Assessments of cloud liquid water contributions to GPS radio occultation refractivity using measurements from COSMIC and CloudSat

Abstract

[1] Global cloud parameters including cloud liquid water, cloud base height, cloud top height, and cloud type are observed from the cloud profiling radar onboard CloudSat. Global Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) data during a 3 year period from 2007 to 2009 are collocated in space and time with CloudSat data. The collocated data set is then classified into seven groups: one clear-sky condition and six different cloud types with liquid water content (LWC) measurements. For each group, atmospheric refractivity, temperature, and water vapor derived from COSMIC GPS ROs are compared with those of the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. It is found that the COSMIC GPS RO refractivity observations are systematically greater than the refractivity calculated from ECMWF analyses, which is to be referred as a positive N bias in clouds. The fractional N bias is as high as 1.2% depending on cloud types. Using CloudSat LWC, it is demonstrated that LWC can contribute 0.8% of the total refractivity by individual clouds and 0.16% of the positive N bias. The 0.16% positive N bias is comparable in magnitude to the retrieval uncertainty quantified by the mean difference (DN O-R ) between the observed refractivity with LWC contribution subtracted and the refractivity calculated using GPS retrieved profiles of temperature, pressure, and humidity. The values of DN O-R increase linearly with LWC as anticipated theoretically. Positive N biases are spatially correlated with positive water vapor biases, negative temperature biases, and large liquid water contents.Citation: Yang, S., and X. Zou (2012), Assessments of cloud liquid water contributions to GPS radio occultation refractivity using measurements from COSMIC and CloudSat,

Comparison of global observations and trends of total precipitable water derived from microwave radiometers and COSMIC radio occultation from 2006 to 2013

“…The last two terms generally contribute less than 1 % to the refractivity and may be ignored . However, they can be significant for some applications under conditions of high cloud liquid or ice water content, as shown by Lin et al (2010), Yang and Zou (2012), and Zou et al (2012). We will neglect these terms in this study, but because we are looking at small differences between MW and RO TPW in cloudy and precipitating conditions in this paper, we estimate the possible contribution of these terms to RO TPW and the consequences of neglecting them here.…”

Global climatological variability of quasi-two-day waves revealed by TIMED/SABER observations

“…Satellites can almost provide whole global wind fields, temperature and atmospheric compositions from the upper troposphere to the lower thermosphere. For example, the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) can measure the lower atmospheric temperature (He et al, 2009;Yang and Zou, 2012); the Upper Atmospheric Research Satellite/High Resolution Doppler Imager (UARS/HRDI) measures temperature, zonal and meridional wind, water vapor and ozone concentration in the middle and upper atmosphere (Wu et al, 1993(Wu et al, , 1996; the Thermosphere Ionosphere Mesosphere Energetics Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) is used for temperature measurements in the upper atmosphere (Xu et al, 2007a, b); and the line-of-sight (LOS) winds can be surveyed by the Microwave Limb Sounder (MLS) on Aura (Tunbridge et al, 2011). Satellite observations contribute greatly to our knowledge of global morphology of QTDWs.…”

Dependence of positive refractivity bias of GPS RO cloudy profiles on cloud fraction along GPS RO limb tracks

“…A more detailed description of the typical geometry of the RO observations can be found in Poli (2004) and Zou (2012). This study extends the work by Yang and Zou (2012) to consider the impacts of cloud fraction along a GPS occulted radio signal propagation track.…”

“…By using LWC observations made available by CloudSat CPR, Yang and Zou (2012) quantified contributions to atmospheric refractivity from LWC to show that GPS signals could be modulated by cloud and precipitation to a level exceeding the GPS RO observation error. Evaluating the GPS N-biases by using a collocated single CloudSat profile to represent a cloud environment along a GPS RO limb track has limitation.…”

“…Any bias must be removed from the data before such data be assimilated into NWP data systems (Tarantola 1987). When compared with the ECMWF (European Centre for Medium-Range Weather Forecasts) analyses, atmospheric refractivity measurements from COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) GPS radio occultation (RO) mission were found to be positively biased in cloudy conditions, and those in clear-sky conditions were not biased (Lin et al 2010;Zou et al 2012;Yang and Zou 2012). In these early studies, cloudy ROs were identified through their collocation with clouds observed by the Cloud Profiling Radar (CPR) onboard CloudSat.…”