Katrina S. Virts

Highlights of a New Ground-Based, Hourly Global Lightning Climatology

The seasonally and diurnally varying frequency of lightning flashes provides a measure of the frequency of occurrence of intense convection and, as such, is useful in describing the Earths climate. Here we present a few highlights of a global lightning climatology based on data from the ground-based World Wide Lightning Location Network (WWLLN), for which global observations began in 2004. Because WWLLN monitors global lightning continuously, it samples ~100 times as many lightning strokes/flashes per year as the Tropical Rainfall Measuring Missions (TRMM) Lightning Imaging Sensor (LIS). Using WWLLN data it is possible to generate a global lightning climatology that captures seasonal variations, including those associated with the midlatitude storm tracks, and resolves the diurnal cycle, thereby illuminating the interplay between sea breezes, mountain–valley wind systems, and remotely forced gravity waves in touching off thunderstorms in a wide variety of geographical settings. The text of the paper sho...

Annual, Interannual, and Intraseasonal Variability of Tropical Tropopause Transition Layer Cirrus

Cloud fields based on the first three years of data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission are used to investigate the relationship between cirrus within the tropical tropopause transition layer (TTL) and the Madden-Julian oscillation (MJO), the annual cycle, and El Nino-Southern Oscillation (ENSO). The TTL cirrus signature observed in association with the MJO resembles convectively induced, mixed Kelvin-RossbywavesolutionsabovethePacificwarmpoolregion.Thissignatureiscenteredtotheeastofthe peak convection and propagates eastward more rapidly than the convection; it exhibits a pronounced east- ward tilt with height, suggestive of downward phase propagation and upward energy dispersion. A cirrus maximumisobservedoverequatorialAfricaandSouthAmericawhentheenhancedMJO-relatedconvection enters the western Pacific. Tropical-mean TTL cirrus is modulatedby the MJO, with more than twice as much TTL cirrus fractional coverage equatorward of 108 latitude when the enhanced convection enters the Pacific than a few weeks earlier, when the convection is over the Indian Ocean. The annual cycle in cirrus clouds around the base of the TTL is equatorially asymmetric, with more cirrus observed in the summer hemisphere. Higher in the TTL, the annual cycle in cirrus clouds is more equatorially symmetric, with a maximum in the boreal winter throughout most of the tropics. The ENSO signature in TTL cirrus is marked by a zonal shift of the peak cloudiness toward the central Pacific during El Nino and toward the Maritime Continent during La Nina.

Tropical Tropopause Transition Layer Cirrus as Represented by CALIPSO Lidar Observations

The spatial and temporal variability of cirrus cloud fraction within the tropical tropopause transition layer (TTL) is investigated based on three years of data from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, analyzed in conjunction with fields from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA)-Interim and temperature profiles from radiosondes launched at Manus Island, Papua New Guinea (28S, 1478E). TTL cirrus is found to be mainly confined to the rising branch of the Hadley cell within ;158 of the equator, with maximum cloud fraction between 14 and 15 km. The time-varying spatial pattern ofcloud fraction within this beltdoesnot resemble the pattern of cloud fractionin the layer below, aswould be expected ifthe TTL cirruswere formed bythe spreading ofthe anvils of convective clouds. On the contrary, within the stably stratified layer above ;13 km, cirrus cloud fraction and temperaturebothappear tobemodulatedbytheplanetary-scalevertical velocityfield. Thetime-varyingspatial patterns arereminiscentofthevertical-propagatingKelvinwaveresponsetoanequatorialheatsource,withthe coldest, cloudiest air in the TTL centered approximately 308 of longitude to the east of the strongest heating.

Observations of Temperature, Wind, Cirrus, and Trace Gases in the Tropical Tropopause Transition Layer during the MJO*

AbstractSatellite observations of temperature, optically thin cirrus clouds, and trace gases derived from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and the Microwave Limb Sounder (MLS) are analyzed in combination with Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) wind and humidity fields in the tropical tropopause transition layer (TTL), using the Madden–Julian oscillation (MJO) as a carrier signal. MJO-related deep convection induces planetary-scale Kelvin and Rossby waves in the stably stratified TTL. Regions of ascent in these waves are associated with anomalously low temperatures, high radiative heating rates, enhanced cirrus occurrence, and high carbon monoxide and low ozone concentrations. Low water vapor mixing ratio anomalies lag the low temperature anomalies by about 1–2 weeks. The anomalies in all fields propagate eastward, circum...

Diurnal and Seasonal Lightning Variability over the Gulf Stream and the Gulf of Mexico

AbstractRecent observations from the World Wide Lightning Location Network (WWLLN) reveal a pronounced lightning maximum over the warm waters of the Gulf Stream that exhibits distinct diurnal and seasonal variability. Lightning is most frequent during summer (June–August). During afternoon and early evening, lightning is enhanced just onshore of the coast of the southeastern United States because of daytime heating of the land surface and the resulting sea-breeze circulations and convection. Near-surface wind observations from the Quick Scatterometer (QuikSCAT) satellite indicate divergence over the Gulf of Mexico and portions of the Gulf Stream at 1800 LT, at which time lightning activity is suppressed there. Lightning frequency exhibits a broad maximum over the Gulf Stream from evening through noon of the following day, and QuikSCAT wind observations at 0600 LT indicate low-level winds blowing away from the continent and converging over the Gulf Stream. Over the northern Gulf of Mexico, lightning is mos...

Diurnal Lightning Variability over the Maritime Continent: Impact of Low-Level Winds, Cloudiness, and the MJO

AbstractLightning over the Maritime Continent exhibits a pronounced diurnal cycle. Daytime and evening lightning occurs near coastlines and over mountain slopes, driven by sea and valley breezes. Nocturnal and morning thunderstorms are touched off where land breezes or mountain breezes converge or by gravity waves propagating away from regions of vigorous afternoon convection. In this study, the modulation of the diurnal cycle of lightning and precipitation by 850-hPa winds, cloudiness, and the Madden–Julian oscillation (MJO) is investigated using observations from the World Wide Lightning Location Network (WWLLN) and the Tropical Rainfall Measuring Mission (TRMM) satellite. The 850-hPa wind speed and area-averaged cloudiness are shown to be negatively correlated with day-to-day lightning frequency over land, and thunderstorm occurrence is suppressed windward of, and enhanced leeward of, mountain ranges. Lightning and environmental conditions are similarly related in the MJO. During break periods, the reg...

Variation of Lightning and Convective Rain Fraction in Mesoscale Convective Systems of the MJO

Characteristics of mesoscale convective systems (MCSs) in regions affected by the Madden‐Julian oscillation (MJO) are investigated using a database of MCSs observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). Lightning occurrence detected by the World-Wide Lightning Location Network (WWLLN) is composited in a frameworkcentered on theMCSs. DuringMJO active periods, MCSs are more numerous and larger, as the convective features persist and attain greater horizontal scales. Anomalies of the lifted index, derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) fields, indicate that MCS environments are more stable during MJO active periods. Over the Indian Ocean, Maritime Continent, and western Pacific, lightning density in an MCS maximizes during the time that the total number of systems begins to increase as the MJO is beginning to be more active, implyingbothmore vigorous convection andless extensivestratiformrainareasat this transitionaltime ofthe MJO. The peak in MJO precipitation coincides with peak occurrence of interconnected MCSs with larger stratiform rain fraction, shown by the Tropical Rainfall Measuring Mission satellite, while composites of lightning frequency show that during MJO active periods the zone of lightning is contracted around the centers of MCSs, and flashes are less frequent.

Daily and intraseasonal relationships between lightning and NO2 over the Maritime Continent

[1] The relationship between lightning and NO2 over Indonesia is examined on daily and intraseasonal time scales based on lightning observations from the World Wide Lightning Location Network (WWLLN) and tropospheric NO2 column densities from the Global Ozone Monitoring Experiment (GOME‐2) satellite mission. Composites of the daily NO2 observations regressed onto lightning frequency reveal a plume of enhanced NO2 following a day of enhanced lightning. Lightning and NO2 also vary coherently with the intraseasonal Madden‐Julian Oscillation (MJO) in a manner distinct from the cloudiness signature, with variations of up to ∼50% of the annual mean. Citation: Virts, K. S., J. A. Thornton, J. M. Wallace, M. L. Hutchins, R. H. Holzworth, and A. R. Jacobson (2011), Daily and intraseasonal relationships between lightning and NO2 over the Maritime Continent, Geophys. Res. Lett., 38, L19803, doi:10.1029/2011GL048578.

Clouds and Water Vapor in the Tropical Tropopause Transition Layer over Mesoscale Convective Systems

AbstractObservations from A-Train satellites and other datasets show that mesoscale convective systems (MCSs) affect the water vapor and ice content of the tropical tropopause transition layer (TTL). The largest MCSs with radar reflectivity characteristics consistent with the presence of large stratiform and anvil regions have the greatest impact. Most MCSs are associated with clouds in the TTL. Composites in MCS-relative coordinates indicate enhanced cloudiness and ice water content (IWC) extending toward the cold-point tropopause (CPT), particularly in large and connected MCSs. Widespread anvils in the lower TTL are evident in the peak cloudiness diverging outward at those levels. Upper-tropospheric water vapor concentrations are enhanced near MCSs. Close to the centers of MCSs, water vapor is suppressed at TTL base, likely because of the combined effects of reduced moistening or dehydration at the higher TTL relative humidities and subsidence above cloud top. Weak moistening is observed near the CPT, c...

Seasonal and Intraseasonal Variability of Mesoscale Convective Systems over the South Asian Monsoon Region

AbstractSeasonal and intraseasonal differences in mesoscale convective systems (MCSs) over South Asia are examined using A-Train satellites, a ground-based lightning network, and reanalysis fields. Premonsoon (April–May) MCSs occur primarily over Bangladesh and the eastern Bay of Bengal. During the monsoon (June–September), small MCSs occur over the Meghalaya Plateau and northeast Himalayan notch, while large and connected MCSs are most widespread over the Bay of Bengal. Monsoon MCSs produce less lightning and exhibit more extensive stratiform and anvil reflectivity structures in CloudSat observations than do premonsoon MCSs.During the monsoon, Bay of Bengal and Meghalaya Plateau MCSs vary with the 30–60-day northward-propagating intraseasonal oscillation, while northeast Himalayan notch MCSs are associated with weak large-scale anomalies but locally enhanced CAPE. During intraseasonal active periods, a zone of enhanced large and connected MCSs, precipitation, and lightning extends from the northeastern A...