Scott Landolt

How Well Are We Measuring Snow: The NOAA/FAA/NCAR Winter Precipitation Test Bed

This paper presents recent efforts to understand the relative accuracies of different instrumentation and gauges with various windshield configurations to measure snowfall. Results from the National Center for Atmospheric Research (NCAR) Marshall Field Site will be highlighted. This site hosts a test bed to assess various solid precipitation measurement techniques and is a joint collaboration between the National Oceanic and Atmospheric Administration (NOAA), NCAR, the National Weather Service (NWS), and Federal Aviation Administration (FAA). The collaboration involves testing new gauges and other solid precipitation measurement techniques in comparison with World Meteorological Organization (WMO) reference snowfall measurements. This assessment is critical for any ongoing studies and applications, such as climate monitoring and aircraft deicing, that rely on accurate and consistent precipitation measurements.

Dependence of Snow Gauge Collection Efficiency on Snowflake Characteristics

Accurate snowfall measurements are critical for a wide variety of research fields, including snowpack monitoring, climate variability, and hydrological applications. It has been recognized that systematic errors in snowfall measurements are often observed as a result of the gauge geometry and the weather conditions. The goal of this study is to understand better the scatter in the snowfall precipitation rate measured by a gauge. To address this issue, field observations and numerical simulations were carried out. First, a theoretical study using finite-element modeling was used to simulate the flow around the gauge. The snowflake trajectories were investigated using a Lagrangian model, and the derived flow field was used to compute a theoretical collection efficiency for different types of snowflakes. Second, field observations were undertaken to determine how different types, shapes, and sizes of snowflakes are collected inside a Geonor, Inc., precipitation gauge. The results show that the collection efficiency is influenced by the type of snowflakes as well as by their size distribution. Different types of snowflakes, which fall at different terminal velocities, interact differently with the airflow around the gauge. Fast-falling snowflakes are more efficiently collected by the gauge than slow-falling ones. The correction factor used to correct the data for the wind speed is improved by adding a parameter for each type of snowflake. The results show that accurate measure of snow depends on the wind speed as well as the type of snowflake observed during a snowstorm.

The Hotplate Precipitation Gauge

A new instrument designed to measure precipitation, the ‘‘hotplate precipitation gauge,’’ is described. The instrument consists of a heated thin disk that provides a reliable, low-maintenance method to measure precipitation rate every minute without the use of a wind shield. The disk consists of two heated, thermally isolated identical aluminum plates—one facing upward and the other downward. The two plates are heated independently, and both are maintained at constant temperatureabove 758C by electronic circuitry that heats the plates depending on the deviation from the set temperature. Precipitation rate is estimated by calculating the power required to either melt or evaporate snow or to evaporate rain on the upward-facing plate, compensated for wind effects by subtracting out the power on the lower, downward-facing plate. Data from the World Meteorological Organization reference standard for liquid-equivalent snowfall rate measurements, the Double Fence Intercomparison Reference (DFIR) shield system, were used as the truth to develop the hotplate algorithm. The hotplate measures the liquid-equivalent precipitation rate from 0.25 to 35 mm h 21 within the National Weather Service standard for solid precipitation measurement. The hotplate was also shown to measure wind speed during severe icing conditions and during vibration. The high update rate (precipitation rate, wind speed, and temperature every 1 min), make this an ideal gauge for real-time applications, such as aircraft deicing and road weather conditions. It serves as an accumulation gauge by integrating the 1-min rates over time. It can also be used as a rain gauge for rainfall rates up to 35 mm h 21 .

New Ground Deicing Hazard Associated with Freezing Drizzle Ingestion by Jet Engines

A new ground deicing hazard is described, consisting of the accretion of freezing drizzle onto jet engine fan blades and cowlings, and subsequent shedding of the accreted ice during takeoff leading to damage to jet engine fan blades. Cases of damage to aircraft from hazardous surface icing conditions at Denver, Colorado and Oslo, Norway are described. The two cases at Denver cost United Airlines over

Weather Support for Terminal Area Icing Weather Information

2 million in damage to 12 B737-300 engines. The hazard is identified as heavy freezing drizzle through examination of National Weather Service observations of upper level temperature and humidity, satellite, radar, and freezing rain sensor data. The official National Weather Service observation during these cases, however, was either light snow and mist or light freezing drizzle. The reason for this misreport and underestimate of intensity lies in the current reporting rules for determining freezing drizzle intensity by visibility and not by precipitation-rate. Theoretical relationships are presented that show that the variation in drizzle size distribution and the difference in determining visibility from day and night is the cause of the poor correlation of drizzle rate with visibility.

The quantification and correction of wind-induced precipitation measurement errors

The Federal Aviation Administration (FAA) recently published a Notice of Proposed Rulemaking on certification of aircraft for operation in supercooled large drop (SLD) icing conditions with a final rule expected in 2014. One likely consequence of the rule is new limitations on takeoff andlanding in freezing drizzle and/or freezing rain conditions for new aircraft that are not fully certified to operate in these conditions as described in the new regulation. This will impact terminal area operations for airports in icing weather conditions and will require that local, highly-resolved, real-time icing condition information be made available for aircraft take-off and landing guidance for all decision-makers: air traffic control, weather dispatchers, ground de-icing facilities, airline operators, and flight crews. To address these issues, the FAA has begun funding research under a new project known as the Terminal Area Icing Weather Information for NextGen (TAIWIN), the goal of which is to improve measurements of icing conditions within the airport terminal area both at the ground and aloft. The initial focus of this project has been on three different areas; feasibility of automated detection of freezing drizzle at the surface using the Automated Surface Observing System (ASOS), development of new data quality control procedures for liquid water equivalent (LWE) gauge measurements, and assessment of snowfall variability across the terminal area and. Initial results from all three areas are presented.