A41D-01
A Novel Approach for Representing Ice Microphysics in Models
A novel approach has been developed for representing ice microphysics in numerical models. In this approach, the ice particle mass-dimension and projected-area-dimension relationships vary as a function of particle size and rimed mass fraction. All ice microphysical processes and parameters are calculated in a self-consistent manner in terms of these mass-dimension and area-dimension relationships. The rimed mass fraction is predicted locally by separately prognosing the ice mixing ratios acquired through water vapor deposition and through riming.This approach allows representing in a natural way gradual transition from small to large ice particles due to growth by water vapor deposition and aggregation, and from unrimed crystals to graupel due to riming. In traditional approach to ice microphysics, these processes are treated by separating ice particles into predefined categories (such as cloud ice, snow, and graupel) using fairly arbitrary conversion thresholds and conversion rates. The new scheme is applied to an idealized 2D kinematic framework with a specified flow field mimicking mixed- phase shallow cumulus. The new scheme is compared to a version of the scheme that uses the traditional approach for ice microphysics; that is, unrimed ice/snow and graupel are separate species, with threshold-based conversion rates between the former and the latter. The new and traditional schemes produce similar results, although the traditional scheme, unlike the new scheme, produces a distinct double maximum in the surface precipitation rate, corresponding to precipitation shafts consisting of either ice/snow or graupel. The relative magnitude of these peaks, as well as the ice water path and optical depth of the simulated cloud, are highly sensitive to the threshold for converting unrimed ice to graupel. In contrast, the new scheme does not require any conversion threshold and predicts formation of ice particles with wide range of rimed fractions.
A41D-02
An Improved Bulk Microphysical Scheme for Studying Precipitation Processes: Comparisons with Other Schemes
Cloud microphysical processes play an important role in non-hydrostatic high-resolution simulations. Over the
past decade both research and operational numerical weather prediction models have started using more
complex cloud microphysical schemes that were originally developed for high-resolution cloud-resolving models.
An improved bulk microphysical parameterization (adopted from the Goddard microphysics schemes) has
recently implemented into the Weather Research and Forecasting (WRF) model. This bulk microphysical scheme
has three different options --- 2ICE (cloud ice & snow), 3ICE-graupel (cloud ice, snow & graupel) and 3ICE-hail
(cloud ice, snow & hail). High-resolution model simulations are conducted to examine the impact of
microphysical schemes on two different weather events (a midlatitude linear convective system and an Atlantic
hurricane). In addition, this bulk microphysical parameterization is compared with WRF's three other bulk
microphysical schemes. The results suggest that microphysics has a major impact on the organization and
precipitation processes associated with a summer midlatitude convective system. The 3ICE scheme with a cloud
ice-snow-hail configuration led to a better simulation of the summer midlatitude convective line system than the
other schemes since the scheme includes dense ice precipitating (hail) particle with very fast fall speed (over 10
m s-1). For an Atlantic hurricane case, varying the microphysical schemes had no significant impact on the track
forecast but did affect the intensity (important for air-sea interaction) and the vertical distribution of cloud species
(important for satellite retrieval). Results also suggest that different configuration of ice schemes are required to
simulate the midlatitude Mesoscale Convective System (MCS) and the hurricane.
http:atmospheres.gsfc.nasa.gov/cloud_modeling
A41D-03
Effect of Cloud Microphysics on Storm Dynamics: PRE-STORM Case Study
A 2D cloud-resolving model, the Goddard Cloud Ensemble (GCE) model, is used to simulate a mid-latitude summertime squall line during the PRE-STORM field campaign on June 10-11, 1985. Two microphysical schemes, a simple bulk scheme and a detailed spectral bin scheme, using identical environmental conditions and initialization, produce storms with different characteristics in terms of storm structures and temporal variations. During the mature stage of the squall line, the bulk scheme produces a multi-cell storm with convective cells, which are remnants of previous leading cells, embedded well into its stratiform region. The leading cell simulated by the bulk scheme has a distinct lifecycle with each new leading cell generated as an independent entity. In contrast, the bin scheme produces a uni-cell storm with a homogeneous stratiform region. The single convective cell at the leading edge has a weak evolution mode with little temporal variation. These characteristics have been observed in storms formed in different environmental conditions, but are simulated here by two self-consistent microphysical schemes. This indicates the significance of cloud microphysics in shaping the storm structure and dynamics. Sensitivity tests using the simple bulk microphysical scheme reveal two major contributors to the sensitivities simulated by the control bulk and bin scheme, that is, the artificial enhancement of the rain evaporation rate simulated in the bulk scheme, and the different partitioning of precipitable ice particles in these two schemes. Strong rain evaporation in the bulk scheme results in a strong near surface cool pool, which overwhelms the horizontal vorticity generated by the near surface wind shear and causes the leading convection to lean backward. The excessive backward tilting is the reason for the splitting and rearward traveling of the leading convection. Reduced rain evaporation in the bulk scheme produces an upright leading convection with little temporal variation, but failed to form an extensive trailing stratiform region due to the assumptions on the forms (hail vs. graupel) and partitioning (snow vs. hail/graupel) of precipitable ice particles. Future work will focus on improving the rain evaporation rate and ice microphysics in the bulk scheme using both the observations and the detailed bin scheme simulations.
A41D-04
Cloud-Precipitation Microphysical Characteristics of Tropical Storm Bilis (2006). Part II: Simulations
Tropical Storm Bilis (2006) made a landfall on Fujian, China on 14 July 2006, and caused direct economic losses of 34.83B Chinese dollars, 654 deaths and 208 missing. In this paper, the Advanced Regional Prediction System (ARPS) along with a two-dimensional cloud-resolving model are used to simulate TS Bilis (2006)'s life cycle. The model simulation data are validated with satellite and radar data and their retrievals as well as the data from all the conventional and non-conventional meteorological soundings including the data from the AWSs, Doppler radars and FY-2C. The verified simulation data are further used to analyze cloud-precipitation microphysical properties and processes over different regions (convective/stratiform regions) during different development stages by calculating surface rainfall equation and cloud microphysics budgets.
A41D-05
Advanced Research WRF (ARW) Modeled Low-level Jet Climatology Compared to Observed Climatologies
Nocturnal low-level jets (LLJs) are common features observed in the Great Plains region of the United States. LLJs play a key factor in initiating and sustaining mesoscale convective systems and other severe convective storm modes in the Great Plains. The LLJ in the Great Plains can also be a key source of moisture transport into the region which is crucial for severe weather development and shown important for widespread flooding. Knowing the climatology of such events is important so an understanding of the importance of the LLJ in severe weather can be furthered. Several observational studies have been conducted to determine the climatology of LLJs over the Great Plains. However these studies are limited due to the spatial restraints. Using point measurements makes it nearly impossible to determine the spatial structure of LLJs. Using a NWP model lessens this restraint, though grid spacing and frequency of the model output is still problematic using operational forecasts. This study investigates how well the operational Advanced Research WRF (ARW) forecasts represent the LLJ climatology of the region centered on the ARM site along the Kansas/Oklahoma border using 6 months (June- Sept.) of 3 hourly outputs with 12 km grid spacing. The NCAR's operational ARW is run in 36/12 km two-way nested configuration and provides a 48 h forecast from 00 Z initialization utilizing 40 km Eta fields. Preliminary results indicate that the ARW has similar climatology characteristics (i.e. frequency, max time occurrence, dominant direction) to the previous observational studies. To forecast LLJs, accurate representation of the PBL is crucial, which is also important for being able to forecast many high impact events. If the WRF can be shown to produce a similar climatology to that what has been observed, we gain more confidence in WRF and its PBL parameterizations. Since the ARW shows promise of representing the LLJ climatology of the ARM site closely to what has been found in the observational studies, the ARW could be used to get a better understanding on the frequency of the LLJ over other sites within the Great Plains. This information could be further used to understand the importance of the LLJ in moisture transportation. It is also possible for the ARW to be used to further investigate the forcing mechanisms of LLJs which is yet not fully understood. A better understanding of LLJ development could lead to greater improvement in forecasts and severe weather initiation.
A41D-06
Microphysical Characteristics of Developing Tropical Cyclones During NAMMA 2006
Aerosols interacting with clouds can have profound impacts on the microphysical structure of these clouds and can affect precipitation formation processes. A modeling study in the 1990's (Rogers et al 1994) noted that a 100- fold increase in ice nuclei (IN) concentration resulted in cloud water depletion and the prevention of homogeneous freezing in maritime cumulus cloud. Several studies conducted during CRYSTAL-FACE (DeMott et al 2003; Sassen et al 2003) noted IN concentrations 20-100 above normal values and glaciation of altocumulus clouds at unusually warm temperatures, both due to Saharan dust. Finally, Koren et al (2005) denoted changes in convective cloud properties due to the effects of aerosols. These effects, particularly on convective clouds, might be important for larger-scale processes, such as tropical cyclogenesis. The microphysical characteristics (liquid water content, particle concentration, etc.) of two developing tropical cyclones (Tropical Depression 8 and Tropical Storm Debby from the 2006 Atlantic Hurricane Season) will be presented. In particular, the possible effects of dust on the microphysics, as well as implications for cyclogenesis will be explored. These data were taken on board the NASA DC-8 during the NAMMA 2006 field campaign. Preliminary results suggest a relationship between increased aerosol concentrations and higher amounts of cloud ice water content and cloud-sized particle concentrations.
A41D-07
Numerical Modeling of Topography-Related Heterogeneity in the Atmospheric Boundary Layer: Implication for Area-Averaged Estimates of Surface Fluxes
Numerical simulations with the Regional Atmospheric Modeling System (RAMS) were used to study surface fluxes perturbations within the atmospheric boundary layer, caused by topography variations at horizontal scales less than 100 meters. Previous studies have shown that the integral effects, averaged over typical distances of 1 km or more, of these surface fluxes perturbations might be as high as 10% as compared with those over a flat surface. We examined area-averaged effects of these perturbations simulated with RAMS over a surface topography represented by an isolated hill and a series of bell-shaped low hills. Both 2D and 3D RAMS simulations were used with different background meteorological conditions and slope values. Particular attention will be paid on potential decomposition of fluxes integral changes into different parts associated with a direct effect of a surface area increase and wind speed, air temperature and humidity changes.