Measurements Of Ice Water Content Research Articles

Variations of tropical upper tropospheric clouds with sea surface temperature and implications for radiative effects

The variations of tropical upper tropospheric (UT) clouds with sea surface temperature (SST) are analyzed using effective cloud fraction from the Atmospheric Infrared Sounder (AIRS) on Aqua and ice water content (IWC) from the Microwave Limb Sounder (MLS) on Aura. The analyses are limited to UT clouds above 300 hPa. Our analyses do not suggest a negative correlation of tropical‐mean UT cloud fraction with the cloud‐weighted SST (CWT). Instead, both tropical‐mean UT cloud fraction and IWC are found to increase with CWT, although their correlations with CWT are rather weak. The rate of increase of UT cloud fraction with CWT is comparable to that of precipitation, while the UT IWC and ice water path (IWP) increase more strongly with CWT. The radiative effect of UT clouds is investigated, and they are shown to provide a net warming at the top of the atmosphere. An increase of IWP with SST yields an increase of net warming that corresponds to a positive feedback, until the UT IWP exceeds a value about 50% greater than presently observed by MLS. Further increases of the UT IWP would favor the shortwave cooling effect, causing a negative feedback. Sensitivities of UT cloud forcing to the uncertainties in UT CFR and IWC measurements are discussed.

Validation of the Aura MLS cloud ice water content measurements

Retrieval and validation of upper tropospheric ice water content (IWC) measurements with the Aura Microwave Limb Sounder (MLS) are described. The MLS version 2.2 (V2.2) IWC, derived from 240‐GHz cloud‐induced radiances (Tcir) at high tangent heights, is scientifically useful at 215–83 hPa. The V2.2 IWC represents a bulk cloud property averaged over a ∼300 × 7 × 4 km3 volume near the pointing tangent height. Precision, accuracy, and spatial resolution of the V2.2 IWC are determined through model simulations and comparisons with CloudSat observations. Comparisons of MLS V2.2 and CloudSat R03 IWCs are made for the months of January and July in terms of normalized probability density function (PDF). The differences between MLS and CloudSat IWC PDFs are generally less than 50% over the IWC range where the MLS technique is valid. At pressures <177 hPa and extratropical latitudes, the MLS V2.2 IWC exhibits a slightly low bias compared to CloudSat, part of which can be attributed to systematic errors in the MLS retrieval. Cloud inhomogeneity and particle size distribution are the leading sources of uncertainties in the V2.2 IWC.

Determination of the Combined Ventilation Factor and Capacitance for Ice Crystal Aggregates from Airborne Observations in a Tropical Anvil Cloud

Abstract The ventilation factor and capacitance used in numerical models to represent ice crystal aggregates directly affects the growth rate of the ice crystal aggregates, and consequently the sink of atmospheric water vapor. Currently, numerical models that prognose ice water content (IWC) and water vapor mixing ratio represent the capacitance and ventilation factor of precipitation-sized particles with simplified geometries, such as hexagonal plates. The geometries of actual precipitation-sized particles are often more complex, and a test of the values being employed is needed. Aircraft observations obtained during a Lagrangian spiral descent through the sublimation zone of a tropical anvil cloud have been used to determine an estimate of combined dimensionless capacitance and ventilation factor for the nonpristine geometries exhibited by ice crystal aggregates. By combining measurements of bulk ice water content, the particle size distribution, and environmental subsaturation, the change in ice water content was modeled throughout the spiral descent and compared with observations of the change in ice water content. Uncertainties resulting from potential systematic biases in the measurements and parameterizations used in the analysis were investigated with sensitivity tests. Most of the uncertainty was related to an assumed maximum potential bias in the measurement of IWC of ±45%. The resulting combined ventilation factor and dimensionless capacitance value was 1.3 (with a range of 0.6–1.9, defined by 68% of sensitivity test trials) for a particle size–weighted mean value of (Sc)1/3(Re)1/2 = 14.9 ± 1.7, where Sc is the Schmidt number and Re is the Reynolds number. Results from commonly adopted combinations of ventilation factor relations and capacitances are compared with the observations presented here, and, finally, surrogate dimensionless capacitances are suggested that when combined with commonly used ventilation factor relations are consistent with the results presented herein.

Comparisons of in situ measurements of cirrus cloud ice water content

Aircraft‐based in situ measurements of cirrus cloud ice water content (IWC) are important for cloud microphysical/radiative modeling and satellite validation studies, yet the measurements have proven challenging due, in part, to the large dynamic range of IWC values present in cirrus clouds. To date, three instruments designed for the measurement of IWC have been flown aboard the NASA WB‐57F research aircraft: the University of Colorado closed‐path laser hygrometer, the Droplet Measurement Technologies Cloud Spectrometer and Impactor, and the Harvard University Lyman‐α total water photofragment‐fluorescence hygrometer. This paper compares IWC measurements from these three instruments taken during the Midlatitude Cirrus Experiment (MidCiX) in 2004. At larger values of IWC (IWC > 10 mg m−3), the three instruments agree, on average, to within 20%, which is of the order of their estimated instrumental uncertainties. At smaller IWC values (<10 mg m−3), the agreement is worse, in part due to increasing instrument uncertainties. These results have implications for measurements in thin and subvisual cirrus. Particle sampling and evaporation, instrument background levels, and hysteresis are not found to be significant contributors to discrepancies among the measurements. For remote sensing validation studies where IWC data are vertically integrated to obtain ice water path, the agreement between the instruments is better than 20% for thick cirrus (τ > 1), implying that IWC measurements in thicker clouds are of sufficient accuracy for validation studies.

Refinements to Ice Particle Mass Dimensional and Terminal Velocity Relationships for Ice Clouds. Part I: Temperature Dependence

Abstract This two-part study attempts to find appropriate mass dimension and terminal velocity relationships that, when considered together with particle size distributions (PSD), agree with coincident measurements of ice water content (IWC), and with variables related to higher moments such as the mean mass-weighted fall speed. Reliable relationships are required for improving microphysical parameterizations for weather forecast models and developing methods for evaluating them, subjects addressed in detail in Part II of this study. Here, a range of values from 1.5 to 2.3 is assumed for the exponent b in the mass dimension relationship, m = aDb, where D is the maximum particle dimension, to bound its likely value for sizes above about 100 μm. Measured IWC and size spectra are used to find appropriate values for the coefficient a. It is demonstrated that all values of the exponent b, with appropriate a coefficients, can fit the IWC measurements. Coincident information on particle cross-sectional areas with the m(D) relationships is used to develop general fall velocity relationships of the form Vt = ADB. These assessments use five midlatitude, synoptically generated ice layers, and 10 low-latitude, convectively generated ice cloud layers, spanning the temperature range from −60° to 0°C. The coefficients a and A and exponent B are represented in terms of the exponent b and are shown to be temperature-dependent for the synoptic clouds and relatively independent of it in the convective clouds, a result of particle mixing through the cloud column. Consistency is found with earlier results and with analytic considerations. It is found that the fall velocity is inversely proportional to the air density to approximately the exponent 0.54, close to values assumed in earlier studies.

Ice water content and precipitation rate as a function of equivalent radar reflectivity and temperature based on in situ observations

Using ice particle spectra measured in stratiform ice clouds in midlatitude and Arctic regions, ice water content (IWC) and precipitation rate (Fm) retrieval algorithms as a function of temperature and radar reflectivity factor (Z) have been developed. These parameterizations were compared with (1) direct measurements of IWC using a Nevzorov probe, (2) precipitation retrieved using an X‐band Doppler scanning radar and a Precipitation Occurrence Sensor System (POSS), (3) the Canadian Global Environmental Multiscale (GEM) and High‐Resolution Model Application Project (HIMAP) models, and (4) derived IWC and precipitation from measured ice spectra during four field projects. The derived IWC and Fm from measured spectra have a correlation coefficient (r) better than 0.8. The IWC retrieved using the X‐band scanning radar during the First Alliance Icing Research Studies (AIRS I) agreed with IWC measured using the Nevzorov Probe much better than conventional IWC retrieval schemes. The retrieved precipitation rate based on the new algorithm using the POSS reflectivity measurements during the Second Alliance Icing Research Study (AIRS II) project agreed reasonably well with the precipitation rates predicted by the GEM and HIMAP models. This study clearly demonstrated that conventional IWC‐Ze relationships fail to replicate the IWC measured with Nevzorov probe. Furthermore, the Fm –Z relationship currently in use in the Canadian operational radar network appears to underestimate the ice precipitation rate. However, since there are some uncertainties in the direct measurements of IWC and no reliable direct measurements of precipitation, further studies are required to validate these parameterizations.

Comparisons of EOS MLS cloud ice measurements with ECMWF analyses and GCM simulations: Initial results

To assess the status of global climate models (GCMs) in simulating upper‐tropospheric ice water content (IWC), a new set of IWC measurements from the Earth Observing System's Microwave Limb Sounder (MLS) are used. Comparisons are made with ECMWF analyses and simulations from several GCMs, including two with multi‐scale‐modeling framework. For January 2005 monthly and daily mean values, the spatial agreement between MLS and ECMWF is quite good, although MLS estimates are higher by a factor of 2–3 over the Western Pacific, tropical Africa and South America. For the GCMs, the model‐data agreement is within a factor of 2–4 with larger values of disagreement occurring over Eastern Pacific and Atlantic ITCZs, tropical Africa and South America. The implications arising from sampling and uncertainties in the observations, the modeled values and their comparison are discussed. These initial results demonstrate the potential usefulness of this data set for evaluating GCM performance and guiding development efforts.

Improved Radar Ice Water Content Retrieval Algorithms Using Coincident Microphysical and Radar Measurements

Abstract Airborne radar reflectivity measurements at frequencies of 9.6 and 94 GHz, with collocated, in situ particle size distribution and ice water content measurements from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) in Florida in July 2002, offer one of the first opportunities to evaluate and improve algorithms for retrieving ice water content from single-wavelength spaceborne radar measurements. Both ice water content and radar reflectivity depend on the distribution of particle mass with size. It is demonstrated that single, power-law, mass dimensional relationships are unable to adequately account for the dominating contribution of small particles at lower reflectivities and large particles at higher reflectivities. To circumvent the need for multiple, or complex, mass dimensional relationships, analytic expressions that use particle ensemble mean ice particle densities that are derived from the coincident microphysical and radar observations are developed. These expressions, together with more than 5000 CRYSTAL FACE size distributions, are used to develop radar reflectivity–ice water content relationships for the two radar wavelengths that appear to provide improvements over earlier relationships, at least for convectively generated stratiform ice clouds.

Measurements of ice water content in tropopause region Arctic cirrus during the SAGE III Ozone Loss and Validation Experiment (SOLVE)

A new instrument, the closed‐path laser hygrometer (CLH), was flown on the NASA DC‐8 aircraft during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) campaign in 1999–2000 to measure condensed‐phase water. The ice water content (IWC) of Arctic cirrus was determined from in situ measurements of condensed‐ and gas‐phase water. The IWC values obtained from the CLH observations are compared to those determined by integrating particle size distributions measured by a Forward Scattering Spectrometer Probe (FSSP) also flown on the DC‐8. The considerably greater IWC seen by the CLH implies the presence of particles with diameters greater than the FSSP's upper limit of 20 μm. The evidence for and implications of the presence of large ice crystals in Arctic cirrus is discussed.

Effective Ice Particle Densities Derived from Aircraft Data

In this study, aircraft data are used to derive effective ice particle densities. This density is defined as the ice particle mass divided by the volume of an equivalent diameter sphere. Measured ice particle size distributions and total ice water contents are used to derive effective ice densities for ice particle populations (ρe) as a function of particle size [ρe(D)]. The density values are critical for modeling and remote sensing applications. The method uses particle size distributions (PSDs) measured by several particle spectrometers to compute the total particle volume per unit volume of air, assuming that the particles are spheres. Simultaneous direct measurements of ice water content from a counterflow virtual impactor (CVI) yield values for the number of grams of ice per unit volume of air, enabling the overall effective ice density for a population to be calculated. The measured PSD together with the CVI measurements are used to derive mass–dimension relationships. The methods are applied to measurements acquired in two field programs. More than 1200 population densities were derived from the Atmospheric Radiation Measurement (ARM) program and more than 5500 for the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) in southern Florida during July 2002. The population densities are represented in terms of two properties of particle size distributions: the spectral slope and the median mass diameter. The datasets have been divided into populations associated with predominantly synoptically generated ice cloud regions, convectively generated ice cloud regions, regions with moderately to heavily rimed and graupel particles, and those within the melting layer. Average particle density relationships are derived for each regime. Values of ρe are generally higher in synoptically than convectively generated cloud layers, and rimed particles are denser than unrimed ones. Values of ρe also decrease systematically downward within the ice clouds except in the melting layer, where they increase downward.

A General Approach for Deriving the Properties of Cirrus and Stratiform Ice Cloud Particles

A new approach is described for calculating the mass (m) and terminal velocity (Vt) of ice particles from airborne and balloon-borne imaging probe data as well as its applications for remote sensing and modeling studies. Unlike past studies that derived these parameters from the maximum (projected) dimension (D) and habit alone, the “two-parameter approach” uses D and the particle's projected cross-sectional area (A). Expressions were developed that relate the area ratio (Ar; the projected area of an ice particle normalized by the area of a circle with diameter D) to its effective density (ρe) and to Vt. Habit-dependent, power-law relationships between ρe and Ar were developed using analytic representations of the geometry of various types of planar and spatial ice crystals. Relationships were also derived from new or reanalyzed data for single ice particles and aggregates observed in clouds and at the ground. The mass relationships were evaluated by comparing calculations to direct measurements of ice water content (IWC). The calculations were from Particle Measuring Systems (PMS) 2D-C and 2D-P probes of particle size distributions in ice cloud layers on 3 days during an Atmospheric Radiation Measurement (ARM) field campaign in Oklahoma; the direct measurements were from counterflow virtual impactor (CVI) observations in ice cloud layers during the field campaign. Agreement was generally to within 20%, whereas using previous mass–dimension relationship approaches usually produced larger differences. Comparison of ground-based measurements of radar reflectivity with calculations from collocated balloon-borne ice crystal measurements also showed that the new method accurately captured the vertical reflectivity structure. Improvements in the accuracy of the estimates from the earlier mass–dimension relationships were achieved by converting them to the new form. A new, more accurate mass–dimension relationship for spatial, cirrus-type crystals was deduced from the comparison. The relationship between Vt and Ar was derived from a combination of theory and observations. A new expression accounting for the drag coefficients of large aggregates was developed from observational data. Explicit relationships for calculating Vt as a function of D for aggregates with a variety of component crystals were developed.

Measurements of wave‐cloud microphysical properties with two new aircraft probes

Measurements of ice water content (IWC) and mean ice‐crystal size and concentration made by two in‐situ probes, CVI and PVM, were compared on the DC‐8 aircraft during SUCCESS flights in orographic ice clouds. The comparison of IWC in these wave clouds, that formed at temperatures of about −38 °C on April 30 and −62 °C on May 2, 1996, showed good agreement. The comparison of ice crystal concentrations agreed better for the April‐30 clouds than for the May‐2 clouds; and the effective radius compared for both probes and for remote retrievals from aircraft and satellite for a segment of the Berthoud wave cloud (May 2) agreed within 30%. The measured parameters of the ice crystals were similar to earlier measurements and recent modeling of cold wave clouds.

Improved Measurements of the Ice Water Content in Cirrus Using a Total-Water Probe

This note describes an improved method for the measurement of the ice water content (IWC) of cirrus cloud using a total water content probe. A previous version of this technique assumed that the air in cloud-containing regions was saturated with respect to ice. This assumption has now been replaced with measurements of the water vapor content from a fast-response Lyman-α fluorescence water vapor sensor. The improved measurement of the vapor phase resolves anomalies in the earlier measurements that were due to the assumption of saturation with respect to ice everywhere within cloud. The comparison of IWC measurements made by this new method with those from a 2D optical array probe is greatly improved. The new measurements may now be used to provide much more stringent tests of the algorithms used for the derivation of crystal mass from measured size in 2D probe data.

An observational and theoretical study of the radiative properties of cirrus: Some results from ICE'89

Observations of the microphysical and radiative properties of two cirrus clouds are reported. The measurements were taken from the C-130 aircraft of the UK Meteorological Research Flight as part of the field campaign of the International Cirrus Experiment, ICE'89. Cirrus microphysical properties are analysed in terms of ice water content and a suitably defined ice crystal ‘effective radius’. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2D-C cloud probe and a Lyman-α total water content (TWC) hygrometer. With both flights large discrepancies are seen to occur between the two instruments near the cloud base and top, with the ice water contents derived from the TWC probe being consistently greater than those from the 2D-C probe. Evidence is presented which suggests that the differences near the cloud base are due to a significant number of large (>800 μm) crystals, which the 2D-C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (<100μm) crystals, which the 2D-C probe is again unable to sample correctly. Thus more weight has been given to the TWC probe measurements of ice water content, with the measurements of ice crystal effective radius being adjusted accordingly. Model clouds, constructed from the microphysical measurements, are used as input to a number of radiation parametrizations, both in the long-wave and short-wave regions of the spectrum. In the long wave, model agreement with both narrow-band and broad-band measurements is reasonably good, although discrepancies exist near the cloud tops. Some evidence is presented which suggests that the measured effective radii from one of the flights are too large to explain the radiative characteristics of the cloud. The effect of long-wave scattering is shown to be small. The inferred long-wave broad-band mass-absorption coefficient varies between 0.002 m2g-1 and 0.087 m2g−1, and it shows a marked inverse dependence on the ice crystal effective radius, as anticipated by simple theoretical calculations. In the short wave, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited by the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence of a relationship between ice water content and effective radius is considered.

An observational and theoretical study of the radiative properties of cirrus: Some results from ICE'89

AbstractObservations of the microphysical and radiative properties of two cirrus clouds are reported. The measurements were taken from the C‐130 aircraft of the UK Meteorological Research Flight as part of the field campaign of the International Cirrus Experiment, ICE'89.Cirrus microphysical properties are analysed in terms of ice water content and a suitably defined ice crystal ‘effective radius’. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2D‐C cloud probe and a Lyman‐α total water content (TWC) hygrometer. With both flights large discrepancies are seen to occur between the two instruments near the cloud base and top, with the ice water contents derived from the TWC probe being consistently greater than those from the 2D‐C probe. Evidence is presented which suggests that the differences near the cloud base are due to a significant number of large (&gt;800 μm) crystals, which the 2D‐C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (&lt;100μm) crystals, which the 2D‐C probe is again unable to sample correctly. Thus more weight has been given to the TWC probe measurements of ice water content, with the measurements of ice crystal effective radius being adjusted accordingly.Model clouds, constructed from the microphysical measurements, are used as input to a number of radiation parametrizations, both in the long‐wave and short‐wave regions of the spectrum. In the long wave, model agreement with both narrow‐band and broad‐band measurements is reasonably good, although discrepancies exist near the cloud tops. Some evidence is presented which suggests that the measured effective radii from one of the flights are too large to explain the radiative characteristics of the cloud. The effect of long‐wave scattering is shown to be small. The inferred long‐wave broad‐band mass‐absorption coefficient varies between 0.002 m2g‐1 and 0.087 m2g−1, and it shows a marked inverse dependence on the ice crystal effective radius, as anticipated by simple theoretical calculations. In the short wave, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited by the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence of a relationship between ice water content and effective radius is considered.

Measurements of the Ice Water Content in Cirrus Using an Evaporative Technique

Abstract A technique for the measurement of the ice water content (IWC) of cirrus clouds is described. The IWC is obtained by the measurement of the total water content (TWC) and the subtraction of the saturation specific humidity with respect to ice at the ambient pressure and temperature. The method is independent of the measurement of the crystal size spectrum and also of any assumptions about the bulk densities of various crystal habits. Examples of IWC measurements made during the International Cirrus Experiment are presented and compared with conventional measurements from a 2D optical array probe. The prime sources of error are the accuracy of the calibration of the TWC probe and the occurrence of subsaturated air, which invalidates one of the main principles of the technique.