| NSF Org: |
AGS Division of Atmospheric and Geospace Sciences |
| Recipient: |
|
| Initial Amendment Date: | September 17, 2012 |
| Latest Amendment Date: | September 17, 2012 |
| Award Number: | 1139743 |
| Award Instrument: | Standard Grant |
| Program Manager: |
edward bensman
AGS Division of Atmospheric and Geospace Sciences GEO Directorate for Geosciences |
| Start Date: | October 1, 2012 |
| End Date: | September 30, 2017 (Estimated) |
| Total Intended Award Amount: | $264,908.00 |
| Total Awarded Amount to Date: | $264,908.00 |
| Funds Obligated to Date: |
|
| History of Investigator: |
|
| Recipient Sponsored Research Office: |
550 S COLLEGE AVE NEWARK DE US 19713-1324 (302)831-2136 |
| Sponsor Congressional District: |
|
| Primary Place of Performance: |
126 Spencer Lab Newark DE US 19716-3140 |
| Primary Place of
Performance Congressional District: |
|
| Unique Entity Identifier (UEI): |
|
| Parent UEI: |
|
| NSF Program(s): | Physical & Dynamic Meteorology |
| Primary Program Source: |
|
| Program Reference Code(s): |
|
| Program Element Code(s): |
|
| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
While the "warm rain" process--in which ice-phase microphysics play little or no role in development of precipitation-size particles--is thought to account for as much as one-third of all tropical precipitation, yet a number of uncertainties remain regarding its exact mechanisms. In particular, the classic theory of condensational growth followed by collision-coalescence accompanying gravitational settling is unable to explain the oft-observed rapid development of rain in clouds confined to temperatures warmer than 0 degC. It has gradually become recognized that turbulence could play a critical role in accelerating this process, but realizable predictions have been difficult to achieve because quantitative research approaches are lacking. Recent advances in theory and computational capacity have enabled more quantitative assessment of turbulence effects on the collision-coalescence rate, and several preliminary parameterizations of turbulent collection have been developed. The goals of this research are to (a) incorporate representation of realistic turbulence-driven collection (i.e. droplet growth) in a large-eddy simulation (LES) model of cloud behavior using a bin microphysical scheme, and subsequently (b) to evaluate resulting predictions using existing in situ observations of real clouds to identify those conditions under which this newly-represented process affects and improves model predictions. Owing to their long lifecycle and a large amount of high-quality observational data available to facilitate real world-model comparisons, marine stratocumulus clouds will be emphasized. A hybrid direct numerical simulation (DNS) approach will be used to develop more accurate parameterization of turbulent collision-coalescence in conditions of low-to-intermediate mean flow dissipation rates, and statistical methods to incorporate such a parameterization into the LES framework will be explored. This approach will be complemented by phenomenological modeling of Reynolds number effects on airflow, and in so doing the LES model will be improved to allow for differing cloud entrainment mixing scenarios. The intellectual merit of this study will center on a more accurate and systematic evolution of the effects of turbulence on realistic stratocumulus conditions and improved assessment of our current ability to represent such effects quantitatively in a predictive mode. Broader Impacts of the effort will include development of findings that should ultimately be applicable to other types of clouds (e.g., more vigorous cumulus) and improved quantitative representation for warm-rain development in more coarse-grained numerical weather prediction and climate models, as well as through enhanced collaboration across the cloud microphysics and computational sciences. This setting will provide a vibrant and multifaceted education and training ground for a mix of undergraduate and graduate students at the two involved institutions.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
Note:
When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external
site maintained by the publisher. Some full text articles may not yet be available without a
charge during the embargo (administrative interval).
Some links on this page may take you to non-federal websites. Their policies may differ from
this site.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This award supported the University of Delaware team led by Dr. Lian-Ping Wang to work on three inter-related research topics. The first is hybrid direct numerical simulation (HDNS) of collision rates of cloud droplets at low flow dissipation rates relevant to stratocumulus clouds. To obtain reliable results, we revisited the collision detection algorithm. The standard deviation (or the uncertainty) of the average collision statistics was also examined analytically in terms of time correlation function of the data. We reported new HDNS results of collision statistics, showing how air turbulence increases the geometric collision statistics and the collision efficiency. We found that the collision-rate enhancement due to turbulence depends nonlinearly on the flow dissipation rate. This result calls for a more careful parameterization of the collision statistics in stratocumulus clouds. The work has led to a Masters thesis. The second topic was to work with Professor Patrick Chuang’s group at UC Santa Cruz, to incorporate our turbulent collection kernel into a large-eddy simulation model of stratocumulus clouds. This led to a journal paper titled “Estimating collision-coalescence rates from observations of marine stratocumulus” (Q. J. R. Meteorol. Soc. 143, 2755–2763, 2017). Here we seek to retrieve collision rates in the bottleneck size regime (droplets of 20 to 60 microns in diameter) from in situ aircraft observations of stratocumulus and explore the role of small-scale variability in the droplet size distributions in controlling retrieved rates. We compare rate constants estimated from observations with reference rate constants derived from a collision–coalescence box model, the result of which is termed the enhancement factor. For any given drop size or averaging length-scale, there is about one order of magnitude variability in the enhancement factors. These results suggest that spatial variability on length-scales smaller than 1.5 km prevents accurate retrieval of rate constants from large-scale average drop-size distributions. We also found that the turbulent kernel could only partially explain the enhancement. Finally, we developed a droplet-interface resolved simulation method and used the method to investigate the collision efficiency of cloud droplets. For the first time, the weak fluid inertia and droplet wake effect were considered in our simulations. We found these two effects can lead to 3 to 4 times increase in collision efficiency for droplets of similar sizes (around 30 microns). These results together demonstrate the complexity in quantifying the collision rate and collision efficiency of cloud droplets.
Last Modified: 03/11/2018
Modified by: Lian-Ping Wang
Please report errors in award information by writing to: awardsearch@nsf.gov.
