Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

Numerical study of convection observed during the Winter Monsoon Experiment using a mesoscale two-dimensional model [presentation]

A new bulk microphysical parameterization (BMP) has been developed for use with the Weather Research and Forecasting (WRF) Model or other mesoscale models. As compared with earlier single-moment BMPs, the new scheme incorporates a large number of improvements to both physical processes and computer coding, and it employs many techniques found in far more sophisticated spectral/bin schemes using lookup tables. Unlike any other BMP, the assumed snow size distribution depends on both ice water content and temperature and is represented as a sum of exponential and gamma distributions. Furthermore, snow assumes a nonspherical shape with a bulk density that varies inversely with diameter as found in observations and in contrast to nearly all other BMPs that assume spherical snow with constant density. The new scheme’s snow category was readily modified to match previous research in sensitivity experiments designed to test the sphericity and distribution shape characteristics. From analysis of four idea...

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Explicit Forecasts of Winter Precipitation Using an Improved Bulk Microphysics Scheme. Part II: Implementation of a New Snow Parameterization

The Prediction in Ungauged Basins (PUB) initiative of the International Association of Hydrological Sciences (IAHS), launched in 2003 and concluded by the PUB Symposium 2012 held in Delft (23–25 October 2012), set out to shift the scientific culture of hydrology towards improved scientific understanding of hydrological processes, as well as associated uncertainties and the development of models with increasing realism and predictive power. This paper reviews the work that has been done under the six science themes of the PUB Decade and outlines the challenges ahead for the hydrological sciences community.Editor D. KoutsoyiannisCitation Hrachowitz, M., Savenije, H.H.G., Bloschl, G., McDonnell, J.J., Sivapalan, M., Pomeroy, J.W., Arheimer, B., Blume, T., Clark, M.P., Ehret, U., Fenicia, F., Freer, J.E., Gelfan, A., Gupta, H.V., Hughes, D.A., Hut, R.W., Montanari, A., Pande, S., Tetzlaff, D., Troch, P.A., Uhlenbrook, S., Wagener, T., Winsemius, H.C., Woods, R.A., Zehe, E., and Cudennec, C., 2013. A d...

/pdf/a-decade-of-predictions-in-ungauged-basins-pub-a-review-2g78unzpne.pdf

A decade of Predictions in Ungauged Basins (PUB)—a review

The application of polarimetric radar data to the retrieval of raindrop size distribution parameters and rain rate in samples of convective and stratiform rain types is presented. Data from the Colorado State University (CSU), CHILL, NCAR S-band polarimetric (S-Pol), and NASA Kwajalein radars are analyzed for the statistics and functional relation of these parameters with rain rate. Surface drop size distribution measurements using two different disdrometers (2D video and RD-69) from a number of climatic regimes are analyzed and compared with the radar retrievals in a statistical and functional approach. The composite statistics based on disdrometer and radar retrievals suggest that, on average, the two parameters (generalized intercept and median volume diameter) for stratiform rain distributions lie on a straight line with negative slope, which appears to be consistent with variations in the microphysics of stratiform precipitation (melting of larger, dry snow particles versus smaller, rimed ic...

/pdf/raindrop-size-distribution-in-different-climatic-regimes-2kh1jgz6g8.pdf

Raindrop Size Distribution in Different Climatic Regimes from Disdrometer and Dual-Polarized Radar Analysis

The shape of the drop size distribution (DSD) reflects the physics of rain. The DSD is the result of the microphysical processes that transform the condensed water into rain. The question of the DSD is also central in radar meteorology, because it rules the relationships between the radar reflectivity and the rainfall rate R. Normalizing raindrop spectra is the only way to identify the shape of the distribution. The concept of normalization of DSD developed in this paper is founded upon two reference variables, the liquid water content LWC and the mean volume diameter Dm. It is shown mathematically that it is appropriate to normalize by N0* ∝ LWC/Dm4 with respect to particle concentration and by Dm with respect to drop diameter. Also, N0* may be defined as the intercept parameter that would have an exponential DSD with the same LWC and Dm as the real one. The major point of the authors' approach is that it is totally free of any assumption about the shape of the DSD. This new normalization has be...

https://journals.ametsoc.org/doi/pdf/10.1175/1520-0450(2001)040%3C1118%3ATCONDT%3E2.0.CO%3B2

The Concept of “Normalized” Distribution to Describe Raindrop Spectra: A Tool for Cloud Physics and Cloud Remote Sensing

In this study, 600 h of vertically pointing X-band radar data and 50 h of UHF boundary layer wind profiler data were processed and analyzed to characterize quantitatively the structure and the causes of the radar signature from melting precipitation. Five classes of vertical profiles of reflectivity in rain were identified, with three of them having precipitation undergoing a transition between the solid and liquid phase. Only one of them, albeit the most common, showed a radar brightband signature. In-depth study of the bright band and its dependence on precipitation intensity reveals that the ratio of the brightband peak reflectivity to the rainfall reflectivity is constant at 8 dB below 0.5 mm h−1 and then increases to reach 13 dB at 2.5 mm h−1 and 16 dB at 5 mm h−1. The equivalent reflectivity factor of snow just above the melting layer is on average 1–2 dB below the reflectivity of rain just below the melting layer, independent of precipitation intensity. The classical brightband explanation...

Long-Term Radar Observations of the Melting Layer of Precipitation and Their Interpretation

High resolution rainfall measurements by radar for very small basins: the sampling problem reexamined

The authors consider the simplest radar equation involving the index of refraction n: the time t taken by electromagnetic waves to reach a target at range r and return to the radar is r=2rn/c/sub 0/, with c/sub 0/, being the speed of light in a vacuum. For fixed targets, r is constant and only n varies; hence if t could be measured precisely for such targets, the average value of the refractive index over the path between the radar and these targets could be determined. Unfortunately, most radars cannot measure t and site surveys are not accurate enough to determine r with the part-per-million accuracy required to obtain useful information about n. However, if the range to the target is fixed, but only known to a fair accuracy, say better than 1%, it would be enough to allow us to relate changes in t to changes in n; the absolute calibration would then have to be done by other means. With this scheme, we are only required to determine changes in t that can be obtained by measuring the phase of the target.

On the Extraction of Near-Surface Index of Refraction Using Radar Phase Measurements from Ground Targets

To complement the meteorological modeling of the melting layer, a model of the scattering properties at microwave frequencies for snow, melting snow, and rain is implemented. The scattering model, running in tandem with a meteorological model, generates the reflectivity fields associated with the hydrometeors in the model to facilitate comparisons with available observations. Several existing and a few new approaches for the scattering of melting snow are attempted. In addition, the models are run using several relationships for the density of snowflakes as a function of their size. A large variability in the prediction of the brightband intensity is observed as a function of the scattering model. However, the scattering model whose melting snow morphology resembles most the one of real snowflakes reproduces the available observations with the highest accuracy. Sensitivity to the snowflake density relationship used is found to be less important. Other features like the melting-layer thickness, brightband peak position, and Doppler velocity are also correctly predicted.

Modeling of the Melting Layer. Part II: Electromagnetic

High-spatial-resolution data were collected for nearby convective storms by using a volume scanning weather radar with 200m resolution, and for stratiform events by using a high-resolution (20m) vertically pointing radar. Errors in the estimation of rainfall by the scanning radar, due to the radar beam intersecting or overshooting features such as the bright band, were simulated by using these high-resolution images. It was found that these effects define a maximum useful range for the radar that is a strong function of the prevailing meteorological conditions (e.g. bright-band height, intense cell diameter and height).

Sudden changes in bright-band height over short times and the large scatter observed in its thickness limit the accuracy with which corrections for the vertical profile of reflectivity may be precalculated in stratiform rain events. A scheme for using a vertically pointing radar to produce these corrections in real time is proposed. the convective events show enough significant storm-to-storm variability that caution is required in extending the useful range of the radar too far.

Frédéric Fabry

Papers

Long-Term Radar Observations of the Melting Layer of Precipitation and Their Interpretation

High resolution rainfall measurements by radar for very small basins: the sampling problem reexamined

On the Extraction of Near-Surface Index of Refraction Using Radar Phase Measurements from Ground Targets

Modeling of the Melting Layer. Part II: Electromagnetic

The accuracy of rainfall estimates by radar as a function of range