Heike Kalesse

Climatology of High Cloud Dynamics Using Profiling ARM Doppler Radar Observations

AbstractIce cloud properties are influenced by cloud-scale vertical air motion. Dynamical properties of ice clouds can be determined via Doppler measurements from ground-based, profiling cloud radars. Here, the decomposition of the Doppler velocities into reflectivity-weighted particle velocity Vt and vertical air motion w is described. The methodology is applied to high clouds observations from 35-GHz profiling millimeter wavelength radars at the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) climate research facility in Oklahoma (January 1997–December 2010) and the ARM Tropical Western Pacific (TWP) site in Manus (July 1999–December 2010). The Doppler velocity measurements are used to detect gravity waves (GW), whose correlation with high cloud macrophysical properties is investigated. Cloud turbulence is studied in the absence and presence of GW. High clouds are less turbulent when GW are observed. Probability density functions of Vt, w, and high cloud macrophysical propert...

Vertical velocities and turbulence in midlatitude anvil cirrus: A comparison between in situ aircraft measurements and ground‐based Doppler cloud radar retrievals

This study introduces a statistical comparison of vertical velocity observations within cirrus from aircraft and ground-based Doppler cloud radar. Two cases of midlatitude anvil cirrus forming under very similar environmental conditions are examined. The case studies benefit from simultaneous observations of vertical velocities in cirrus collected at and around the Atmospheric Radiation Measurement Southern Great Plains site during the U.S. Department of Energy Small Particles in Cirrus field campaign. Observations from both platforms suggest that the majority of vertical velocities in the examined midlatitude anvil cirrus cases are roughly within ±1 m s−1 although higher vertical velocities are occasionally observed. The quality of the vertical velocity comparison between in situ aircraft measurements and ground-based Doppler radar retrievals depends on the case. For the first case on 23 April 2010, the comparison suggests that the radar retrieval may underestimate vertical velocities in the range between roughly 50 cm s−1 and 1 m s−1. For the second case on 14 June 2010, the agreement between radar and aircraft is excellent, and the differences are largely within the observed variability of vertical velocities within cirrus. Differences in the spatial scales of vertical velocities and turbulence sampled by the aircraft and Doppler radar, which arise due to differences in the temporal resolution of the observational platforms are not found to explain the observed discrepancies. Estimates for the dissipation rate of turbulent kinetic energy agree to within 1 order of magnitude between the two observational platforms.

First observations of triple-frequency radar Doppler spectra in snowfall: Interpretation and applications

The potential of multifrequency Doppler spectra to constrain precipitation microphysics has so far only been exploited for dual-frequency spectra in rain. In this study, we extend the dual-frequency concept to triple-frequency Doppler radar spectra obtained during a snowfall event which included rimed and unrimed snow aggregates. A large selection of spectra obtained from low-turbulence regions within the cloud reveals distinctly different signatures of the derived dual spectral ratios. Due to the third frequency, a characteristic curve can be derived which is almost independent of the underlying particle size distribution and velocity-size relation. This approach provides new opportunities for validating existing and future snow scattering models and reveals how the information content of triple-frequency radar data sets can be further exploited for snowfall studies.

The Role of Shallow Cloud Moistening in MJO and Non-MJO Convective Events over the ARM Manus Site

AbstractObservations from the Atmospheric Radiation Measurement Program (ARM) site at Manus Island in the western Pacific and (re)analysis products are used to investigate moistening by shallow cumulus clouds and by the circulation in large-scale convective events. Large-scale convective events are defined as rainfall anomalies larger than one standard deviation for a minimum of three consecutive days over a 10° × 10° domain centered at Manus. These events are categorized into two groups: Madden–Julian oscillation (MJO) events, with eastward propagation, and non-MJO events, without propagation. Shallow cumulus clouds are identified as continuous time–height echoes from 1-min cloud radar observations with their tops below the freezing level and their bases within the boundary layer. Daily moistening tendencies of shallow clouds, estimated from differences between their mean liquid water content and precipitation over their presumed life spans, and those of physical processes and advection from (re)analysis...

Direct estimation of the global distribution of vertical velocity within cirrus clouds

Cirrus clouds determine the radiative balance of the upper troposphere and the transport of water vapor across the tropopause. The representation of vertical wind velocity, W, in atmospheric models constitutes the largest source of uncertainty in the calculation of the cirrus formation rate. Using global atmospheric simulations with a spatial resolution of 7 km we obtain for the first time a direct estimate of the distribution of W at the scale relevant for cirrus formation, validated against long-term observations at two different ground sites. The standard deviation in W, σw, varies widely over the globe with the highest values resulting from orographic uplift and convection, and the lowest occurring in the Arctic. Globally about 90% of the simulated σw values are below 0.1 m s−1 and about one in 104 cloud formation events occur in environments with σw > 0.8 m s−1. Combining our estimate with reanalysis products and an advanced cloud formation scheme results in lower homogeneous ice nucleation frequency than previously reported, and a decreasing average ice crystal concentration with decreasing temperature. These features are in agreement with observations and suggest that the correct parameterization of σw is critical to simulate realistic cirrus properties.

Stratiform and Convective Precipitation Observed by Multiple Radars during the DYNAMO/AMIE Experiment

AbstractIn this study, methods of convective/stratiform precipitation classification and surface rain-rate estimation based on the Atmospheric Radiation Measurement Program (ARM) cloud radar measurements were developed and evaluated. Simultaneous and collocated observations of the Ka-band ARM zenith radar (KAZR), two scanning precipitation radars [NCAR S-band/Ka-band Dual Polarization, Dual Wavelength Doppler Radar (S-PolKa) and Texas A&M University Shared Mobile Atmospheric Research and Teaching Radar (SMART-R)], and surface precipitation during the Dynamics of the Madden–Julian Oscillation/ARM MJO Investigation Experiment (DYNAMO/AMIE) field campaign were used. The motivation of this study is to apply the unique long-term ARM cloud radar observations without accompanying precipitation radars to the study of cloud life cycle and precipitation features under different weather and climate regimes. The resulting convective/stratiform classification from KAZR was evaluated against precipitation radars. Preci...

Understanding Rapid Changes in Phase Partitioning between Cloud Liquid and Ice in Stratiform Mixed-Phase Clouds: An Arctic Case Study

AbstractUnderstanding phase transitions in mixed-phase clouds is of great importance because the hydrometeor phase controls the lifetime and radiative effects of clouds. In high latitudes, these cloud radiative effects have a crucial impact on the surface energy budget and thus on the evolution of the ice cover. For a springtime low-level mixed-phase stratiform cloud case from Barrow, Alaska, a unique combination of instruments and retrieval methods is combined with multiple modeling perspectives to determine key processes that control cloud phase partitioning. The interplay of local cloud-scale versus large-scale processes is considered. Rapid changes in phase partitioning were found to be caused by several main factors. Major influences were the large-scale advection of different air masses with different aerosol concentrations and humidity content, cloud-scale processes such as a change in the thermodynamical coupling state, and local-scale dynamics influencing the residence time of ice particles. Othe...