UAE Research Program for Rain Enhancement Science Award

 “Microphysics of Convective Clouds and the Effects of Hygroscopic Seeding”   

UAE Research Program for Rain Enhancement Science Award

Selected out of 91 international research proposals that included over 398 scientists and researchers affiliated with 180 research institutes from 45 countries, Dr. Paul Lawson is the first U.S. scientist to receive the UAE Research Program for Rain Enhancement Science award in January, 2017.  Under the award, he will lead a research team from SPEC in investigating the role of ice production in cumulus clouds using the company’s Learjet research aircraft.  In this second cycle of the award, winners also included Prof. Hannele Korhonen, from Finland, and Prof. Giles Harrison, from the U.K.  Prof. Korhonen was awarded for her research investigating the role of atmospheric aerosols in precipitation production efficiency.  Prof. Harrison was awarded for his research into the electrical properties of clouds and their relation to rain production. See UAE Awards Video.


Project Overview

Through this award, a team of scientists from SPEC and NCAR will combine field observations and modeling efforts to investigate a new approach to rain enhancement that triggers a natural secondary ice production process in cumulus clouds (see presentation slides).  When large "supercooled" drops – drops that initially remain unfrozen at temperatures below zero degrees Celsius form in clouds and some of them freeze, they emit tiny ice particles.  These tiny ice particles then collide with other large supercooled drops, producing an avalanche process that freezes the remaining large drops, producing small hail stones.  The hail stones then melt into rain after they fall into the warm sub-cloud layer.  Seeding in the updraft at cloud base with water attracting material can facilitate the development of large drops that are required to generate the natural secondary ice process.

Using sophisticated instrumented aircraft and radar, this experiment will study this process in cumulus clouds in the UAE, and evaluate the potential for rain enhancement.

UAE Research Program for Rain Enhancement Science visit to Boulder, Colorado.


Science Background and Proposal Objectives

This work seeks to examine the effects of hygroscopic seeding on the formation of large supercooled cloud drops that are connected with rapid glaciation and heavy rainfall.  The proposed research will explore how hygroscopic seeding affects cumulus cloud drop development and ice formation over a range of cloud base temperatures and drop distributions, focusing on conditions that would not promote large drop formation naturally.   The Learjet is an in situ cloud microphysics aircraft that is a proven platform for rapidly climbing with updraft cores and documenting the development of coalescence.  Cloud base hygroscopic seeding will be conducted using seeding aircraft from the West Texas Weather Modification Association, which has been conducting cloud base seeding for over 30 years.  NexRad radar data will be used in real time to help vector the aircraft to appropriate locations and also in the post-analysis.  Finally, the Morrison and Grabowski (2010) one-dimensional cloud resolving model, which was adapted to include aircraft measurements (Lawson et al. 2015), will be upgraded to three-dimensions and compared with radar data.

Overall Scientific Objective:  Determine if hygroscopic seeding in the updrafts at the base of certain cumulus clouds can increase the drop size distribution sufficiently to initiate a natural secondary ice production process in the supercooled region of  the updraft.    


In-Situ Measurements and Sampling Platform

The SPEC Learjet will be flown in updraft cores to collect in situ data.  As shown in the photographs below, the Learjet is outfitted with a suite of micophysical sensors, which measure particle concentration, area, and size, and collect high resolution images for phase and shape identification.

The specific flight profile conducted by the Learjet is described as follows.  Once suitable cumulus clouds are present in an area, the Learjet makes a few passes just below cloud base to document the cloud base temperature and pressure, which is used to “categorize” the cloud and also for computation of adiabatic liquid water content (LWC) in post analysis. Next, the Learjet makes passes just above cloud base to document the drop size distribution and then climbs to the – 5 °C level. The crew selects a newly-developing cloud turret to penetrate and documents the LWC, drop spectra and updraft characteristics.  The goal is to find a cloud with an ice-free or nearly ice-free updraft core whose updraft exceeds 5 m s-1 and is sustained for at least 3 s (about 0.4 km) along the flight  path.  Once a suitable turret is located, the cloud is categorized as a “candidate” and the Learjet climbs and makes repeated penetrations at the – 8 °C, – 12 °C, and – 15 °C levels.  During the ICE-T and SEAC4RS projects, strong, nearly ice-free cores were frequently identified visually by the crew.   One of the reasons the Learjet is an adept platform for this work is that, with an experienced crew that can identify newly developing cumuli, the Learjet can “dash” to the cloud at 200 m s-1 and then slow to its penetration airspeed of 120 to 130 m s-1 (at the specified altitudes), then quickly climb with the turret as it grows for repeated penetrations at higher altitudes.  Slower, less maneuverable aircraft are often too late in arriving at the candidate cloud, and the ice initiation process has already begun by the time the aircraft penetrates the cloud.


SPEC Learjet outfitted with microphysical instrumentation


Numerical Simulations

Our modeling effort is primarily focused on upgrading and further developing the bin microphysics scheme described by Morrison and Grabowski (2010) and Lawson et al. (2015). We have re-vamped most of the code, including using the two-moment top-hat method of moments for condensation growth and evaporation (Stevens et al. 1996) and the two-moment method for collision-coalescence (Tzivion et al. 1987). Our implementation of these methods is similar to that within the Tel Aviv University bin microphysics model. Ice is represented using a novel shape, density, and aspect ratio-evolving scheme, led by Dr. Jerry Harrington (Penn State University). While most of the liquid microphysics component is finished, considerable development work remains with the ice, including treatments of aggregation and melting.

We have implemented the code into the Weather Research and Forecasting model version 3.6.1. Testing has begun for simple two-dimensional and three-dimensional idealized test cases. Our plan is to use this scheme in WRF to investigate microphysics-dynamics interactions in growing cumulus clouds, focusing on precipitation initiation around the time of first radar echo detection.


Project Timeline: Progress and Outlook

First Year: 

Utilizing the SPEC Learjet and Nexrad radar, in situ microphysical and radar observations were collected of cumulus clouds with a range of cloud base temperatures and drop size distributions to determine what combination of these parameters are most suitable for modification with hygroscopic seeding material.

Second Year:

Once candidate clouds are identified, conduct coordinated randomized seeding experiments to determine if treated clouds develop a significantly broader drop distribution and rapidly glaciate.

Second and Third Year:

Analyze radar data using TITAN software to determine if the seeded clouds produce more rainfall below cloud base. Run the modified Morrison-Grabowski 3-D cloud resolving model to determine if the effects from seeding can be reproduced in the model.

Third Year:

Compile all results and publicize findings in annual reports and peer-reviewed journal papers.


Lawson, R. P., Woods, S., and H. Morrison, 2015: The microphysics of ice and precipitation development in tropical cumulus clouds. J. Atmos. Sci., 72, 2429-2445. 

Morrison, H., and W. W. Grabowski, 2010: An improved representation of rimed snow and conversion to graupel in a mutlicomponent bin microphysics scheme. J. Atmos. Sci., 67, 1337-1360. 

Stevens, B., G. Feingold, W. R. Cotton, and R. L. Walko, 1996a: Elements of the microphysical structure of numerically simulated nonprecipitating stratocumulus. J. Atmos. Sci., 53, 980–1006.


Tzivion, S., G. Feingold, and Z. Levin, 1987: An efficient numerical solution to the stochastic collection equation. J. Atmos. Sci., 44, 3139–3149.