Heat and Moisture Budgets of an Intense Midlatitude Squall Line

William A. Gallus Jr. Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Richard H. Johnson Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

Rawinsonde data from OK PRE-STORM are used to calculate the heat and moisture budgets during the late mature through dissipating stages of an intense squall line system with a trailing stratiform region. Budgets have been computed at three separate times using composited data from approximately 30 rawinsondes over three-hour intervals. Data spacing is marginally adequate to resolve features within the broad (∼200 km) stratiform region, but not the narrow (∼40 km) leading convective line. Low-level radar data and surface accumulated rainfall are used to partition the system into convective line and stratiform regions, and to cheek the accuracy of the heat and moisture budgets.

The squall line had a pronounced descending rear inflow jet that extended toward the front of the system with time, and strong front-to-rear flow above and beneath this jet. The diagnosed vertical motion showed deep ascent in the leading convective line and upper-level ascent above lower-level descent in the trailing stratiform region. The two centers of ascent were somewhat merged together, due in part to resolution limitations. The axes of ascent and descent weakened and became increasingly sloped over time as the trailing stratiform region widened and became separated from the leading convective line. The heat source (Q1) distribution and its time variations are similar to those of vertical motion with heating peaks between 300 and 400 mb in the leading convective line and trailing stratiform region. Cooling due to evaporation and melting occurred in the stratiform region and was most intense just behind the back edge of stratiform echo around 550 mb. The moisture sink (Q2) peaked in the convective line region, but at lower levels than the heat source. Moistening due to evaporation was typically strongest around 700 mb in the stratiform region.

Vertical integrations of both budgets produced rainfall rates generally close to the observed ones for averages over the entire system. Convective line rates were underestimated by 40%, due in large part to inadequate sounding resolution. In the later stages, when the stratiform region was well separated from the leading convective line and could be reasonably well resolved, diagnosed rates underestimated observed rainfall rates by as much as 2–3 mm h−1 as the system decayed. Radar reflectivity data showed that the rearward transport of hydrometeors from the leading convective line could add as much as 2–4 mm h−1 to the diagnosed stratiform precipitation rates. This transport, along with a possible additional contribution from the fallout of hydrometeors stored from earlier times, can well explain the deficiencies in rainfall rates diagnosed from the moisture budget.

Abstract

Rawinsonde data from OK PRE-STORM are used to calculate the heat and moisture budgets during the late mature through dissipating stages of an intense squall line system with a trailing stratiform region. Budgets have been computed at three separate times using composited data from approximately 30 rawinsondes over three-hour intervals. Data spacing is marginally adequate to resolve features within the broad (∼200 km) stratiform region, but not the narrow (∼40 km) leading convective line. Low-level radar data and surface accumulated rainfall are used to partition the system into convective line and stratiform regions, and to cheek the accuracy of the heat and moisture budgets.

The squall line had a pronounced descending rear inflow jet that extended toward the front of the system with time, and strong front-to-rear flow above and beneath this jet. The diagnosed vertical motion showed deep ascent in the leading convective line and upper-level ascent above lower-level descent in the trailing stratiform region. The two centers of ascent were somewhat merged together, due in part to resolution limitations. The axes of ascent and descent weakened and became increasingly sloped over time as the trailing stratiform region widened and became separated from the leading convective line. The heat source (Q1) distribution and its time variations are similar to those of vertical motion with heating peaks between 300 and 400 mb in the leading convective line and trailing stratiform region. Cooling due to evaporation and melting occurred in the stratiform region and was most intense just behind the back edge of stratiform echo around 550 mb. The moisture sink (Q2) peaked in the convective line region, but at lower levels than the heat source. Moistening due to evaporation was typically strongest around 700 mb in the stratiform region.

Vertical integrations of both budgets produced rainfall rates generally close to the observed ones for averages over the entire system. Convective line rates were underestimated by 40%, due in large part to inadequate sounding resolution. In the later stages, when the stratiform region was well separated from the leading convective line and could be reasonably well resolved, diagnosed rates underestimated observed rainfall rates by as much as 2–3 mm h−1 as the system decayed. Radar reflectivity data showed that the rearward transport of hydrometeors from the leading convective line could add as much as 2–4 mm h−1 to the diagnosed stratiform precipitation rates. This transport, along with a possible additional contribution from the fallout of hydrometeors stored from earlier times, can well explain the deficiencies in rainfall rates diagnosed from the moisture budget.

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