorithms, the effects of the bandwidth and beam width biases in liquid water path, integrated water vapor, and temperature are also examined. The largest errors in liquid water path and integrated water vapor are associated with the beam width errors.

scholarly journals MBL drizzle properties and their impact on cloud property retrievals

2015 ◽  
Vol 8 (4) ◽  
pp. 4307-4323
Author(s):  
P. Wu ◽  
X. Dong ◽  
B. Xi
Keyword(s):  
Liquid Water ◽  
Cloud Droplet ◽  
Number Concentration ◽  
Effective Radius ◽  
Liquid Water Path ◽  
Water Path ◽  
Cloud Property ◽  
Annual Means ◽  
Mean Differences ◽  
The Impact

Abstract. In this study, we retrieve and document drizzle properties, and investigate the impact of drizzle on cloud property retrievals from ground-based measurements at the ARM Azores site from June 2009 to December 2010. For the selected cloud and drizzle samples, the drizzle occurrence is 42.6% with a maximum of 55.8% in winter and a minimum of 35.6% in summer. The annual means of drizzle liquid water path LWPd, effective radius rd, and number concentration Nd for the rain (virga) samples are 5.48 (1.29) g m−2, 68.7 (39.5) μm, and 0.14 (0.38) cm−3. The seasonal mean LWPd values are less than 4% of the MWR-retrieved LWP values. The annual mean differences in cloud-droplet effective radius with and without drizzle are 0.12 and 0.38 μm, respectively, for the virga and rain samples. Therefore, we conclude that the impact of drizzle on cloud property retrievals is insignificant at the ARM Azores site.

Variational Assimilation of Cloud Liquid/Ice Water Path and Its Impact on NWP

2015 ◽  
Vol 54 (8) ◽  
pp. 1809-1825 ◽  
Author(s):  
Yaodeng Chen ◽  
Hongli Wang ◽  
Jinzhong Min ◽  
Xiang-Yu Huang ◽  
Patrick Minnis ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Liquid Water ◽  
Weather Prediction ◽  
Wrf Model ◽  
Liquid Water Path ◽  
Cloud Liquid Water ◽  
Water Path ◽  
Ice Water Path ◽  
Ice Water ◽  
The Impact

AbstractAnalysis of the cloud components in numerical weather prediction models using advanced data assimilation techniques has been a prime topic in recent years. In this research, the variational data assimilation (DA) system for the Weather Research and Forecasting (WRF) Model (WRFDA) is further developed to assimilate satellite cloud products that will produce the cloud liquid water and ice water analysis. Observation operators for the cloud liquid water path and cloud ice water path are developed and incorporated into the WRFDA system. The updated system is tested by assimilating cloud liquid water path and cloud ice water path observations from Global Geostationary Gridded Cloud Products at NASA. To assess the impact of cloud liquid/ice water path data assimilation on short-term regional numerical weather prediction (NWP), 3-hourly cycling data assimilation and forecast experiments with and without the use of the cloud liquid/ice water paths are conducted. It is shown that assimilating cloud liquid/ice water paths increases the accuracy of temperature, humidity, and wind analyses at model levels between 300 and 150 hPa after 5 cycles (15 h). It is also shown that assimilating cloud liquid/ice water paths significantly reduces forecast errors in temperature and wind at model levels between 300 and 150 hPa. The precipitation forecast skills are improved as well. One reason that leads to the improved analysis and forecast is that the 3-hourly rapid update cycle carries over the impact of cloud information from the previous cycles spun up by the WRF Model.

scholarly journals The Impact of Low Clouds on Surface Shortwave Radiation in the ECMWF Model

2012 ◽  
Vol 140 (11) ◽  
pp. 3783-3794 ◽  
Author(s):  
Maike Ahlgrimm ◽  
Richard Forbes
Keyword(s):  
Great Plains ◽  
Liquid Water ◽  
Shortwave Radiation ◽  
Fair Weather ◽  
Liquid Water Path ◽  
Water Path ◽  
Surface Irradiance ◽  
Low Clouds ◽  
Low Cloud ◽  
The Impact

Abstract The long-term measurement records from the Atmospheric Radiation Measurement site on the Southern Great Plains show evidence of a bias in the ECMWF model’s surface irradiance. Based on previous studies, which have suggested that summertime shallow clouds may contribute to the bias, an evaluation of 146 days with observed nonprecipitating fair-weather cumulus clouds is performed. In-cloud liquid water path and effective radius are both overestimated in the model with liquid water path dominating to produce clouds that are too reflective. These are compensated by occasional cloud-free days in the model such that the fair-weather cumulus regime overall does not contribute significantly to the multiyear daytime mean surface irradiance bias of 23 W m−2. To further explore the origin of the bias, observed and modeled cloud fraction profiles over 6 years are classified and sorted based on the surface irradiance bias associated with each sample pair. Overcast low cloud conditions during the spring and fall seasons are identified as a major contributor. For samples with low cloud present in both observations and model, opposing surface irradiance biases are found for overcast and broken cloud cover conditions. A reduction of cloud liquid to a third for broken low clouds and an increase by a factor of 1.5 in overcast situations improves agreement with the observed liquid water path distribution. This approach of combining the model shortwave bias with a cloud classification helps to identify compensating errors in the model, providing guidance for a targeted improvement of cloud parameterizations.

scholarly journals Synergetic use of millimeter- and centimeter-wavelength radars for retrievals of cloud and rainfall parameters

2010 ◽  
Vol 10 (7) ◽  
pp. 3321-3331 ◽  
Author(s):  
S. Y. Matrosov
Keyword(s):  
Liquid Water ◽  
Rain Rate ◽  
Wet Season ◽  
Liquid Water Path ◽  
Vertical Column ◽  
Suggested Approach ◽  
Ka Band ◽  
Water Path ◽  
Climate Research ◽  
W Band

Abstract. A remote sensing approach for simultaneous retrievals of cloud and rainfall parameters in the vertical column above the US Department of Energy's (DOE) Climate Research Facility at the Tropical Western Pacific (TWP) Darwin site in Australia is described. This approach uses vertically pointing measurements from a DOE Ka-band radar and scanning measurements from a nearby C-band radar pointing toward the TWP Darwin site. Rainfall retrieval constraints are provided by data from a surface impact disdrometer. The approach is applicable to stratiform precipitating cloud systems when a separation between the liquid hydrometeor layer, which contains rainfall and liquid water clouds, and the ice hydrometeor layer is provided by the radar bright band. Absolute C-band reflectivities and Ka-band vertical reflectivity gradients in the liquid layer are used for retrievals of the mean layer rain rate and cloud liquid water path (CLWP). C-band radar reflectivities are also used to estimate ice water path (IWP) in regions above the melting layer. The retrieval uncertainties of CLWP and IWP for typical stratiform precipitation systems are about 500–800 g m−2 (for CLWP) and a factor of 2 (for IWP). The CLWP retrieval uncertainties increase with rain rate, so retrievals for higher rain rates may be impractical. The expected uncertainties of layer mean rain rate retrievals are around 20%, which, in part, is due to constraints available from the disdrometer data. The applicability of the suggested approach is illustrated for two characteristic events observed at the TWP Darwin site during the wet season of 2007. A future deployment of W-band radars at the DOE tropical Climate Research Facilities can improve CLWP estimation accuracies and provide retrievals for a wider range of stratiform precipitating cloud events.

scholarly journals Aerosol Impacts on the Diurnal Cycle of Marine Stratocumulus

2008 ◽  
Vol 65 (8) ◽  
pp. 2705-2718 ◽  
Author(s):  
Irina Sandu ◽  
Jean-Louis Brenguier ◽  
Olivier Geoffroy ◽  
Odile Thouron ◽  
Valery Masson
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Liquid Water ◽  
Large Scale ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Water Path ◽  
The Impact

Abstract Recent large-eddy simulation (LES) studies of the impact of aerosol on the dynamics of nocturnal marine stratocumulus revealed that, depending on the large-scale forcings, an aerosol-induced increase of the droplet concentration can lead to either an increase or a decrease of the liquid water path, hence contrasting with the cloud thickening that is expected from a reduction of the precipitation efficiency. In this study, the aerosol impacts on cloud microphysics are examined in the context of the boundary-layer diurnal cycle using 36-h LES simulations of pristine and polluted clouds. These simulations corroborate previous findings that during nighttime aerosol-induced liquid water path changes are sensitive to the large-scale forcings via enhancement of cloud-top entrainment such that, ultimately, the liquid water path may be reduced when the free-tropospheric-entrained air is drier. During the day, however, enhanced entrainment, inhibition of drizzle evaporation below cloud base, and reduced sensible heat flux from the surface lead to a more pronounced decoupling of the boundary layer, which significantly amplifies the liquid water path reduction of the polluted clouds. At night the sign of the liquid water path difference between pristine and polluted clouds depends upon large-scale forcings, while during the day the liquid water path of polluted clouds is always smaller than the one of the pristine clouds. Suggestions are made on how observational studies could be designed for validation of these simulations.

scholarly journals How important is the vertical structure for the representation of aerosol impacts on the diurnal cycle of marine stratocumulus?

2009 ◽  
Vol 9 (12) ◽  
pp. 4039-4052 ◽  
Author(s):  
I. Sandu ◽  
J.-L. Brenguier ◽  
O. Thouron ◽  
B. Stevens
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Liquid Water ◽  
Vertical Structure ◽  
Radiative Properties ◽  
Vertical Stratification ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Water Path ◽  
The Impact

Abstract. Large-Eddy Simulations (LES) are performed to examine the impact of hygroscopic aerosols on the diurnal cycle of marine stratocumulus clouds, under varying meteorological forcing conditions. When the cloud condensation nuclei concentration increase is sufficient to inhibit drizzle formation in the cloud layer, the precipitating and the non-precipitating cloud layers exhibit contrasting evolutions, with noticeable differences in liquid water path. Aerosol-induced modifications of the droplet sedimentation and drizzle precipitation result in noticeable changes of the entrainment velocity at cloud top, but also in significant changes of the vertical stratification in the boundary layer. This set of simulations is then used to evaluate whether a model which does not explicitly represent the effects of the interactions occurring within the boundary layer on its vertical stratification (i.e. such as a mixed-layer model) is capable of reproducing at least the sign, if not the amplitude, of these aerosol impacts on the liquid water path. It is shown that the evolution of the vertical structure is key to the responses we simulate, and must be considered in bulk models that wish to predict the impact of aerosol perturbations on the radiative properties of stratocumulus-topped boundary layers.

scholarly journals The impact of horizontal heterogeneities, cloud fraction, and liquid water path on warm cloud effective radii from CERES-like Aqua MODIS retrievals

2013 ◽  
Vol 13 (19) ◽  
pp. 9997-10003 ◽  
Author(s):  
D. Painemal ◽  
P. Minnis ◽  
S. Sun-Mack
Keyword(s):  
Liquid Water ◽  
Vertical Structure ◽  
Cloud Fraction ◽  
Effective Radius ◽  
Liquid Water Path ◽  
Water Path ◽  
Warm Cloud ◽  
Southeast Pacific ◽  
The Impact

Abstract. The impact of horizontal heterogeneities, liquid water path (LWP from AMSR-E), and cloud fraction (CF) on MODIS cloud effective radius (re), retrieved from the 2.1 μm (re2.1) and 3.8 μm (re3.8) channels, is investigated for warm clouds over the southeast Pacific. Values of re retrieved using the CERES algorithms are averaged at the CERES footprint resolution (∼20 km), while heterogeneities (Hσ) are calculated as the ratio between the standard deviation and mean 0.64 μm reflectance. The value of re2.1 strongly depends on CF, with magnitudes up to 5 μm larger than those for overcast scenes, whereas re3.8 remains insensitive to CF. For cloudy scenes, both re2.1 and re3.8 increase with Hσ for any given AMSR-E LWP, but re2.1 changes more than for re3.8. Additionally, re3.8–re2.1 differences are positive (<1 μm) for homogeneous scenes (Hσ < 0.2) and LWP > 45 gm−2, and negative (up to −4 μm) for larger Hσ. While re3.8–re2.1 differences in homogeneous scenes are qualitatively consistent with in situ microphysical observations over the region of study, negative differences – particularly evinced in mean regional maps – are more likely to reflect the dominant bias associated with cloud heterogeneities rather than information about the cloud vertical structure. The consequences for MODIS LWP are also discussed.

scholarly journals Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle

2017 ◽  
Vol 17 (21) ◽  
pp. 13017-13035 ◽  
Author(s):  
Marie Mazoyer ◽  
Christine Lac ◽  
Odile Thouron ◽  
Thierry Bergot ◽  
Valery Masson ◽  
...  
Keyword(s):  
Life Cycle ◽  
Liquid Water ◽  
Horizontal Resolution ◽  
Liquid Water Path ◽  
Radiation Fog ◽  
Near Surface ◽  
Water Path ◽  
Fog Prediction ◽  
Large Eddy ◽  
The Impact

Abstract. Large eddy simulations (LESs) of a radiation fog event occurring during the ParisFog experiment are studied with a view to analyse the impact of the dynamics of the boundary layer on the fog life cycle. The LES, performed with the Meso-NH model at 5 m resolution horizontally and 1 m vertically, and with a 2-moment microphysical scheme, includes the drag effect of a tree barrier and the deposition of droplets on vegetation. The model shows good agreement with measurements of near-surface dynamic and thermodynamic parameters and liquid water path. The blocking effect of the trees induces elevated fog formation, as actually observed, and horizontal heterogeneities during the formation. It also limits cooling and cloud water production. Deposition is found to exert the most significant impact on fog prediction as it not only erodes the fog near the surface but also modifies the fog life cycle and induces vertical heterogeneities. A comparison with the 2 m horizontal resolution simulation reveals small differences, meaning that grid convergence is achieved. Conversely, increasing numerical diffusion through a wind advection operator of lower order leads to an increase in the liquid water path and has a very similar effect to removing the tree barrier. This study allows us to establish the major dynamical ingredients needed to accurately represent the fog life cycle at very high-resolution.

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