R. Boers

Accuracy of absolute precipitable water vapor estimates from GPS observations

We present GPS, radiosonde and microwave radiometer (MWR) estimates of precipitable water vapor (PW) at Cape Grim, Tasmania, during November and December 1995. The rms differences between GPS and radiosonde, MWR and radiosonde and GPS and MWR estimates of PW were 1.5 mm, 1.3 mm and 1.4 mm, respectively, whilst the biases between the three systems were ∼0.2 mm. However, there are occasions when the amount of PW was underestimated by GPS whilst at other times was over-estimated by MWR. The average overlap error of the GPS estimates of PW between adjacent daily solutions is related to the orbit overlap error and we removed a 2 mm bias introduced using International GPS Service orbits by estimating more accurate global orbits. The discrepancies of up to 3–4 mm between the MWR and GPS systems are not caused by rain, waveguide losses, varying waveguide temperature, detector non-linearity or inaccurate estimates of the mean radiating temperature of the atmosphere. However, small differences between mapping functions at low elevations can produce biases comparable with the bias between the two systems. Consequently, we suspect that the biases arise because the mapping functions do not represent the localized atmospheric conditions at Cape Grim. The most accurate GPS estimates are achieved when the GPS analysis contains station separations of more than 2000 km, an elevation cutoff angle of 12° is used and the CFA2.2 wet mapping function is used to map the wet delay at any angle to the delay in the zenith.

Determination of precipitable water and cloud liquid water over oceans from the NOAA 15 advanced microwave sounding unit

The advanced microwave sounding unit (AMSU) was finally launched in May 1998 aboard the NOAA 15 satellite. Algorithms are provided for retrieving the total precipitable water (TPW) and cloud liquid water (CLW) over oceans using the AMSU measurements at 23.8 and 31.4 GHz. Extensive comparisons are made between the AMSU retrievals of CLW and TPW and those obtained using other satellite instruments (Special Sensor Microwave Imager (SSM/I) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI)) and ground-based radiometers. The AMSU TPW is also compared against radiosonde data, where all of the results are in good agreement with rms differences less than 3 mm and biases less than 1 mm over the range between 5 and 60 mm. The CLW comparisons show greater variability, although the time series of the AMSU and ground-based sensors follow each other and cover the same dynamic range of 0 - 0.5 mm. The AMSU CLW also compares well with the other satellite measurements, although a bias exists between AMSU and TMI when the CLW exceeds 0.5 mm.

A modeling study of the effect of drizzle on cloud optical depth and susceptibility

This paper examines the impact of drop spectral broadening, generated by the collection process, on the optical depth, cloud albedo, and susceptibility of marine stratocumulus clouds. The results are arrived at using (1) the output from a simple box model calculation of collection and (2) the output from an eddy-resolving model of stratocumulus clouds that explicitly represents the size distribution of the drops. It is shown that commonly used relationships for cloud optical properties developed for narrow spectra do not generally apply to spectra undergoing spectral broadening. The optical depth dependence on the drop number concentration to the one-third power is shown to be an overestimate of the optical depth when spectra broaden through collection. In addition, the cloud susceptibility dependence on drop number is shown to be larger for spectra experiencing broadening than for narrow spectra.

Ground-Based Atmospheric Remote Sensing in the Netherlands: European Outlook

This paper describes the contours of a Dutch monitoring and research site for climate change and related atmospheric processes. The station has large benefits for atmospheric science, both in The Netherlands and internationally. It provides a platform for collaboration in this important field, and will provide the routine observations needed to assess the impact of the different atmospheric parameters on the local climate. The station fits in directly in the selected group of global monitoring networks that are currently operational or being set up to address the problems of climate. In addition, the station can play a major role in supporting world-wide satellite measurements of climate related parameters. The only way to get a global picture of the essential climate change parameters can be found in the combination of satellite measurements and ground-based stations equipped with advanced remote sensing and in situ instrumentation. Furthermore, the combined expertise of European universities and research institutes, encompassing the whole field of atmospheric research, offers a unique chance for the training of young scientists. The research site is an attractive center for international young scientists to develop and deepen their skills. Copyright

Microphysical properties of boundary layer clouds over the Southern Ocean during ACE 1

A survey is presented of microphysical properties of boundary layer clouds sampled over the Southern Ocean during the First Aerosol Characterization Experiment (ACE 1, November-December 1995). Observations were confined between the latitudes of 40°S and 55°S and between the longitudes of 135°E and 160°E, which covers a region south of the island of Tasmania. Because of the remoteness of the region that was sampled, the microphysical cloud structure represents background conditions in which anthropogenic influences are minimal. Maximum cloud droplet concentrations for individual case studies ranged between 45 and 200 cm -3 . These numbers are consistent with those reported from the summer phase of the Southern Ocean Cloud Experiment (SOCEX II, February 1995) and confirm that the droplet concentrations in an unpolluted atmosphere are largely constrained to values below 200 cm -3 . The often observed weak decoupling of the cloud layer from the surface layer suggests that the entrainment of aerosol from the free atmosphere cannot be ruled out as a significant source of cloud condensation nuclei. The effective radius of the cloud droplets ranged from 4.5 to 16.6 μm. On many occasions the condition for cloud top entrainment instability was satisfied. This implies the occurrence of dry conditions aloft, turbulence generation due to entrainment, and more generally, conditions of broken cloud cover. It is premature to suggest that these conditions are prevalent over the Southern Ocean as the experiment may have inadvertently targeted atmospheric conditions with regions of broken clouds.

Simultaneous retrievals of cloud optical depth and droplet concentration from solar irradiance and microwave liquid water path

A 20-month time series of continuous observations of microwave radiation and solar irradiance was used to estimate the cloud optical depth and droplet number concentration at the Cape Grim Baseline Air Pollution Station, Tasmania, (40°41′S, 144°41′E). The data were selected by wind directions. When the air was from “baseline” origin, i.e., it had travelled over long oceanic distances and was mostly devoid of anthropogenic influences, the retrieved droplet concentration and optical depth were lower than when the air was from “nonbaseline” i.e., continental origin. Therefore the observed variation in cloud microphysical properties reflects the difference between the natural background conditions over the Southern Ocean and continental conditions with elevated droplet counts. Under baseline conditions the retrieved cloud optical depth exhibits a weak but perceptible seasonal cycle that has been previously observed from satellite data with a minimum in the austral winter, and a maximum in the austral summer. The results demonstrate that routine retrievals of cloud microphysical properties are possible using only a pyranometer and a microwave liquid water radiometer.

Ground-Based Observations and Modeling of the Visibility and Radar Reflectivity in a Radiation Fog Layer

The development of a radiation fog layer at the Cabauw Experimental Site for Atmospheric Research(51.97°N, 4.93°E) on 23 March 2011 was observed with ground-based in situ and remote sensing observationsto investigate the relationship between visibility and radar reflectivity. The fog layer thickness was less than200 m. Radar reflectivity values did not exceed 225 dBZ even with visibilities less than 100 m. The onset andevaporation of fog produce different radar reflectivity-visibility relationships. The evolution of the fog layerwas modeled with a droplet activation model tha used the aerosol size distribution observed at the 60-m altitude tower level as input. Radar reflectivity and visibility were calculated from model drop ize spectra using Mie scattering theory. Since radiative cooling rates are small in comparison with cooling rates due to adiabatic lift of aerosol-laden air, the modeled supersaturation remains low so that few aerosol particles are activated to cloud droplets. The modeling results suggest that the different radar reflectivity-v sibility relationshipsare the result of differences in the interplay between water vapor and cloud droplets during formation and evaporation of the fog. During droplet activation, only a few large cloud droplets remain after successfully competing for water vapor with the smaller activated droplets. These small droplets eventually evaporate (deactivate) again. In the fog dissolution/evaporation stage, only these large droplet need to be evaporated.Therefore, to convert radar reflectivity to visibility for traffic safety products, knowledge of the state of local fog evolution is necessary.

Detection of Boundary Layer Water Clouds by Spaceborne Cloud Radar

Abstract A model for the radar reflectivity of boundary layer water clouds is constructed using cloud droplet spectra fitted to a truncated gamma distribution. The spectra were derived from several recent field experiments. Realistic space-based radar returns are simulated that take into account the pulse shape, digitization interval, averaging volume, and variations in droplet concentration, cloud depth, and cloud-top height. The results show that the long pulse length of the proposed radar is responsible for smearing out the real reflectivity spatially so that the space-based detected clouds occupy a volume far exceeding that of the “observed” cloud. However, the effect of smearing is reduced by the limited receiver sensitivity. Cloud volume of boundary layer clouds is overestimated by between 30% and 100% using proposed radar parameters. Even if clouds are detected, the radar reflectivity convoluted by the pulse shape is sufficiently different from the originally observed reflectivity to seriously impe...

Overview of Research and Networking with Ground based Remote Sensing for Atmospheric Profiling at the Cabauw Experimental Site for Atmospheric Research (CESAR) - The Netherlands

CESAR, the Cabauw Experimental Site for Atmospheric Research, is the Dutch focal point for collaboration on climate monitoring and atmospheric research and is situated on the KNMI meteorological research site near Cabauw in the Netherlands. CESAR addresses challenging topics in atmospheric research, especially the questions that are related to the interaction between clouds, aerosols and radiation and questions dealing with land-atmosphere interaction. These topics are approached via process studies, model evaluations, climate monitoring, development of new experimental techniques and supporting activities for satellite missions. For each of these approaches, specific demands are put on the instrumentation, mode of operation and overall infrastructure. This paper gives an overview of CESAR that was recently augmented with a scanning drizzle radar (IDRA) and a multi-wavelength Raman lidar for aerosols, clouds and water vapor (CAELI).

Sensitivity of the shortwave radiative budget to the parameterization of ice crystal effective radius

[1] A new effective particle size (R eff ) parameterization for ice clouds has been formulated based on depth into cloud relative to cloud top. This parameterization has been developed based on an extensive data set of lidar and radar ice cloud retrievals. Using this parameterization within the stand-alone radiation code from the European Centre for Medium Range Weather Forecasting (cy23r4), the performance of the new parameterization is compared with the more commonly used parameterizations based on temperature and/or ice water content. An evaluation is performed on the basis of observed shortwave fluxes for 13 days with persistent ice cloud decks, with no liquid clouds beneath, over the Cabauw Experimental Site for Atmospheric Research in the Netherlands. For each of these clouds the shortwave flux is calculated after which the distribution of the differences between the observed and modeled shortwave fluxes from the combined 13 days are compared with each other. The new parameterization shows a median absolute difference of 0.7 W m -2 relative to the observations. The control parameterization based on temperature shows a median absolute difference of 15 W m -2 . Within the framework of the KNMI regional climate model (RACMO2), the new parameterization yields an effective particle size versus temperature distribution very similar to the observed distributions from lidar and radar retrievals. Results from a 1-year integration indicate that the domain averaged monthly mean planetary albedo and transmissivity change by a maximum of 2.6 and 2.4%, respectively, using the new parameterization compared to the temperature-based parameterization.